Adjustable tone controls allow reproduction to be altered to suit individual taste, or permit some measure of compensation to improve overall frequency response. They are very useful for general purpose equipment which may be used with crystal or magnetic input units, or for radio and tape, etc., but which do not have input circuits designed for these particular purposes.
Three passive tone control circuits are shown in Figure 40. That at A incorporates a preamplifier stage shown in full. With passive tone control circuits of this type, there is an overall loss of audio so that the signal level is reduced. If the amplifier has easily adequate gain, sufficient volume can still be obtained. But if gain is already at maximum, the addition of a tone control network can result in output volume then being insufficient. This depends on the amplifier and other circumstances, arid when it arises the addition of a preamplifier will restore volume.
In A VR1 is the tone control, higher frequencies being reduced as the wiper moves towards C1. VR2 is a gain or volume control. R3 and C3 provide source bias and by-passing, and R2 is the drain audio load, with output from C4. R1 with C2 decouple the positive supply line.
Typical values for A are:
VR1 500k linear VR2 500k log.
R1 1k C1 2.2nF
R2 4.7k C2 470uF
R3 2.2k C3 47uF
FET 2N3819 C4 0.47uF
Operation is from a 12v or similar supply, and R1 can be changed if necessary for higher voltages. In this and similar circuits there is considerable latitude in the choice of values for positions such as C1.
At B VR1 is a top cut control, and VR2 the volume control. C2 is connected to the gate at G, and a 2.2 megohm resistor provides the DC path from gate to negative line, other components being R1, R2, P3, C2, C3 and C4 as at A.
Typical values for B are:
C1 10nF VR1 500k linear
C2 0.47uF VR2 500k log
Yet another top cut control is shown at C. Here, R1 and R2 are the some as R1 and R2 in A, C2 of A being included as at A. In some cases such a tone control can be added to an existing stage without any disturbance to the circuit board. C1 at C can be 47nF, and VR1 25k. Higher values may be fitted for VR1, but tend to make most of the audible effect of this control occupy only a small part of its rotation. C1 can be increased, to give increased top cut. The results obtained with various component values are influenced by the impedance of the circuit.
With such circuits, low frequencies can be made more prominent by reducing treble with VR1, then increasing gain or volume.
Other circuits are able to offer control of both treble and bass frequencies. A passive circuit of this type is shown in Figure 41.
C1 is the input isolating capacitor, and may be unnecessary with some types of input. VR1 is for treble control. Treble is lifted with the wiper towards C2, whose reactance falls as frequency rises. With VR1 wiper towards C3, treble is reduced. VR2 similarly provides lift or cut in bass. Both are linear controls, supplying the volume control VR3 by means of R1. The source and drain circuits for the FET can be as in Figure 40.
Again, values are to some extent a matter of choice, but there is little point in using components which will provide extreme degrees of cut or boost, which wilt never be required. It is also of advantage to have approximately flat reproduction with VR1 and VR2 central.
Typical values for Figure 41 are:
C1 0.47uF R1 270k
C2 470pF R2 390k
C3 1.2nF P2 18k
C4 2nF VR1 500k lin
C5 5nF VR2 1 megohm lin
FET 2N3819 VR3 1 megohm log
This circuit incorporates an amplifier, so can be operated from. Circuits having a signal level similar to about that of a radio tuner or from higher levels. In Figure 39, C1 isolates the circuit from the audio stage monitored. VR1 adjusts the input to the gate of the FET, so acts as a sensitivity control.
Audio power from the drain develops a voltage across R2, and is coupled to the full-wave rectifiers by C3. Current from these flows through the meter, so that a reading is obtained which depends on the strength of the audio signal.
A level meter of this type is useful for recording, a calibrated scale being provided for VR1. It can also be used to monitor the strength of audio signals generally. The loading provided by the circuit C1/VR1 will be of negligible importance for most medium or relatively low impedance circuits. For obvious reasons, AF is best taken at a point after any gain or volume controls in the equipment. Where the signal level is substantially too high, a resistor may be placed in series with C1, at the audio circuit takeoff point. The value of this resistor will depend on the signal voltage, but can be expected to lie between about 470k and 10 megohm.
A 6v to 9v supply is not essential. A resistor may be put in the positive circuit, where current is taken from a higher voltage line.
The use of a 50uA meter allows high sensitivity. Any general purpose or detector type germanium point-contact diodes and similar rectifiers will be satisfactory. Care should be exercised when first testing the circuit, by advancing VR1 slowly from zero, so that a sensitive meter movement is not damaged. If damping of the meter movement is required, this can be obtained by connecting a capacitor across positive and negative terminals of the instrument.
