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Old 12th Feb 2017, 1:11 pm   #141
Argus25
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Default Re: R.F. voltmeter: your thoughts, please

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Originally Posted by Skywave View Post
it seems that option (b) above is superior to option (a).
Doing the rectification directly on the RF signal source results in terrible non linearity for low level signals, by that I mean below 150mV for the schottky or germanium diode (not 10mV). In fact, at 130mV peak RF voltage, a 1N60 diode (with no fwd bias) gives an output level that is already 6dB down compared to say a 1v peak RF signal and 30dB down at 12mV peak RF (useless at these levels without compensations).

The loading on the signal under test with option (a) is not that high though because the diodes only end up conducting a little on peaks to maintain the charge on the filter capacitors, which only discharge a fraction between peaks assuming the diode system feeds a high Z DC amplifier. So option (a) is good if all the measured RF levels are at least 500mV pk and above, that is if you want a nice linear scale.

The solution is (b) (if you don't use the H&H current source drive for the diodes) a high input impedance buffer, then broadband RF amplification with switched attenuation to get the level up to at least a few volts, and then rectification of the high level signal then a suitable DC buffer to drive the whatever meter with a linear scale. The only caveat, is that the input dynamic range is more limited than using diodes directly, because the input device (fet probably) can saturate if over driven.

If suitable precautions are taken with high range power supply voltage +/-10V at least running the input buffer (a jFet probably) and a good design of the broadband amplifier after that, excellent results can be attained.

One trick, almost too obvious, if it didn't have to run from batteries, is to make the input buffer/follower a valve, like an Acorn tube or Nuvistor with a 50 to 70V available HT rail, rather than the jfet, that way it could also accept very high level input signals without non linearity.
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Old 12th Feb 2017, 8:10 pm   #142
Bazz4CQJ
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Default Re: R.F. voltmeter: your thoughts, please

Investigating the use of a Nuvistor is exactly what I’ve been doing the last few weeks. Way back in the late 60’s, I built a 2m pre-amp with a pair of 6CW4’s in cascode and it was a very good piece of kit. One thing I liked about the Nuvistors is that they were very tolerant (“indestructible”) of fairly high levels of RF around them, and I think that is a feature that would be very good in a RF Probe.

For the VM, my idea was to just have a single Nuvistor feeding one or more BJT amps and feeding those in to the diodes. This is very much a “work in progress” as I haven’t been able to spend too much time on it and there has been a little bit of a learning curve to climb. When you start to experiment with Nuvistors, you quickly realise that a valve whose cathode and grid are so very close together bring a whole new meaning to the term “grid leak resistor”.

Recently, the thought has also occurred to me that the best thing to put behind the first Nuvistor would be another Nuvistor!

B
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Old 13th Feb 2017, 12:23 am   #143
Skywave
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Arrow Re: R.F. voltmeter: your thoughts, please

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Originally Posted by Bazz4CQJ View Post
This is very much a 'work in progress' as I haven’t been able to spend too much time on it and there has been a little bit of a learning curve to climb.
I couldn't help but pick-up on that: same here, except in my case the learning curve has been - and continues to be - rather steep, but a fascinating & absorbing one. However, things are beginning to fall into place.
And in that regard, my thanks to David (Radio Wrangler) and Argus 25 for your posts immediately above: each were informative, insightful and helpful.

Al.
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