Quote:
Originally Posted by G0HZU_JMR
Yes, it's going to be risky connecting your analyser to valve stuff. As energy stored in the blocking cap is proportional to voltage squared, a lot of energy can be stored in the blocking caps at high HT voltages, and this can produce damaging transients when the energy eventually wants to escape from the cap into a new home. I'd recommend some form of (fast) clamping limiter at the analyser input to minimise this risk.
I managed to find that thesis online, and the technical content isn't that great. Some of it is OK but I'm not impressed by the way the fet probe was developed. The author placed a 5th order LPF at the output and tweaked the values to try and offset some ripple up at UHF. The end result appears to be a network that has smaller cap values than expected. This will make the output network look inductive within the passband of the LPF.
If you place an inductive load at the output of a FET follower, this will effectively lower the input Rp of the FET quite significantly. So, I'm surprised this was done. Perhaps even worse, at higher UHF frequencies I'd expect that network to look capacitive. Putting small capacitive loads at the output of a follower will make the FET input tend towards negative resistance. Up at UHF this may cause the probe to act as a Q multiplier or even become unstable when probing certain loads even with the 47Ω bias resistor present.
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I didn't bother with his Butterworth LP filter as I am not convinced either. I thought it is not necessary. Funny enough this guy also copied his LP filter idea:
https://electronicprojectsforfun.wor...quency-probes/
Mr Carlson's came up with his spectrum analyzer protector. It was designed for 1M input. I thought his design is over-complicated. He used two strings of BAV99 as voltage clippers. I am not sure if the diodes will add too much stray capacitance at the 50 ohm input and output of the spectrum analyzer with a tracking generator that may cause problem at VHF/ UHF frequencies. His design seems more suitable for relatively low frequency sweep in medium or short wave valve radios.
https://youtu.be/ETKyKC2Zj-M
I will avoid the use of too big value of the blocking capacitor to restrict the stored charge energy. At the moment I use one 1kV 4.7nF blocking cap. I will use three 1KV 10nF capacitors in series giving 3.3nF total and 3kV rating. The 250mV SMT fast fuse has 0.665 ohms dc resistance which will cause some insertion losses but it is probably better be safe than sorry. This guy fired his front end of his Siglent SSA3021x and he repaired it:
https://youtu.be/zQ1lPbTwKaU
At least I have some idea on how to do repair if it ever happens to me. But i rather be very cautious.