The need to observe essential safety precautions has been mentioned from time to time. Otherwise, in some circumstances,shock hazards may arise in unexpected ways. These must be avoided in the interests of the safety of all users of the equipment.
With low voltage battery-operated equipment, there is of course normally no danger whatever of shocks. Preamplifiers or other units can thus be connected without any particular precautions.
In the case of larger amplifiers, current is generally derived from the mains. It is usual to have a transformer, which both reduces the voltage, and isolates the equipment from any direct contact with mains circuits. Where the transformer is of approved double insulated type, in good condition; or where safety earthing and fusing are used, no mains voltages can arise in the amplifier itself. These low voltage circuits thus present no particular hazard.
With thermionic valve equipment, circumstances may be dissimilar. An amplifier may derive HT and heater supplies from a transformer, so that it can have an earthed chassis. This offers maximum safety, though high voltages will be present in anode and other circuits. However, some economically priced amplifiers draw HT and heater current directly from the mains. As a result, a metal chassis, or other items, may be dangerous to touch. It is thus recommended that such appatatus should never be used in conjunction with any circuits in this book, in view of the special isolating and other safety precautions which are essential. Failure to understand or observe such precautions can result in preamplifier cases, microphones, or any other equipment becoming alive at mains voltage, and thus highly dangerous.
The circuit in Figure 38 permits mixing two inputs, and is particularly intended for one high impedance, relatively low-level input, and a lower impedance, higher level input. As example, the “Mike” socket can take a crystal microphone input, and will be, of high impedance. This input receives amplification from the FET. The 2N3819 is suitable. Other low-level inputs may of course be employed here if necessary.
The “Radio” socket is suitable for a tuner, or for other medium or lower impedance inputs having a higher signal level. VR1 control the level of the “Mike” input, and VR2 the level of the “Radio” input. This preamplifier-mixer will be found very useful when employed with a main amplifier where adequate gain is already available for full volume output with a radio or similar input, but which has no mixing facility, and possibly rather low gain for small input levels. Construction, power supply and audio circuits can be arranged as described for Figures 36 and 37. There is quite wide latitude in the supply voltage, but the value of R4 can be modified if necessary, if current is drawn from the main amplifier.
Circuits of this type are excellent for general usage. Where pick-up, tape or other equalisation circuits are required, er other considerations arise, reference can be made to Handbook No. BP35, Babani Press.
It is often convenient to be able to fade in or out or mix at any wanted level the inputs from two audio sources. The circuit in Figure 37 is suitable for this purpose.
One input is to socket 1, and the second to socket 2. Each input is suitable for high or other impedances, and has its own volume control VR1 and VR2. R1 and R2 are to provide isolation of VR1 and VR2 so that a minimum setting for one potentiometer does not ground the other input. This arrangement is suitable for all general purposes, with microphones, pick-up, tuner, tape, etc.
The 2N3819 and other audio and general purpose FETs will be found suitable. Output is to a screened connector, via C4, and is arranged as for Figure 36. Power supplies may also be provided in the same way as already described.
Such a mixer can be constructed in a small screened box,which is grounded to the negative line. The box will carry the input sockets 1 and 24 with associated volume controls. Running can be from an internal battery supply, if a power connection is not wanted to the main amplifier. An on-off switch will then be necessary in the positive battery lead. There is, some latitude in component values, without a very significant change in performance.
Both inputs receive amplification. When one input will be at a relatively high level, as from a radio tuner, it may be preferred to employ a circuit in which extra gain is available for one input only.
Figure 36 is the circuit of a preamplifier with high impedance input and an output circuit allowing coupling to a main amplifier. It will be found extremely useful for purposes such as boosting the signal from a crystal microphone.
The input socket, connected to C1, should take the microphone or other input plug, and a screened lead should be prepared to connect output and main amplifier. R4 and C4 provide smoothing and decoupling, so that current can generally be drawn from the main amplifier. It is preferable to use a 2-way power supply cord, not relying on the outer conductor of the audio lead for the negative circuit. A 12v supply is not essential, and the value of R4 may if necessary be altered in value to suit other voltages. The 2N3819 and other audio and general purpose FETs are suitable.
In some circumstances it may be possible to build this stage on a small insulated board which can be included in the main amplifier. This will greatly ease overall assembly. Alternative sockets can allow the preamplifier to be omitted, or included for greater sensitivity, as necessary. It should be clear of AC, power, speaker or similar circuits carrying high current or audio signals, to avoid any difficulties from unwanted feed-back.
Figure 35 is the circuit of a single stage preamplifier, incorporating a volume control, and intended for headphones. Component values are for the 2N3819 and similar FETs. Input to the gate is high impedance, so audio may be derived from virtually any source.
