Some of you might be interested in the following technique, that I see
mentioned in a professional electronics journal:
P. C. D. Hobbs, "Ultra-Quite High Voltage Source", IEEE Transactions on
Instrumentation and Measurement, Volume 38, number 5, October 1989,
pages 1004 to 1005.
The basic idea is to design a simple HV power supply (either regulated
or not), which has the inevitable noise/ripple on it. The
noise/ripple at the load is AC coupled via a high-voltage blocking
capacitor (0.1 uF, 3 kV used) to the inverting input of an operational
amplifier (they used an LF356). The op-amp is used to generate a
waveform that is in anti-phase to the noise. The original noisy DC is
then put in series with the op-amp, so canceling the noise. The op-amp
sits on the low voltage side of the psu, so the op-amp does not need to
handle the high-voltage, although it does need to handle the current of
the supply.
The authors claim measured noise of 7 V peak to peak on a 1 kV HV
supply, but reduced this to 300 micro-volts peak-to-peak (a reduction of
87 dB), so the wide-band noise was -140 dB down on the DC power !
For reasons of copyright, I'm not willing to scan the original article,
but I hope the above description is sufficient to enable anyone to
understand the basics of the method. Any library should be able to get
the article, where you will find full circuit diagram, which only
consists of 11 parts.
This could I feel have use in grid and screen supplies. Perhaps even
anode supplies, although that is probably a bit OTT.
The authors also point out that by using the technique, it is possible
to reduce the size of the HV DC smoothing capacitors, potentially making
the supply cheaper and smaller.
It might also be of use to anyone using APDs or PMTs in the optical band
- both devices having a gain that rises rapidly with voltage, so noise
is an issue there.
Fell free to copy to /dev/null if you find this of no use.
Dr. David Kirkby (G8WRB),
Senior Research Fellow,
Dept. of Medical Physics,
University College London,
11-20 Capper St,
London,
WC1E 6JA.
Tel: 020 7679 6409.
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