Dr. David Kirkby wrote:
>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 !
>
Interesting - I've noticed the same thing in active shunt stabilizers
for screen supplies.
To handle some of the heat dissipation, I use a resistor in series with
the drain of the MOSFET shunt device. A scope probe on the point between
the resistor and the drain then shows several volts of hum and noise. I
was quite horrified by this, because the regulated output was very
clean.
Finally the penny dropped: this was the *antiphase* hum and noise that
the active regulator was producing, equal and opposite to the *real* hum
and noise on the unregulated input. When added together at the output,
the result is clean DC.
The circuit Dave describes will work the same way. It will clean up all
the AC noise, down as far as mains hum, but being AC-coupled it won't
help the regulation at the lower frequencies of speech and keying.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
New e-mail: g3sek@ifwtech.co.uk
New website: http://www.ifwtech.co.uk/g3sek
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