Re: Looking to build RF detector probe for low frequency
This is a high impedance probe, not unlike a normal 10:1 scope probe, and like the scope probe, it will be sensitive to the length of the earth lead.
On a scope, the effect of the earth lead is visible as distortion and ringing on fast edges. You can see the response change as you move the earth lead. On an RF voltmeter the uneven frequency response may not be obvious.
Decent scope probes have a removable earth lead so that you can use a lead appropriate to the job in hand, and for best performance you would use an earth spike attached at the probe tip which gives a “loop” with the probe tip itself of only half an inch or so across. You say you only need a response up to 3 to 4MHz, so you don’t need to go to such extremes, but it may be worth considering a removable earth lead so you can trade off performance with convenience.
It’s no use going to the trouble of minimising all the lead lengths on the signal input to the last fraction of an inch, and then adding a foot or more of earth lead.
Stuart
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