The DC resistance of the phones will not influence DC operating conditions (assuming C2 does not pass a significant leakage current). However, much the best results are obtained with high impedance headsets - say 2k, or at least not much under 500 ohm. A very useful degree of amplification is available, allowing weak audio signals to be boosted up to good headphone volume.
It is also possible to connect high resistance magnetic phones between drain and positive, omitting VR1. For other transistor types or a substantially lower supply voltage, it can be worth trying alternative values for R2, to secure maximum gain
The use of a 2N5459 or other general purpose RF type FET in the circuit in Figure 34 will be found to give very good results. This circuit provides very little loading for the intermediate frequency transformer. Current drain is very small typically under ½mA - and a 6v to 12v supply is satisfactory. It should be well smoothed, though C2 and R2 will be helpful in avoiding ripple here.
When this type of detector is fitted in an existing receiver, VR1 will be present, and it is not essential that the value is as in Figure 34. In some home built or other receivers, this may be found to be an easy way of boosting performance. The final IFT will usually need slight realignment. In general, there is some latitude in all the component values.
Superregenerative receivers offer high sensitivity, with a very simple circuit when compared to a superhet. Their main limitations lie in the relatively low selectivity, and the superregenerative hiss produced. However, on some of the HF-bands, where a simple receiver is required, such circuits are still practical and can have enough selectivity. The, regenerative hiss also ceases, when a carrier or AM signal is tuned in.
A circuit such as that in Figure 33 can be assembled easily in a small box, and has a range of many miles from even a low powered transmitter. It is most suitable for field and intermittent use, where no interference is likely to arise from it.
L1 couples the whip aerial to the tuning inductor L2. It is quite practical to add an RF stage, using one of the circuits shown earlier, to reduce radiation. VC 1, with the fixed capacitor C3, allows the Whole of the 28MHz band to be covered easily.
TR1 is the superregenerative detector, and a 2N5459 is suggested as allowing smoothest control on these frequencies. However,various VHF FETs will be found to work reasonably well, though it may be necessary to change the value of C6, or omit this item or fit a preset of about 5pF maximum value. VR1 controls regeneration, and quenching is obtained by the correct choice of resistor and capacitor values here.
TR2 is the first audio amplifier, followed by TR3, which will operate a 2½in speaker incorporated in the equipment. A more complete audio amplifier is scarcely justified, though it can of course be used.
L1 and L2 are wound on a 5mm diameter cored former. It is necessary that the core is of material suitable for 30MHz and higher frequencies. L2 has fourteen turns of 32swg enamelled wire, side by side near the top of the former. L1 consists of 6 turns, spaced 1/8in from the grounded end of L2. Too tight aerial coupling may prevent superregeneration.
L1/L2, C3, VC1, TR1 and associated items should be grouped close together, so that short leads are possible. The two AF stages are a little clear of TR1. The RF choke is wound with 42swg wire, on a 7mm or similar insulated rods sufficient turns being used to form a solenoid 7/8in long. The winding should not be doped or varnished, though small touches of adhesive can be used at the ends, as with L1 and L2.
A telescopic aerial which extends to 20 to 30in or so, fitted to the receiver, will be most convenient for portable use. When VR1 is rotated a quite loud hissing should be heard from the receiver. If this does not arise, L1 may be too near L2, or C6 may be of unsuitable value, or TR1 could be a type not intended for these frequencies. Assuming that the receiver will be used in conjunction with an amateur transmitter operating in the 28MHz band, set the core of L2 so that signals fall at about the centre of the range obtained with VC1. VR1 may be adjusted for best results with weak signals, but its setting is relatively uncritical. No hiss or oscillation will accompany normal reception.
Where an alternative and more powerful amplifier is used, VR1 must be fed from an adequately decoupled supply line.
Figure 32 is the circuit of an easily made amplified diode receiver. VC1 may be an ordinary size 500pF or similar air spaced tuning capacitor; or a miniature component if all dimensions are to be kept as small as possible. The tuning coil can have about fifty turns of 26swg to 34swg wire, on a ferrite rod, similar to that described for Figure 31. Or existing medium wave coils may be used. The windings intended for MW transistor portables have a relatively high inductance, and VC1 can then be about 200pF.
For other than short aerials, it is useful to have tappings, as in Figure 32, to allow sharper tuning. These taps may be at about the middle of the winding, and one quarter the total number of turns from the earthed end. Some coils may have a primary, or aerial coupling winding, which can be used instead.
Component values are not very critical. The detector diode D1 would normally work into a load of a few hundred thousand ohms, such as could be provided by a 270k resistor across C1, but this will be found of little practical significance here.
The 2N3819 and other audio or general purpose FETs will be suitable. Very good results can be expected from a medium or high impedance headset, as for Figure 31.
