RF superimposed on HVDC

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Anthony_Zeberoff
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RF superimposed on HVDC

Post by Anthony_Zeberoff » Sun Sep 13, 2020 4:07 pm

I understand that a bias Tee is what is often used for lower voltage applications. I am wondering if anyone here has some experience with superimposing an RF signal on DC? Perhaps some guidelines or literature would be very helpful. Can a simple bias tee work for this, and if so, how do I go about choosing what size of capacitor and inductor (or resistor)?

Specifically, there are two circuits I would like to build:
1) Bias a 10-100kHz signal (+/- 250 V) on 1 kV DC.
2) Bias a 10-100 MHz signal (+/- 10V) on 100 kV DC.

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Rich Feldman
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Re: RF superimposed on HVDC

Post by Rich Feldman » Sun Sep 13, 2020 6:30 pm

You need to start by specifying details like the RF impedance of the HV load, HV power source, and RF signal source.
The series capacitor on RF-input branch obviously needs to hold off some kV, and will conduct glitches from HV arcs and sparks back to the RF signal source. The traditional inductor on DC-input branch is to block RF power from going that way. Its sizing depends on frequency and the aforementioned external impedances. It may need to hold off some kV in the event of arcs and sparks that are fast compared to the corner frequency.

If this is just for signaling, there might be fiber-optic-coupled alternatives.
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John Futter
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Re: RF superimposed on HVDC

Post by John Futter » Sun Sep 13, 2020 7:15 pm

Look at how the output tube in ham linear amplifiers feed the anode of the output tube.
The DC isolation choke is quite special and is designed so as not to have any resonances in the rf pass band of the amplifier.
This done by seriesing several inductors of differing vaues usually on the same former ( a stack of differing dimension pi wound air cored inductors) the dc side of the choke is decoupled with several capacitors again to stop resonances with the choke 0.1uf, 10nf, 470pf are typical

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Bob Reite
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Re: RF superimposed on HVDC

Post by Bob Reite » Mon Sep 14, 2020 12:30 am

Or a specially wound choke in which the turns get spaced further apart toward one end. Also helps with arcing over issues.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

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