Measuring frequency counter input sensitivity, splitter needed? - Page 1
Forum section: electronics / projects

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 02, 2024, 08:01:15 am
content: I'm looking to measure the input sensitivity of an input to a frequency counter across its frequency operating range.I see in the 5335A manual, this test is done by splitting the input frequency using a 11667A to allow the attachment of an 8481A or 8482A power sensor (I have an N8482A) on one output of the splitter and the other output to the counter.This requires the use of a 11667A (which I also have), but I was wondering, why can't I just connect a standard T connector instead of the splitter? Is there a reason that the splitter is used here? I'm only working with a sinewave up to 3GHz and the power sensor will be connected right there near the frequency counter input.

author: berke
author_rank: Frequent Contributor
timestamp: February 02, 2024, 09:37:46 am
content: You're dealing with a transmission line where waves can travel both ways. Whenever there is a discontinuity such as a change in impedance in the transmission line, waves will split, some will be reflected some will keep going. A power splitter has matched impedances at all ports and (ideally) creates no reflections, whereas a T will create a big discontinuity in your transmission line and therefore standing waves. The resulting amplitude at the power meter and at the frequency counter will depend on the phase relations and will depend on the lengths of the different segments, the mismatch etc. so it will be a big mess.

author: jonpaul
author_rank: Super Contributor
timestamp: February 02, 2024, 02:11:43 pm
content: What range of freq, voltage?Simple menthod:Gen>>Mini Ckts 50 Ohm BNC splitter>>Counter and TRMS meter or probeWe use an HP 8640B RF gen with calibrated freq readout and precision atten512kHz..1024 MHz, 3V RMS>>1 uV RMSJonJon

author: ch_scr
author_rank: Frequent Contributor
timestamp: February 02, 2024, 02:15:48 pm
content: With the splitter, all ports will see 50Ohm Impedance.With a tee, the generator will still be 50 Ohms but it will be terminated into 25Ohm (both counter and power sensor in parallel) at the end of a 50Ohm transmission line.That's what leads to reflections and it's associated problems. For these to arise the frequency has to be high enough (or the connecting cable long enough) to count as a transmission line.At "bench setup lengths" 10MHz might not yet show the effect, but at 100MHz you'll start to see it. And it'll quickly get worse from there on.If one input is high-Z (and the frequency is below, say, like, 500MHz) you might get away with it.The limit arises when the capacitive load of the "high-Z" input become a "low impedance" for the frequencies involved, again causing mismatch.This video's a pretty good demonstration:

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 02, 2024, 02:28:30 pm
content: The range is 10MHz to 3GHz, I'm checking the sensitivity, my N8482A power sensor goes down to -35dBm, and the HP spec on the frequency counter input says it should work down to at least -27 to -21dBm depending on frequency, so around there is where I'll be testing.I have a 11667A splitter and the required attachments and stuff, so I'll dig it all out and hook it u for some measurements done good and proper.

author: HalFoster
author_rank: Frequent Contributor
timestamp: February 02, 2024, 04:06:16 pm
content: I've done a lot of the 5335A's - and other counters - and you really don't need the splitter. One thing to keep in mind when working on HP equipment is that HP wanted to sell more equipment - so usually the calibration and service instructions are loaded with gear (HP, of course) that is not strictly needed. One reason the I much prefer the military TM's if they are available. Anyway, on the 5335A - I don't know what you are using for the signal source, but if it is accurate enough then just use the indicated value. Or, just set to the specified frequency and increase the attenuation until the counter starts missing counts - then disconnect and measure with the powers sensor. Just make sure the 5335A is set to 50R input impedance.Oh, if you haven't already - increase A4C36 from the default 10uF to 22-27uF. This will increase the manual gate time enough that you get 10 digits resolution instead of the default 9.Hal

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 02, 2024, 04:15:32 pm
content: Yeah, that was the other way I was thinking of doing it, but I have the 11667A now, so I need to play with new toy.I've already done the capacitor mod, and the counter happily displays 10 digits.I also have replaced a couple capacitors in the PSU section (there's a little silver Vishay one there that I have found was dead on two of two units) and the relay/fan service upgrade etc has been completed too.The last thing I need to make this unit 100% perfect is a new display window. Someone used some nasty chemical on the two units I have, one was cracked, warped, frosted and unusable, the other is a little warped and frosted and is borderline usable.If I could find a source of appropriately sized red plastic (or some replacement windows), I'd be so happy....

author: HalFoster
author_rank: Frequent Contributor
timestamp: February 02, 2024, 05:28:30 pm
content: You can buy the acrylic sheets off Amazon for pretty much nothing and then they can be easily cut with straight edge and a razor knife. Won't have the lettering, but that can be done with stick on lettering - fiddly but it can be done. Or, most sign shops can do clear stick-on vinyl with anything you want pre-printed.

author: jonpaul
author_rank: Super Contributor
timestamp: February 02, 2024, 11:25:59 pm
content: Filter :Tap Plastics has many retail stores and web, for sure red acrylic of any thickness.https://www.tapplastics.com/j