With crystal earpieces, which cannot be employed directly in the drain circuit, R3 and C4 may be added. The earpiece is then used between C4 and the negative line. This will also allow resistance capacitor coupling into an amplifier for loudspeaker reception. A 9v supply is preferred when R3 is present, but this depends also on the volume required.
Figure 31 is a circuit giving good headphone reception for persons listening, and it can if wished be constructed as a miniature receiver with a short throw out aerial. Alternatively, it can be used with reduced range by relying on the ferrite rod alone for signal pick-up.
TR1 is the detector, and regeneration is obtained by tapping the source up the tuning coil. The use of regeneration greatly improves selectivity, and also sensitivity to weak signals. The potentiometer VR1 allows manual adjustment of the drain potential of TR1, and so acts as a regeneration control.
Audio output from TR1 is coupled to TR2 by C5. This FET is an audio amplifier, operating the headphones. A complete headset is preferable for general listening, and phones of about 500 ohms DC resistance, or about 2k impedance, will give very good results here. If a miniature earpiece is wanted, this should be a medium or high impedance magnetic unit. A crystal earpiece will require resistance capacity coupling, Figure 32.
The tuning inductor is fifty turns of 26swg wire, on a ferrite rod about 5in x 3/8in. If the turns are wound on a thin card sleeve which can be moved on the rod, this will allow adjustment of band coverage. The winding begins at A, and aerial tapping B is at about twenty-five turns. D is the grounded end of the coil. The best position of the tapping C depends somewhat on the actual FET, on the battery voltage, and on whether the receiver is to be used with an external aerial wire or not. Should the tapping C be too near to end D, no regeneration will be obtained, or regeneration will be weak, even with VR1 rotated for maximum voltage. On the other hand, with too many turns between C and D, oscillation will begin with VR1 only slightly advanced, and signals will be weak.
Best results are expected when regeneration begins smoothly, with VR1 about half way through its rotation. It was found that only one to two turns were required between C and D. As changing the whole coil by a turn or so has little practical effect on frequency coverage, the best method is to make C two turns from D. Then if necessary unwind half a turn or more at D.
When regeneration is obtained, a heterodyne will be heard if the receiver is tuned through a transmission. VR1 should then be turned back very slightly. Maximum possible sensitivity is achieved when TR1 is almost in the oscillating condition. VR1 has to be set to suit the frequency tuned by VC1, so that final critical adjustment can be made. It is useless to regard VR1 as a gain control, and set it at maximum.
A metal case is suitable where an external aerial wire will be used. Where the ferrite rod only will be employed, for local signals, the box or case must be of plastic or other insulating material.
A tuner allows radio reception with the aid of any amplifier, so can greatly increase the scope of the latter. In many cases reception of this kind will be of the more local or powerful stations only and a superhet tuner is not then necessary. The circuit in Figure 30 has a single FET as RF stage, followed by diode detector, and is intended for medium wave reception.
L1 is the aerial coil, and L2 the diode coil, and a matched pair of TRF inductors is required here. Pin numbering is for the Denco (Clacton) valve type coils, “Blue” Range 2 for the aerial position, and “Yellow” Range 2 for the diode stage. VC1/VC2 maybe a 2 x 310pF or 2 x 365pF ganged capacitor, with integral trimmers. Separate 50pF trimmers may be added if these are not present.
It is probable that other matched coils, such as those used in a discarded valve receiver, would be found satisfactory here.
Numerous general purpose RF FETs will be found suitable in this circuit. The constructional layout should separate L1 and L2, in the way explained for earlier RF amplifiers. Coupling between L2 and L1 causes the stage to oscillate, so that proper reception is impossible.
Where the amplifier has an input volume control which will be used, VR1 can be omitted. Audio output from the tuner is at a much higher level, than that from a pick-up or microphone. It may thus be convenient to retain VR1 so that output can easily be set at a suitable level.
Operation is from a separate 9v battery, so that the tuner can be connected to any amplifier without needing to draw power from the latter. It would in many cases be a straightforward matter to arrange that current for the tuner is derived from the amplifier.
Such a tuner will normally be used with a transistor amplifier. It can, of course, operate with valve amplifiers, but it is essential to refer to the earlier notes on safety, in such circumstances.
The aerial may be indoors or outdoors, depending on signal pick-up required, freedom or otherwise from local interference, and similar points. The cores of L1 and L2, and trimmers for VC1 and VC2, are initially set in approximately similar positions. Subsequently adjust the trimmers at a quite high frequency in the band covered, and the coil cores at a quite low frequency, for best results. As these settings interact to some extent, repeat the adjustments a few times.
