"Tony" <tonycox01@...> writes:
I am troubleshooting a signal generator and the service manual says to check TP1 for 500MHz at +2-3dBm.
TP1 appears to be just a probe location on a trace (see attached image).
My question is what sort of probe should I attach to my spectrum analyser (8566B) to check the test point on the trace? Can I use a oscilloscope probe? If so, do I attach the probe ground lead?
I¡¯m used to using straight BNC-based input so this has me a bit unsure how to proceed.
I don¡¯t want to get an incorrect reading or worse, damage my SA!
If you have it, I'd probably use a high-frequency passive probe for
this. For example, the PMK PML series:
I think Keysight and others just rebrand this for their high-frequency
passive probes.
The 50ohm-input terminated 10:1 and 100:1 versions will get you up to
1.5 GHz. If you use this with a scope you'll have to use it's 50ohm
input or add a 50ohm pass-through termination. Of course, your SA is
already 50ohm input so nothing additional needed there. If you use 100:1
division, the resulting signal will be on the weak side for an oscope,
but your SA will see it fine.
Of course you can also use a suitable active probe if you have that, but
check it's damage levels first and make sure the signal is within those
before using it (that actually goes for the 10:1 passive probe too,
which can only handle 12V peak).
In the likely scenario you don't have these, I'd just use something you
do have (as others have suggested), such as a normal 10x probe and
realize the attenuation at 500 MHz will be significant. You can solder
in a coax cable too, but realize that you'll be loading down the circuit
pretty badly. The capacitance of the cable will also present a
significant load to the circuit. You might be able to solder a resistor
in series to decrease the loading, but I don't know how easy it will be
to get high-frequency performance doing this.
As for damaging the SA, put a good DC block in series with the
input. That should prevent damage in the case of DC voltages being
present. I actually just leave DC blocks on my 8566's at all times and
take them off if I ever need to see below the 10 MHz cutoff. If you're
still worried, you can probe the signal first with other means (eg an
oscope) to check for large DC voltages or even large AC swings, but this
all seems unlikely to me.
Matt
