On Fri, 26 Apr 2019 21:17:09 -0400, you wrote:
Hi Raymond,
Thanks for your ever so prompt response.?? I am not missing the ABC's of
probes and in fact have previously read it a few times. Rather than look
for my paper copy, I just downloaded it and skimmed it again and I am
afraid that I am still at a loss as to the usefulness of either of the 1
ghz probes I mentioned at a 1ghz frequency under test.? If you could
describe what I am looking for in the primer, I will certainly go take
another look.? I also understand the convenience and utility of using 50
ohm cable and loads at higher frequencies.?
that depends on the circuit. Lots of RF circuits are 50 ohms, some
75.
However, high speed TTL, ECL, and the like circuits fake being RF (and
quite well, since the boards often have to be designed like RF
circuits). So the scope loads the circuit, as all things do.
Have I happened on a couple
of "dogs" for the probes I have investigated and mentioned?? The 6243
active probe has only 1 pf of capacitance at the tip and looks like a 50
ohm input to the scope.?
what it looks like to the scope is meant to match the scope. What
does it look like to the circuit? The P6201 probes look like few pf
to the circuit, 100k ohms, but present the plugin with it's required
50 ohm input.
Are virtually all 1 ghz circuits of such? low
impedance that throwing say 500 ohms capacitive across them in parallel
doesn't perturb them??
for 50 ohms, that's about 10%.
At 1 GHz, what's the reactance of 20 pf of a standard high bandwidth
probe?
Is there a mysterious phase angle thing here that
I don't fathom?? Should I be looking at other probes?? Tektronix was or
is selling these things as 1 ghz capable probes.? Of what use are they?
how would you connect a scope to a circuit that is running at 500 Mhz
to 1 Ghz, or has equivalent bandwidth?
take the best bandwidth 10x probe you have, put it on a relatively
high impedance high frequency circuit (not a signal generator with a
low output impedance), then throw a 1x probe across that and see what
happens.
Harvey
On 4/26/2019 7:57 PM, Raymond Domp Frank wrote:
Hi Jack,
The only thing you're missing is a recent edition of Tektronix' obligatory document "ABC of Probes", downloadable from e..g. <>.
Yes, a probe presents quite a load at high frequencies and it depends on the signal source whether that is a problem.
Where source impedance can be controlled, a low-Z probe (usually 50 Ohm impedance) is preferable.
It's all in ABC of Probes.
Raymond
