Unliab 10M FM Conversion - Exciter Board

If you have been following along we pick up from the last instalment and detail what changes are needed once the transmit VCO board is working. The big issue with this project is the output spectrum. We have taken a 80MHz broadband design and pushed it down to 30MHz. As a result, the output spectrum resembles that of a tripler!

The problem stems from the amplifier stage immediately after the VCO board. This is the final stage in the exciter and it really works well as a tripler. And it is quite good as a doubler too! What I did was to alter the circuit back to a narrow band design with some selectivity.

1. Pad C324, the 100pF input capacitor, with 1000pF
2. Remove C328 and C329, the two 1000pF emitter bypass capacitors
3. Remove L305, the tapped bifialar transformer wound on a ferrite bead.
4. Remove the resistor that was in parallel with this. Mine was a 680ohm resistor.
5. Clean out the holes and solder in 3 pins from a 0.1" header strip.
6. Take a T50-2 toroid, wind 16turns with a tap at 4 turns. (I have plenty of these if you're stuck)
7. Solder the toroid across the outside of the three pins. The cold end, that closest to the tap, goes to the pin towards the centre of the board. 
8. The tap goes to the middle of the three pins.
9. Solder a trim cap, say 40pF, across the two outside pins of the header.
10. Adjust FVR301 for maximum D voltage on TP303
11. Tune the trim cap for strongest output on 10m.

I'm still not entirely happy with the results. These changes are a vast improvement over where I started but the second harmonic is sill only 16dB down, and the third harmonic is 28dB down. There was a significant improvement with the higher harmonics and these are of no concern at present.

I now have a 100mW exciter but I will have to consider the output spectrum more carefully as I progress to rebuild the PA.

73's
Richard