General purpose receivers have a diode detector, usually fitted for AM reception (and often to supply an automatic volume control voltage for an early IF amplifier). This type of detector can only operate well with CW or SSB when signals are not so strong that they swamp the BFO carrier. The experimenter or constructor may thus wish to provide a product detector, and Figure 29 can be used.
Two inputs are required to the FET. That to C1 is taken from the secondary of the final IFT in the receiver. C4 is connected to 3 in Figure 28
The audio filter is optional, and is designed to reduce the higher frequencies, not necessary for communication purposes. If employed, it can have two 2nF capacitors, with a 100mH choke. The choke, if not screened, should not be located where it may pick up hum from a mains power supply.
After adding this detector, a slight readjustment of the tuning of the final IFT may be necessary. The associated BFO or CO is used in the manner already explained.
Where resolution of strong SSB signals fails merely because the transmission is too strong, an easy solution to this difficulty can be found in fitting an aerial attenuator. Figure 7 shows how a potentiometer can be employed, with no changes to receiver circuits.
To resolve SSB or CW, a BFO signal may be coupled into the intermediate frequency amplifier. In these circumstances, the BFO has to operate near the intermediate frequency of the receiver, for transmissions of all frequencies. So with a BFO of this type, signals may be received at any frequency throughout the ranges which are provided for the receiver.
Some short wave receivers have an IF of about 1.6MHz. The BFO must then be constructed for this. With most general purpose or ordinary domestic receivers, the IF is near 455/470k Hz. If so, the BFO has to be arranged with this in view. Except for this point of providing the correct frequency, both 1.6MHz and 455-470kHz BFO units are the same, and employ the circuit in Figure 28.
455-470kHz Beat Frequency Oscillator
A general purpose RF FET is used as oscillator, Its frequency is determined by VC1, with the associated winding and parallel capacitor. The smaller winding is for drain feedback. A stabilised supply with 6.2v 400mW Zener diode allows the unit to be run from the receiver, if wished.
For frequencies around 1.6MHz, a Denco (Clacton) IFT17 intermediate frequency transformer is used. For frequencies in the 455-470kHz range, the IFT14 is fitted. Each has the parallel capacitor present inside the screening can, in the usual way, and pin connections are shown in Figure 28.
Components may be assembled on a small insulated board, and it may be possible to fit this somewhere inside the receiver. An on/off switch is included in the positive supply lead, as the BFO must not be in use for ordinary AM reception.
A control knob is necessary on VC1, as this must be adjustable during reception. If it is not practical to include this control on the receiver, then the BFO can be constructed as a separate unit.
The best degree of coupling from 2 of the BFO coil to the receiver IF circuits needs to be found by trial. It should be, sufficient to run an insulated lead from 2, to the vicinity of the first transistor in the IF amplifier. Alternatively, a capacitor of 8.2pF can be included here, as shown, and the output lead can be connected to the receiver diode, at the final IF transformer.
If coupling is too loose, only relatively weak CW or SSB stations will be resolved, and stronger SSB will sound like badly distorted and over modulated AM. However, very tight coupling is not wanted and will tend to swamp weaker transmissions.
Operation is similar to that described for the signal frequency resolver, except that as all signals are converted to the intermediate frequency in the receiver, it is only necessary to make trifling adjustments to VC1. For CW, VC1 will have two positions (above and below the station frequency). But for SSB, there is only one suitable frequency setting for the BFO, as if this is placed the wrong side the SSB signal, speech will be inverted, or unintelligible.
Initially, tune in an AM signal, and set VC1 to about half open position. Then rotate the core of the oscillator coil until a heterodyne is heard, and can be set to zero. Then rotating VC1 either way should cause an audio tone which rises in pitch. Check that this happens with signals on any frequency, to make sure harmonics of the BFO in the aerial circuit are not responsible.
Many general purpose receivers will have frequency bands which cover some of the amateur allocations, but which are not able to resolve single sideband or CW (Morse) signals. With a
communications type receiver intended for SSB or CW reception, a beat frequency or carrier oscillator is provided. With its aid, SSB or CW signals can be received. When a general purpose receiver is employed, the BFO or CO can be constructed as a separate unit.
The output of the BFO or CO may be taken to the intermediate frequency circuit of the receiver, as shown later. Alternatively, the beat frequency or carrier oscillator can operate at the frequency of the band being tuned. If so, no actual connection at all is necessary to the receiver, as the wanted signal and oscillator signal are both fed into the receiver aerial circuit.
This method is convenient for the low frequency amateur bands, especially 1.8-2.0MHz and 3.5-3.8MHz. It also has some advantage where the receiver tuning control is of a type which cannot easily provide critical tuning, as the latter will now depend on the signal frequency SSB/CW resolver.
