136 kHz transmitter vxo

By Claudio Pozzi, IK2PII - August 2001


download pdf schematic

1.  Introduction

After some test with my Xtal controlled transmitted I decided that my chances of two ways QSO should be improved by a tunable VFO. Some useful projects where found in the G3LDO (Peter Dodd) "LOW FREQUENCY EXPERIMENTER'S HANDBOOK", RSGB editor, but I'm not able to copy other's projects without some modifications (I call them improvements, HI).
My mixture uses easy to find crystals, one transistor tunable oscillator for the higher frequency crystal, one CMOS IC for the other oscillator, for mixer and as output buffer/amplifier. The best from other projects found in the aforementioned book.
The output frequency is 272 kHz; the transmitter exciter divides it by two.

2.  Circuit description

The transistor Q2 is the tunable oscillator. L2 and L3 are 100 microhenry molded inductors, CV2 is the tuning capacitor. Tuning range is sufficient to cover the 137.600 to 137.800 QRSS portion of the band. The U1A is the buffer/amplifier for driving the U1C mixer.
The U1B port and associated components form the fixed frequency oscillator. Adjust the trimmer CV1 for a secure start of the oscillations. Don?t care the exact frequency.
The port U1C forms the mixer, at the output are present all the mixing products and their harmonics products. The low pass filter (L1, C5 and C6) clean the signal. The FET Q2 adapts the 272 kHz sinusoid to the following buffer/amplifier. The output is a square wave about 12 volts peak to peak.
The circuit was mounted wit the dead bug technique into a 30-year-old 144 MHz VFO box, recycling the good variable demultiplied capacitor (do you remember amplitude modulation and the QQE03/12?).

3.  Performance test

The test setup was entirely built with 136 kHz equipments. The VXO connected to the transmitter exciter (dividing by 2), my direct conversion Xtal controlled receiver, the PC running ARGO in 30 seconds dot mode. After some warm up I tuned the VXO to 137699.53 Hz on the waterfall screen. After half hour the frequency was 137699.49 Hz, after another hour 137699.42 Hz. The measuring chain have two unknown points, the frequency stability of RX Xtal and of PC audio board Xtal, but for now I can't improve my test bed. I think that 0.1 Hz in 90 minutes is enough for QRSS traffic.

136 kHz VFO schematic

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