Unilab 10m FM conversion - Transmitter VCO Board

In the quest to get the harmonic content of the transmitter under control I returned my attention to the VCO board. Sure enough, when I measured the output of the mmic I found it was around 900mVpp and the flat topping of the waveform was clearly seen on the CRO. This equates to 3dBm for a 50 ohm load and is right on the limit for the mmic used. You may recall that the coupling capacitor from the VCO to the mmic was a parallel combination of 1p5 and 2p5. I removed the 2p5 and replaced it with a 1p5, giving 3pF in total. Did it make a difference?

Measured at the exciter output into my spectrum analyser (all readings in dBm):

	Before	After	Difference
Fundamental	22	20	-2
2nd Harmonic	13	3	-10
3rd Harmonic	-6	-7	-1
4th Harmonic	-7	-30	-23

So yes, it made a difference. While there was little change to the output or the third harmonic, the second and fourth harmonics were significantly reduced.

While the original mmic was fine when the VCO was running at 80MHz and the pre-scaler had plenty of sensitivity, the reduced pre-scaler sensitivity at 30MHz means the mmic has to be run at the limit to get enough signal into the pre-scaler for reliable counting to occur.

I see two alternative courses of action: substitute a mmic with a higher output rating and similar gain, or put a small amplifier stage between the mmic and the pre-scaler so the output level from the mmic can be reduced still further. A different mmic is perhaps the most expedient solution.

The existing mmic has a gain of 18dB at 30MHz and an output level of 3dBm. The suitable alternatives I know of are a MAR8 or a BGA616. Neither is a simple drop in replacement though.

Type Po -1dB Freq Range Gain at 30MHz uPC1651G (existing) 3 10MHz-1GHz 18 MAR8 12.5 DC-1Ghz 32 BGA616 18 DC-2.7Ghz 20

The BGA616 would be my first choice. It is capable of a higher output level then the MAR8 and this should result in even less distortion, hence harmonic output, than the MAR8 will give. The biasing of the BGA616 would also be easier given the 8V rail available on the VCO board. A 100ohm resistor and inductor will suffice. The MAR8 runs at a nominal 7.8V and I suspect the 5.6ohm biasing resistor it will need is too low in the event the temperature rises.

In some ways a small amplifier stage between the mmic and the pre-scaler is less critical. A voltage gain of say +4 would be ample and there is no need to re-jig the board for accepting a different mmic. And just about any small signal transistor could be pressed into service.

So I built this onto a piece of vero-board and mounted it as shown in the photo below:

And after removing one of the two 1p5 caps that coupled the oscillator to the mmic the results were:

Which confirms the mmic was definitely being over-driven. The relationship of fundamental to harmonic output levels now "appears" better. However, when I consider that these are measured after the tuned tank circuit I added on the main exciter board I think the results should be better.

I'm going to ponder this over the next few days as we go through a heat wave. I am going to rebuild the driver stage onto a small pcb which will fit in the cut-out where the BGY32 used to go. It will include a pre-driver stage to lift the transmit power from 20W to something closer to 100W. And since this involves etching I will also rebuild the buffer onto a small pcb which will look a lot neater. If you'd like either board just let me know!

Regards
Richard