Probe for frequency counter - Page 1
Forum section: electronics / beginners

author: Homer J Simpson
author_rank: Super Contributor
timestamp: January 26, 2013, 02:58:08 pm
content: I did some searching on using an oscilloscope probe for your frequency counter.I understand the signal attenuation issue, 1x vs 10x.Other questions I could not find and answer........What function if any would the probe ground lead have on the reading? In my tests it measures fine with it not connected.Does the frequency rating of the probe matter when used on a frequency counter? For example would a 100 MHz probe still work OK with higher frequencies?I have the BNC to crocodile clips and have read that are only ok to about 20MHz?Also....My current frequency counter goes to 120MHz. I have coming from EBay a 2.7GHz counter. This meter has the "N" connector for the higher frequencies. I bought some N to BNC adaptors. Would the higher frequencies be effected because of the adaptor?Thanks

author: Unknown
author_rank: Unknown
timestamp: January 26, 2013, 03:06:34 pm
content: Bandwidth and signal integrity is usually less critical for counters than for scopes, since you're not worried about accurate signal shape or levels. At some point attenuation will become a problem, so I wouldn't count on using that 100 MHz scope for 300 MHz, but it should be fine for 120 MHz.You may need the ground lead if the signal has fast edges. Try probing the same signal with a scope without connecting the ground lead to get an approximation of what the counter will see. Fast square waves will produce a fair amount of oscillation without a ground lead. This may lead to double triggering.The BNC to crocodile clips will have quite a lot of parasitic capacitance. This will load down your circuit (eg. a crystal may stop oscillating or shift in frequency) and will attenuate the signal.The 2.7 GHz input will be 50 ohm, so normal scope probes won't work. A BNC connector will perform worse than N for high frequencies, but I wouldn't expect any problems with a 2.7 GHz counter. A good BNC connector should be good up to about 3-4 GHz.

author: Homer J Simpson
author_rank: Super Contributor
timestamp: January 26, 2013, 03:17:55 pm
content: Thanks.What is recommended for a "probe" attached to the high frequency input? or all of the inputs for that matterI have 400MHz probe. That would be good to maybe 600MHz??

author: Unknown
author_rank: Unknown
timestamp: January 26, 2013, 03:47:17 pm
content: Direct connection to a 50 ohm output with a coax cable is always the best option. Other options for 50 ohm inputs would be inductive pickup, resistive probes (Howard Johnson and Doug Smith have written about them) or even $$$ active probes for some applications.I would expect a 400 MHz scope probe to be OK at 600 MHz for a counter. Just try it and see. Very few, if any, counters have 1 Mohm inputs capable of 600 MHz, however.

author: mamalala
author_rank: Unknown
timestamp: January 26, 2013, 05:01:34 pm
content: Quote from: alm on January 26, 2013, 03:47:17 pmThe HP 5345A that i have has a switchable input impedance, 1MOhm or 50Ohm. But then, it only goes up to 500MHzGreetings,Chris

author: Homer J Simpson
author_rank: Super Contributor
timestamp: January 26, 2013, 06:53:00 pm
content: Does anybody have instructions and or a link on how to make your own probes?I read a few posts on some ham radio forums and they talk of doing this.They mention adding capacitance and or resistance to the probe but their values are all different.Another style they talk about is a pick up loop. All of these were made from an BNC cable.Would one probe work with both the 1 Megaohm and 50 Ohm inputs?Thanks

author: Unknown
author_rank: Unknown
timestamp: January 26, 2013, 07:04:48 pm
content: No point in constructing your own 10x probe for 1 Mohm inputs in my opinion, performance will be inferior to even cheap no-name Chinese probes because you won't have access to the resistive coax they use (no, this is not standard RG-174). Mechanical construction will also be tricky.Instructions for constructing resistive divider probes (good up to maybe 1 GHz, but high DC loading) are on Doug Smith website, searching for 'Doug Smith probe' should find it. Some instructions for a wire loop probe arehere. Both will only work with 50 ohm inputs.The only way to have a probe with decent bandwidth work with both 1 Mohm and 50 ohm inputs is by using a feed-through terminator to terminate the 1 Mohm input into 50 ohms. Not sure why you'd want to use this instead of a real 50 ohm input, though. Performance will be inferior.