Oscillators

I assembled the two transponder oscillators yesterday. One is for 82 MHz and the other for 114.2 mHz. They are based on the initial design I did last summer. I thought I had these all figured out. The 82 MHz oscillator just sucked. The harmonic content was very high. Hell, I did not need the multiplier stages I designed. I had a 738 MHz harmonic from the oscillator directly only 30 DB below the fundamental!! As it turned out the little MMIC amp I had at the end of the oscillator was being overdriven. I just removed the MMIC and jumpered the board from the end of the buffer to the SMA connector. I have a good -5 to 0 DBm which was the initial design goal. I also removed the filter components after the buffer. I was getting a spur around 300 MHz. I don't know if it would have gone away by removing the MMIC. I removed the filter parts before removing the MMIC. I suspect circuit board traces in the filter caused the instability. I probably had stray capacitance and/or inductance somewhere in the filter physical layout that screwed it all up.

Other changes I made:
1. Removed the tuned transformer between the oscillator and buffer. Replaced it with a short to the DC line.
2. I moved the input capacitor of the buffer that was on the transformer link to the top of the oscillator emitter resistor.
3. I also reduced the buffer emitter resistor from 220 to 100 ohms. This helped reduce the harmonics a little. I think the buffer was also being overdriven.

I experimented with the tapped capacitors in the oscillator tank circuit. Increased them to 82 and 33 from 68 and 22 pf. No real difference. The oscillator may be slightly more stable, from a start up perspective, with the original caps so I put them back to the way they were.

In the end the 2nd harmonic is 10 db down and the others are 30 db down or more. The 5th harmonic is the last one I see on the spectrum analyzer. The input tuning on the multiplier may make these spurs a moot point. If not I will have to add an extra low pass filter after the oscillator. The final oscillator is much simpler design.

I will incorporate the design changes into the 114.2 MHz oscillator and test it out tomorrow night.

I guess I will have to go back and update the design document on my web page. That will have to wait until after Hamvention. I need to get this project done by Hamvention.