The circuit in Figure 27 will operate at a fundamental frequency of approximately 1.75-2.0MHz, or 3.5-4.0MHz. The former range is suitable for both 160m and 80m, as the second harmonic of
the 160m range will provide resolution on the 80m band. However, the 3.5-4.0MHz coverage is more suitable when 80m alone is wanted, or some reception on perhaps 7MHz or 14MHz, with harmonics.
For the lower frequency range, C1 is 250pF and VC1 is 75pF. L1 has fifty-seven turns in all, tapped at twelve turns from the grounded end. The winding is of 32swg enamelled wire, on a 7/ 16th in or 11mm diameter former, with adjustable core. Turns are side by side. To cover the higher frequency range, L1 has thirty turns, tapped at 6 turns, with wire and former as described. C1 and VC1 are as shown.
Stabilisation of the oscillator can be omitted when the current is to be drawn from a separate battery, but becomes necessary if the circuit is operated from the same battery as the receiver. Otherwise fluctuations of supply voltage, especially at other than low loudspeaker volume, will cause frequency modulation of the oscillator.
Construction of the resolver should be in a metal case, with rigid mounting of components. A reduction drive is necessary for VC1.
As the output from the oscillator passes into the receiver aerial circuit, and is amplified with other signals, very loose coupling must be used. It may be found adequate merely to place the resolver unit near the receiver aerial lead. If more coupling than this proves necessary, a lead a few inches long can be taken from the tap on L1, out of the screening box, and near to the aerial lead. The, degree of coupling is not too critical. But if this is too great, signals will be swamped by the oscillator carrier. Alternatively, very loose coupling will give too low an input to the receiver, so that strong SSB or CW cannot be resolved.
To operate the unit, tune in the wanted SSB or CW signal with the receiver. Final tuning is now done with the resolver control, at the point where the oscillator signal is heard to beat or mix with the transmission tuned in. With CW, tuning is not too critical, as adjustments around the transmission frequency will merely alter the audio pitch. But for good reception of SSB, tuning is quite critical, as the resolver carrier should replace the carrier which was suppressed before transmission as correctly as possible.
Tight coupling of the oscillator is not necessary, and should not be made to an aerial, as this could cause interference to near by listeners.
Adding T1, TR2 and associated items, shown in Figure 16, will allow modulation of the RF signal produced by the generator. The first stage in Figure 16 consists of the RF generator, as in Figure 15.
TR2 is an audio oscillator, with feedback from drain to gate by means of the transformer windings. A small driver transformer, as used to couple the driver stage to ½-watt and similar push-pull output stages in transistor portable and other receivers, will be found suitable for T1. The original primary is used for the drain circuit. The whole of the secondary is employed for the gate, the centre-tap being ignored. If no oscillation is secured, reverse the connections to one winding. It will be found that considerable control over the tone produced can be exercised by the choice of values for R1, R2 and C1. A clear tone of middle frequency (say 400 hertz) is most satisfactory. TR2 is a general purpose or audio FET.
The RF produced by TR1 will now be heard to carry an audio tone, so can be tuned in by ear with a receiver. For trimming and similar adjustments, take the generator output to the aerial circuit of the receiver, or to a loop of a few turns near or on its ferrite rod. Signal level must be kept down with VR1, or the receiver automatic volume control response will mask exact adjustments.
A signal generator is a most useful instrument for aligning and checking the RF and IF sections of a receiver. Figure 15 shows the RF section of a generator, to which modulation can be added as explained later.
L1 and VC1 can be selected from a wide range of values, tosecure the coverage required. It is convenient to fit the Denco(Clacton) “Red” oscillator coils, of valve type. With these, pin 8 is taken to drain (S1) and pin 9 to C1. Pin 1 goes to S2 for all coils except Range 1, when pin 7 is used. The appropriate ground pin for each range is as shown below, which indicates the bands over which these coils can be used, with a capacitance swing of approximately 20-350pF for VC1.
Range 1. ( 5 ) 380 - 1600kHz.
Range 2. ( 2 ) 0.8 - 3.5MHz.
Range 3. ( 3) 2 - 8.5MHz.
Range 4. ( 4 ) 6 - 22MHz.
Range 5. ( 6) 15 - 45MHz.
If other coils are used, allowance has to be made for the omissionof the usual padders and trimmers, as well as for the difference in receiver aerial and oscillator frequencies.
S1/S2 can have as many ways as bands required. However, the higher frequency coils need short connections, and coils must be reasonably separated from each other. Numerous general
purpose and RF type FETs will be found suitable. The presence of oscillation throughout each band can be checked with a meter in one battery lead - current should change if VC1 is shorted. The drain feedback winding must be correctly phased to obtain oscillation.
VR1 is an output attenuator. Construction should be in a metal cabinet, with a reasonably large scale for VC1. Calibration can be by the methods described later.
