We are talking about a 3 phase full wave rectifier here so the ripple
frequency is 300Hz.
After a string of minor problems (including discovering DC blowers with
built in drivers do not like strong RF fields) I finally managed to get the
amplifier to work correctly and I did take some very interesting RF
measurements.
I have not measured the actual amount of ripple on the anode supply. I was
quite tired as it was quite late and handling 3kV directly is the last
thing I want to do when tired and mistake-prone.
But I did do the RF measurements and I have to report no signs of the 300Hz
signal on CW anywhere. Likewise I cannot hear it on Ssb voice, nor I could
spot it on the SDR.
I opted for LimeSDR+Spyverter up converter, instead of my tinySA or the
Tektronix spectrum analyser, because the SDR has much better resolution (of
course I pay for it with more noise, less repeatability and so on).
But to see a tiny signal 300Hz away from the carrier I had to use the SDR.
Still, nothing was found.
I was also very surprised , the non stabilised screen supply (or should I
call it choke stabilised?) resulted in 26dB of third order IMD on maximum
possible power. Pretty much the same as my highly stabilised op-amp based
circuit... I start thinking the trx may be the limiting factor and not the
amplifier.
The 9th order was -46dB, so I believe similar to a typical commercial
amplifier set at maximum power while shouting into the mic. Not great,but
perhaps usable? What do you think? I'll be repeating the measurements at
lower power too.
I also measured the voice bandwidth and I saw no splatter (in 45dB dynamic
range between -60dB and -105dB) despite setting it as high as possible
while staying within the tube spec.
This is my first time using a SDR for these kind of measurements and it was
quite late so I might have made a mistake somewhere. I post the pictures of
the screen for the key points so my work can be checked. Sorry for the lack
of the scale on some pictures.I only noticed it after I reviewed them much
later.
So this is the IMD two tone measurement at 1613W and 3.7MHz. The tones are
1000Hz and 1575Hz as generated by my Kenwood sm-220 station monitor.
Each line is 2dB , the second photo shows the scale,but the left side of
the passband is cut a bit.
https://ibb.co/4VcCVyC
https://ibb.co/6tBjy3J
And here is voice bandwidth when talking normally. I'm not sure what the
occasional peaks on the right are. I suspect some noise or rx distortion
perhaps?
Here every horizontal line is 5dB and the top level I think is -60dB, the
highlighted part is the 2.4kHz.
https://ibb.co/sR5BKW8
So overall it appears far from excellent ,but definitely usable. When I
have some time I'll repeat measurements on higher bands.
One extra bit of info is, the input circuit is not really 50 ohms. I
believe it is 140ohm -25J at 1.5Mhz and 20ohm -9j at 30Mhz. And I had to
use 30dB attenuation on my trx output not to overdrive it. I'm not sure if
this discrepancy is responsible for some distortion I could perhaps remove.
Also, for anyone interested how did I get the highly attenuated signal from
the amp output. I used a 2GHz splitter that happens to pick up HF signals
at -90~120dB so it is great to emulate receiving them from air.
73,
Łukasz - SP4IT
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