[VK-Microwave] 13cm tvtr oscillation

Robin Szemeti - G1YFG

OK, I think I see my problem .. changing the caps didn't help.

I built the kit with everything on the top side ... including the DC connectors and the LED, load resistor ... and I mounted the board "top side up".

My oscillation is almost certainly because I have a high gain QRO amplifier on the output (+15dBm in, 170W out) ... and the feedback loop includes the PA and the strip on the input.  Without the PA, into a pad and a spectrum analyser the output is clean and stable.   With a 10dB pad between the tvtr and the amp, it still oscillates, but moving my hand over board, I can reduce or stop the oscillation, do I do think it is RF from the PA getting back into that stripline on the amp input, because I have the board mounted "topside" up! The 10dB pad should ensure the tvtr is seeing a good match, so I really don't think it is the tvtr stages oscillating on their own, it is the overall loop gain with the PA, and the board mounted "topside up" is exposing the transmit input stripline.

I shall swap the connectors, LEDs and load resistor to the back of the board and mount it "topside down", which should shield the input to the TX chain effectively from stray RF.  I strongly suspect that is my problem .. not following the instructions!  Failing that, I'll try a shileding can, but I suspect simply mounting it as designed will sort it!


On Sat, 26 Sep 2020 at 02:07, Robin Szemeti - G1YFG via <> wrote:
Hi Rob,

That's good to hear, it's probably just me then. I shall replace all the C's just in case, if others have had success, then there is no reason mine should be any different.

On Sat, 26 Sep 2020 at 01:57, Rob Whitmore <vandwhit@...> wrote:
Hi Robin,

I have constructed one of these kits and it performs exactly as
described in the construction notes without oscillations.

May I suggest that you check all the bypass caps for any cracks or any
poor solder joints. The components are tiny and errors are easily made.

A tinplate box may not be the answer as this could create another issue
of "wave guiding" within the structure.

Hope this helps.


 On Sat, 2020-09-26 at 01:35 +0100, Robin Szemeti - G1YFG wrote:
> Hi,
> Has anyone any experience of this kit oscillating on transmit?
> Even with no LO and no IF signal applied, the output pair of devices
> bursts into oscillation.   When you transmit an FM or reasonably
> strong SSB signal, it is enough to "swamp" the oscillation and make
> it appear to work correctly, but on quiet SSB sections it oscillates
> again.
> If I remove the input coupling capacitor from the first amp,
> disconnecting it from the stripline feed that runs down the middle of
> the board, it becomes stable, but with the stripline connected, even
> if I remove the bias from the PIN diode T/R switch on the mixer it
> burst into oscillation still.
> I'm beginning to think that having a 40dB gain amplifier with such a
> long length of exposed stripline across the board may not be the
> worlds greatest plan.
> Any thoughts?  I'm tempted to construct a tin-plate box around the
> board, but I am not convinced it will help.
> --
> Best regards,
> Robin Szemeti G1YFG

Robin Szemeti - G1YFG

Robin Szemeti - G1YFG