The signal produced by this generator is unmodulated, or silent. Unmodulated RF is used for tuning sharp crystal filters, and can be used for alignment of receivers which have a tuning meter or indicator, or where a meter is clipped to the AVC circuit to show the effect of adjustments. For other purposes, it is necessary that an audio tone is present with the carrier or RF signal.
This converter allows short wave reception with a receiver having a medium wave range which can be tuned to about 1.4MHz to 1.6MHz, or 1400-1600kHz. It has a single range, covering approximately 5 to 15MHz, or 60 to 20 metres, and this includes the most important short wave broadcast bands.
Two transistors are used, Figure 14. TR1 is the mixer, and TR2 the oscillator. Tuning is by the ganged capacitor VC1/2, and output at a fixed frequency in the 1.4-1.6MHz range passes to the receiver from TR1 drain at X.
L1 numbering is for the Denco (Clacton) “Blue” Range 4 aerial coil, L4 being the “White” (1.6MHz IF oscillator) Range 4 coil. Both are valve type coils. C4 is the oscillator padder, and it will be found that 960pF, 970pF or 1000pF may be used, though 960pF is specified by the coil maker.
Trimmers T1 and T2 can be integral with the ganged capacitor VC1/2, or may be separate trimmers of about 50pF.
It is convenient to use a layout in which L1 and TR1 are near the front section of the tuning, capacitor. VC1. L2 and TR2, with associated items, can then be adjacent to the rear section, VC2.
One of several possible methods can be used to couple the drain of TR1 to the receiver. A radio frequency choke between X and the positive line will allow capacitor coupling, as in Figure 7.
It is also feasible to provide a coupling winding on the receiver ferrite rod, as in Figure 12, or use an existing external aerial coupling winding here, in some receivers. A resonant coupling, as in
Figure 13, is also practicable, this being arranged for the frequency to be used with the receiver. This should not be too far removed from 1.6MHz or 1600kHz, or alignment difficulties can arise in the converter. If the receiver is of the type which must have an external aerial, it should not be too difficult to find a frequency near the high frequency end of the MW band where no unwanted signals break through. More care in finding a frequency may be necessary with a portable receiver with ferrite aerial, especially during the hours of darkness.
When a tuning position has been found on the receiver which gives no reception, note this for future use. Switch on the converter and align aerial and oscillator circuits in the usual way.
The low frequency band end reached depends largely on the position of the core of L2, while the high frequency band end depends on T2. Initially set L2 core at about middle position; with
T2 about half open. L1 and T1 must then be adjusted for best reception. L1 core is always adjusted near the LF end of the range, and T1 near the HF end. That is, rotate L1 core for maximum volume with VC1/2 nearly closed, and set T1 with VC1/2 nearly fully open.
Should L2 or T2 be altered, to modify the band tuned, then L1 and T1 will have to be readjusted to match.
If wished, T1 may be omitted, and a 50pF panel trimmer can be used instead. This will allow signals to be peaked up critically with any aerial, and eases alignment of aerial and oscillator stages.
With an RF amplifier having two tuned circuits (as example,Figure 10) both coils must tune simultaneously to the same frequency. But in Figure 14 L1 and L2 must maintain a frequency
difference throughout the tuning range, this corresponding to the output frequency at X. As example, if the output is to be 1.6MHz, and L1 is tuned to 10MHz, then L2 is tuned to 11.6MHz. As far as possible, this same frequency difference should be maintained for all tuning positions of VC1/2. This is achieved by L2 being of lower inductance than L1, and by the series padder C4.
The reception of 2 metre signals is generally with a converter and short wave receiver, preferably of communications type. The latter will have sensitivity and selectivity better than average. With such an arrangement of equipment, the 144MHz or other VHF signal is changed in frequency so that the converter output falls within the tuning range of the receiver.
A converter of this type often has its own RF amplifier, and a relatively low frequency crystal controlled oscillator, followed by frequency multipliers. This allows high sensitivity and excellent
frequency stability, but is a relatively complicated and expensive item. Bearing in mind that at this frequency the RF amplifier will not, contribute very much gain, and that tunable VHF oscillators
are used in many domestic VHF receivers, it is possible to use the much simpler circuit in Figure 13.
L1 is broadly tuned to the wanted frequency band by T1, and signal input is to gate 1 of TR1. TR2 is the local oscillator, and the operating frequency here is determined by L2 and T2. Oscillator injection is via C3 to gate 2 of TR1. The frequency of the output from the drain of the mixer TR1 is the difference between G1 and G2 frequencies. Thus if the signal at G1 is 144MHz, and TR2 is tuned to oscillate at 116MHz, output will be at 144 minus 116MHz, or 28MHz. Similarly, with the oscillator set at 116MHz, an input at 146MHz to G1 will give an output of 30MHz. Therefore 144-146MHz can be covered by tuning the receiver from 28MHz to 30MHz. L3 is broadly tuned to this band, and L4 couples the signal to the short wave receiver.
