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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.
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It seems that many people popped the two RF switches at the input of the Siglent SSA3021x in spite of the presence of two internal ESD diodes. The SSA3021x has a massive 1 microfarads DC blocker at the input. I guess the large capacitance value helps to improve the low frequency response but it comes at a price of discharging lots of energy like a firecracker in the presence of transient voltage surges. So it is very risky to use it in vintage valve receivers:
http://emccompliance.co.uk/fixing-an...ctrum-analyzer
Commercial microwave transient limiters are very expensive and they seem to use carefully designed Schottky-PIN limiter with built-in attenuator to act as another layer of protection. The limiter consists of a Schottky diode in parallel with a matched PIN diode. The Schottky diode lowers the turn-on threshold voltage and the PIN limiter diode protects the Schottky diode at higher power levels (see attachment). The pair have to be matched and optimized for the best S11 and S21 across broadband frequencies. So the Schottky-PIN limiter are specialized and hard-to-find components.
I have ordered some BAV99 diodes and will try to measure their total capacitance versus frequency in a parallel strings as in Mr Carlson's idea. They work like a bi-directional voltage clipper.
I have looked at some datasheets of SMT ceramic gas discharge tubes rated at 70V (the lowest voltage available) and they have capacitance of 0.5pF. Probably it is rated too high voltage for my need.