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.