The oscillator can actually be tuned above or below the aerial circuit frequency of the converter, as it is the difference between converter signal input and oscillator frequencies which determines the converter output frequency. It is also possible to choose other reception and output frequencies, provided L1, L2 and L3 are chosen to suit.
L1 and L2 are wound in the same way, except that L1 is tapped one turn from its grounded end. Each coil has five turns of 18swg wire, self supporting, formed by winding the turns on an object 7mm in diameter. Space turns so that each coil is ½in or about 12mm long.
L3 is fifteen turns of 26swg enamelled wire, side by side on a 7mm former with adjustable core. L4 is four turns, overwound on the earthed (positive line) end of L3.
Layout should allow very short connections in the VHF circuits. A co-axial aerial socket is fitted near L1. A screened coaxial lead is preferred from L4 to the receiver, to avoid unnecessary pick-up of signals in, the 28-30MHz range. The converter will operate from a 9v to 12v supply.
L3 should first be peaked at about 29MHz. If a signal generatoris available (that described later can be used) couple this to TR1 drain by placing the output lead near the drain circuit. Tune
generator and receiver to 29MHz, and adjust the core of L3 for best results. Otherwise, couple an aerial by means of a small capacitor to the drain circuit, and tune in some signal in the 28- 30MHz range, to allow adjustment of the core of L3.
It is now necessary to tune L1 to about 145MHz, and L2 to 116MHz, or 174MHz. If an absorption frequency indicator is available, this will permit an approximate setting of T2. A dip oscillator will also allow T1 to be adjusted. (The circuits shown later may be used here.) Subsequently adjust L2 to bring the wanted signals in at the required frequencies on the receiver, and peak these for best volume with T1, and check the setting of L2 core.
The converter is best assembled in a small aluminium box, completely closed, which can be placed behind the receiver. Note that if TR2 is not oscillating, no reception is possible through the converter. TR2 should be a VHF FET, such as the BF244, MPF102, and similar types, and if necessary T3 may be adjusted to secure oscillation here. The 40602, 40673, and similar VHF types will be satisfactory for TR1. If needed, frequencies can be brought within the swing of T1 and T2 by stretching or compressing L1 or L2.
The aerial may be about 38½in long, constructed as a simple self-supporting or wire dipole, with a feeder descending to the converter.
Amateur activity is most likely to be greatest at week ends, and in many areas a whip or very short wire aerial will provide local reception.
The range or volume of a domestic portable receiver can be increased considerably by adding an external aerial, and the circuit in Figure 12 does this, and provides a tuned booster stage also. With an external aerial, this can allow a small transistor portable or similar receiver to give good reception of signals which may be virtually inaudible otherwise. The booster is not required for normal or local reception, and it is not permanently connected to the receiver in any way.
Medium Wave coverage is most useful, and the coil connection numbers are for the Denco (Clacton) “Blue” Range 2 valve type coil. This can be fitted in a 9-pin valveholder, or can be mounted by its threaded end, with leads soldered directly to the pins. Coverage in this circuit is approximately 1.6MHz to 550kHz, but can be varied to suit the receiver by altering the position of the coil core.
TR1, R1, C1 and C2 can be assembled on a small insulated board, fitted near VC1 and the coil. Two thin flexible leads run from TR1 drain and C2 (positive) to the coupling loop L1. This consists of fifteen turns of insulated wire, about 1in in diameter. Wind these turns on a suitable object, slip the winding off, and bind it with cotton to keep the turns together. Connections from
the booster to the loop should not be unnecessarily long - about 6in to 9in or so. The loop and its leads may all be one uncut length of thin flexible wire.
If it is convenient to place L1 on the receiver ferrite rod, without in any way disturbing the existing windings there, this provides the closest coupling. Otherwise, position L1 near the rod and its medium wave coil, but outside the receiver. Tune in a weak signal, with VC1 adjusted to peak this up to best volume, and check that L1 is providing effective coupling, as shown by best sensitivity. It is necessary to operate VC1 in conjunction with the receiver tuning, so its scale can be calibrated to agree with the receiver, by tuning in transmissions throughout the band.
L1 and the receiver should not be placed so that there is coupling back to the aerial coil or aerial, or instability may arise. This will cause whistles on all signals.
Best signal pick up will be by an outdoor aerial, and this can be some 20ft to 50ft or so of wire, as high and clear of buildings as possible. However, even a poor or short indoor aerial can be expected to give an increase in volume, especially if an earth connection is also available.
