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An Oscillator / Mixer with a CMOS IC and a Balanced Mixer
The Crystal Oscillator circuit . . .
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Fig. 1
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| The second circuit was designed around a 14.944 MHz crystal and a CMOS oscillator. This circuit works stable down to less than 10 Volts supply voltage. The 467 kHz output is taken from the divide by 32 output on pin 5. Use a 1 MOhm resistor across the crystal for CMOS Bias.
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. . . and the following Balanced Mixer and 12 kHz Output
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Fig. 2
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The 467 kHz output from the CMOS oscillator / divider is fed via a small capacitor to an IF Transformer tuned to the oscillator frequency and fed to one of the the balanced mixer's inputs, pins 11 and 13. The IF can be picked up at a low impedance point in the Yaesu FRG 7700. The IF voltage level at this point is about +90 dB µV.
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Fig. 3
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A second 455 kHz IF transformer feeds the 455 kHz IF to the balanced input to the S042P on the pins 7 and 8. The 12 kHz IF output is recovered on pin 2. The isolation between the receiver and the PC is again provided by the interstage transformer found in an old CB transceiver. To further increase the bandwidth of the IF signal, a 6,8 kOhm resistor is soldered in parallel to the primary winding of the transformer. A small ceramic capacitor is soldered to the secondary winding of the transformer.
The S042P provides ample 12 kHz output to feed the Line Input of the computer's Sound Card. Be sure to mute the Line Input in the computer's Volume Control Panel, otherwise the 12 kHz signal will be heard in the speakers.
If you connect to the computer's Microphone Input, then it would be wise to add a 5 kOhm potentiometer to the secondary winding of the transformer.
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The finished Oscillator and Mixer
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Fig. 4
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Some changes which improve the SNR ratio:
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- Hook up the 455 kHz IF transformer of the 12 kHz mixer board to the drain of Q24 via a 22 pF - 47 pF capacitor instead at VR02. Compensate the loss in input level with the volume control on the secondary winding of the output transformer.
- Use as SLOW AGC as possible. I slowed mine even more by adding 47 uF across C131 (10 uF).
- Increase the external supply voltage to the FRG-7700 from 13.5 Volt to 15 Volt. (The internal voltage of the FRG-7700 is supposed to be 13.5 Volts and 11 Volts stabilised).
- Feed DC to the VFO from 13 Volts via a 7808 IC instead of the internal 8 Volt rail.
- Modify the DX / LOC attenuator to a "real" 10 dB attenuator and use it when receiving DRM signals, if they are strong enough. (My original attenuator was about 28 dB on short wave).
- If the input signal is in the S9 + range, use the ATT on the front with care to set the S-meter around S9 +10 dB.
- Use the coaxial UHF (SO-239) connector as the Antenna Terminal instead of the SW / BC terminal.
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