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The Test Oscillator

2016-08-08 17:20  
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The Test Oscillator
All of the various PN-junction tuning configurations were evaluated in the test oscillator shown in Figure 1. The basic principles and rules of stable low-drift VFO design are still required for the best possible frequency stability. I have covered many of these at the bottom of the following web page from the 40 meter super het project VFO schematic. All the VFO stability hints except for number 6 apply. When building a PN junction tuned VFO, it is probably best to build the entire VFO, ensure that it is working and then add your tuning circuit. This way you can ensure the circuit oscillates and is stable before you add more circuitry which may make troubleshooting more difficult in the event of problems. I built my test VFO with just the first and second 51 pf NP0 capacitor soldered in place and watched the output in the scope and on the frequency counter. I saw a sine wave and observed the frequency to be ~8.6 MHz. I then soldered the third 51 pf NP0 ceramic capacitor in place and watched the frequency drop to ~7.1 MHz. This is perfect as I wanted my VFO for the 40 meter band and knew that the added tuning circuitry should drop the frequency to somewhere around 7.0 MHz. I listened to the VFO output as an audio tone on my commercial super het tuned to 7.1 MHz after connecting a 1 meter piece of wire to the emitter of Q3 for an antenna . The audio tone after about 5 minutes was quite stable considering the oscillator was not encased in an air tight enclosure. This was also confirmed with the frequency counter. Is is unlikely that the exact same frequency would be measured in a reproduction of this circuit, however it should be close. The tapped Hartley was chosen for reproducibility and because it is a rare-to-fail design. The voltage across the tank circuit in a Hartley oscillator is many volts peak to peak and I wonder if there is a better VFO design for using tuning diodes with. This web page created much more questions than answers I am afraid. L1 was wound and then boiled in water for ~6 minutes. It was glued to a single-sided circuit board with Polystyrene Q-Dope from GC Electronics. The coil does not lay on copper as I used a motor tool to grind away the copper in the shape of a circle with a diameter slightly larger than the wound coil. The inductor was doped face down on this circuit and covered with 2 separate coats. It is vital that your inductor is secured so that it is unmovable. A small trimmer variable capacitor for setting the lower band edge frequency would normally be added to this circuit. This was emitted for control purposes when testing the various semiconductors for frequency range in the Figure 1 circuit. The Q2-Q3 buffer has too high an output voltage for 7 dBm diode ring mixers. For diode rings, a more suitable buffer may be used and two examples may be found on this web site.