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Old 20th Apr 2021, 8:26 am   #1
6SN7WGTB
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Default Can an RF probe circuit be remote from test point?

Having been enthused by the super repository of information on this site, my recapping of an old SONY World band receiver has led to the purchase and restoration of an old Nombrex RF SG.

I do have a (20MHz) 'scope but to get the most from the SG decided to make a simple (probably voltage doubler) RF probe/detector, using some germanium diodes from the never-thought-I'd-find-a-use-for-them box.

My question is:

If I use a shielded probe (e.g. the 'scope one for example - which is ideal with its attached earth croc clip etc.) can I mount the RF probe circuit at the DMM end of the (shielded) cable?

I can appreciate the impedance of the cable will have an effect, but to what extent would this detract from the usefulness of the probe?

Sorry if this is an obvious Q, but as a life-long 'audio' guy, I have never done anything up at the RF end aside some SDR which was all plug and play.

Thank you.
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Old 20th Apr 2021, 10:38 am   #2
Radio Wrangler
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Default Re: Can an RF probe circuit be remote from test point?

You really do want the little detector circuit close to the test point you are probing. The effect of a bit of cable is much more significant at RF than at audio, also the capacitance of that cable will load the circuitry the test point is in quite signicifantly, so what you see will be affected. So you minimise the length of the RF connection and have your cable on the easy side of things.

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Old 20th Apr 2021, 10:52 am   #3
6SN7WGTB
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Default Re: Can an RF probe circuit be remote from test point?

Thanks David, that all makes sense. I'll pot it in a bit of brass tube probably.
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Old 24th Apr 2021, 4:12 am   #4
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Default Re: Can an RF probe circuit be remote from test point?

You used to be able to buy ready-made AM-Demod / peak-detector probes, that looked just like normal 'scope probes but were green-colour plastic to distinguish from their standard probes.
So they didn't have lots of shielded casing (or it was inside outer plastic casing)
Although I don't think they were voltage-doubling, but just a single low Vf diode and envelope-smoothing capacitor in the probe to avoid loading-effects from cable to 'scope (as 10:1 & 100:1 probes try to reduce)

And could probably mod the insides of an old 10:1 probe, to change it to one of these.
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