Fig, 28, 25 watt modulator uses readily available commercial transformers. Transistors QI and Q2 are 2NII72, 2N301, 2NI560, SK3009, GE-9 or HEP-232; Q3 and Q4 are 2N!74t 2N278, SK3012, GE-4 or HEP-233*
The frequency response ol this circuit is quite good and is essentially flat from 200 Hz to 7 kHz.
Circuits for Receivers
If you are interested in building a dual conversion receiver for single channel operation on MARS, 2 meter F\1 or WWV, the single oscillator circuit which provides two outputs illustrated in Fig. 29 should be of interest This single oscillator circuit results in a reduction of components without sacrificing receiver performance. Basically it consists of a conventional common base transistor oscillator which provides the injection voltage for the second mixen "he output of the oscillator is fed into a diode harmonic generator and resonant tank which is tuned to the desired harmonic; this harmonic is used for injection into the first mixer. For example, for a dual conversion 15 MHz WWV receiver with a 455 kHz if, a 3636 kHz cystal would be used along with its third harmonic at 10,908 MHz. The 10.908 MHz signal would be mixed with the 15 MHz WWV signal in the first mixer to provide an output at 4092 MHz; this signal would in turn be mixed with the 3636 kliz oscillator output in the second mixer to provide the 455 kHz if. The only consideration in c hoosing the crystal and harmonic frequencies is that only odd harmonics should be used. This is because when even harmonics are used in this scheme, poor second if image rejection will be a problem.
In most receivers the oscillator injection frequencies are below that of the signal frequencies. This is usually desirable since it results in a lower first if frequency which will provide better image rejection. In this case the necessary crystal frequency may be found from the following formula:
Where: f* = Crystal frequency
u = Signal frequency fir — if frequency h = Harmonic to which diode tank circuit is tuned.
In those cases where it might be desirable to have the injection frequencies higher than the signal frequencies, the following formula may be used:
The simple BFO in Fig, 30 may be added to an existing receiver with a minimum of cost and effort. Essentially it is a tuned collector oscillator with an if transformer being used for the tuned circuit inductance. Just pick a transformer that is compatible with the if in your receiver; it doesn't make any difference to the transistor. Anything between 85 kHz and 1600 kHz will work
Fig. 29. Single oscillator and diode provide two injection frequencies for dual conversion receivers. Transistor QI is a 2NI745, 2N2I88, TIM 10, GE-9 or HEP-2: the diode should be a 1N82A or similar.
FSg. 30. This beat frequency oscillator may be added to existing receivers with a minimum of difficulty, The BFO frequency is determined by the if transformer which provides feedback from collector to emitter. Transistor Ql should be a 2N384, 2N1749, 2N2362f TIM 10, SK3008, GE-9 ar HEP-2, well in this circuit Before you can use the transformer though, remove all of the fixed tuning capacitors from the unit; usually these are readily available on the bottom of the transformer. If the circuit does not oscillate when voltage is applied, reverse the transformer leads going to the emitter of the transistor. :'o connect the BFO into the receiver, run a piece of small coaxial cable from the BFO output to the base (or grid) of the detector, (n some cases sufficient injection will be obtained by just placing the coax lead in the immediate vicinity of the detector. Adjust the core in the BFO transformer so that the variable tuning capacitor allows the BFO output to swing to either side of the receiver if; then the variable capacitor will operate as a pitch control
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