View Single Post
  #6   Report Post  
Old December 30th 08, 07:32 AM posted to rec.radio.amateur.homebrew
K7ITM K7ITM is offline
external usenet poster
 
First recorded activity by RadioBanter: Jul 2006
Posts: 644
Default RF probe for millivolts?

On Dec 29, 9:29*pm, "Tio Pedro" wrote:
" DOH! In fact I do have access to one via a friend. Why didn't I think

of that in the first place? Too much wine and egg nog over Christmas I
guess. But also, thanks to v\everyone for all their comments - all
very useful and if the scope doesn't work for me I will try some of
the other ideas out. Cheers...


Larry VE7EA


don't forget the scope is going to give you a very high reading,
since the display is peak-to-peak. *You'll have to take the
reading from the waveform's *0 voltage cross-over
poiint to the peak of on one side of the full
waveform, and multiply by .707 to get the RMS value.

Pete


Another point about scopes and their probes: they commonly put quite
a bit of capacitance (several pF) at the point they're measuring. A
10:1 passive probe with a couple meters of cable will have several
tens of pF capacitance in the cable, and the capacitive part of the
divider at the probe body can't drop that by more than 10:1. I'm not
sure where the OP will be probing; it may be low enough frequency and
low enough impedance that it doesn't matter. But it's quite possible
for a decent diode-based RF probe to present lower capacitance than a
scope probe. Probing things without disturbing them can be very
tricky at times.

Cheers,
Tom