OK to use my `scope probes with a spectruum analyser? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 10, 2016, 07:49:50 am
content: If I want to probe a transceiver that's incredibly deaf on RX to trace an injected signal, can I use my 100 MHz scope probe and a DC block with my SA? I guess it's OK, but thought I'd check. The service manual states to inject a -110dBm 14.2 MHz signal at the antenna port of the transceiver, a signal too small to see on my scope. Thanks.

author: cowana
author_rank: Frequent Contributor
timestamp: May 10, 2016, 08:09:58 am
content: You can - however you need to think about impedances.The SA will have an input impedance of 50R, while the scope probe (in normal 10x mode) will have an impedance of 9M.This will cause a huge potential divider attenuating the signal - so as long as you factor that attenuation into your measurements, it'll at least show you the frequencies of signals on the board.Better to use a 50R probe if its a low amplitude signal.

author: tggzzz
author_rank: Super Contributor
timestamp: May 10, 2016, 08:13:10 am
content: Your 100MHz probe probably works with a high impedance scope input, not a 50 ohm input.A "low impedance Z0" probe will work. As with anything, the probe's response will become part of the measurement.Consider making a probe out of a 50 ohm cable and putting a coil antenna on the end, to act as a non-contact "sniffer". A DC block is a wise precaution.

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 10, 2016, 09:25:50 am
content: Thanks for the replies, I really hoped to be able to take actual level measurements with my SA, as a matter of interest, if I were to buy a new Rigol or other modern hobbyist SA, what probes would it come with? Could I buy them as "spare parts£ to use with my boat anchor SA? It's an old HP8568BI am using this level diagram to make initial tests, hopefully it will be readable.

author: cowana
author_rank: Frequent Contributor
timestamp: May 10, 2016, 09:34:08 am
content: Typically spectrum analysers don't come with probes.Due to the low input impedance (50R) needed, you're not able to probe in the same way you would poke around with a scope probe - connecting the probe anywhere on the circuit is going to alter the operation of the circuit under test.Typically you'd connect the output of your circuit (which is designed to drive a 50R load) to the SA using an RF coax cable. Alternatively you could either make a pickup coil as tggzzz suggested, or use an active probe that buffers the signal - however high impedance active probes can get expensive very quickly!https://www.rohde-schwarz.com/uk/faq/use-of-high-impedance-active-probes-faq_78704-30148.html

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 10, 2016, 12:04:31 pm
content: If you want to probe certain sections of a circuit that don't have a 50R output impedance you can insert a series 1-10k resistor. This will give you a large attenuation but won't load the circuit very much. You will have to account for the insertion loss on the SA but that's not a problem unless the signals are very small.

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 10, 2016, 01:35:21 pm
content: These replies have been most helpful and with your assistance I think I have found the issue in the 1st mixer. I have questions regarding how a mixer fault may pull the signal level down, but I think I should start a different thread, thanks again all!