measuring frequency - Page 1
Forum section: electronics / rf-microwave

author: yalect
author_rank: Regular Contributor
timestamp: January 25, 2021, 09:29:09 pm
content: Hi,I would like to ask you if I can measure high frequency voltage withsimple or ordinary multimetre,.maybe a picture or vedio to palnation?, let's say100mvand75Mhz.Thank you

author: radiolistener
author_rank: Super Contributor
timestamp: January 25, 2021, 09:42:23 pm
content: Quote from: yalect on January 25, 2021, 09:29:09 pmno, you cannot measure high frequency with multimeter.You're needs frequency counter for that.Some multimeters have frequency measurement feature, but it is usable for very low frequency.Usually it works for 10-100 kHz. Some of them can work up to 2-3 MHz. But not more.If you're needs to measure 75 MHz there is no multimeter for so high frequency. You're needs frequency counter.Also you can use RTLSDRv3 receiver. It allows to see frequency spike on the spectrum, so you can see exact frequency value with pretty high precision (about 2-4 Hz error per 10 MHz).100 mV is safe for RTLSDRv3, but a little too high, so it's better to put some attenuator, or use some inductive/capacitive coupling to avoid overload (for example just wind up two wires in a twisted pair with no galvanic connection). As alternative, you can make some little wire loop as antenna for RTLSDRv3 and put it close to your RF source.

author: coppercone2
author_rank: Super Contributor
timestamp: January 26, 2021, 02:05:56 am
content: Was there a F-V converter that was high frequency? I know of a V to F converter.This is the best F-V converter I foundhttps://www.edn.com/f-v-converter-has-high-accuracy/There might be a 100MHz versionYou can modify the circuit’s prescaler to adapt the circuit to considerably higher frequency ranges.Thats the only way I can think of something like a hand held DMM measuring 100MHz.an article about doing what you wanthttps://www.elektormagazine.com/magazine/elektor-197907/58463/

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 26, 2021, 02:14:19 am
content: It's not clear OP wants to measure the frequency. I read the question as wanting to measure the amplitude of the signal.

author: coppercone2
author_rank: Super Contributor
timestamp: January 26, 2021, 02:26:31 am
content: then you need two multimeters, one to measure FRQ and one for voltage with something like thishttps://www.amazon.com/Fluke-85RF-II-Frequency-Probe/dp/B00937VAW8

author: joeqsmith
author_rank: Super Contributor
timestamp: January 26, 2021, 03:18:29 am
content: I have a cheap CEM that can read frequency up to about 200MHz.We used to use detector probes with scopes to look at signals that were higher than what the scopes could handle. You could also look up demodulator probes or traveling detectors. Maybe check out the following thread:https://www.eevblog.com/forum/testgear/video-build-a-simple-rf-demodulator-detector-probe-for-your-dmmscope/

author: yalect
author_rank: Regular Contributor
timestamp: January 26, 2021, 03:24:25 pm
content: thank youSorry, I mean voltage not frequency, when I set Multimetre toACposition.Can I measure voltage of75Mhzwith simple one (Multimetre)? not frequency.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 26, 2021, 03:59:30 pm
content: I think your subject "measuring frequency" made it a bit unclear. No matter.If you use the probing technique I mention, you would be looking at using the DCV mode, not AC. I made a probe like this with a 50ohm terminator in the head. I'll drag it out. The 100mV would be the problem.There are lots of articles on the subject. Just fire up Google.http://techlib.com/electronics/detect.htm****Watching that video I linked, he claims he was detecting down to 15mV.

author: radiolistener
author_rank: Super Contributor
timestamp: January 26, 2021, 06:03:09 pm
content: Quote from: yalect on January 26, 2021, 03:24:25 pmNo. Multimeter is not intented for high frequency. It works with very low frequency.Voltage measurement on TrueRMS multimeter works up to 10-100 kHz and not more.Frequency limit for DMM may vary for different models. Cheap DMM usually 1-10 kHz and more expensive usually 100 kHz.If you want to measure amplitude for more than 50 kHz, then you're needs RF voltmeter.Also, you can use RTLSDRv3 for that, but it also needs reference RF source at the same frequency range in order to do calibration.Another option is logarithmic detector modules available on aliexpress for about 3-5 USD and usual DC multimeter. But it also needs RF reference source for calibration.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: January 26, 2021, 06:30:57 pm
content: just from trying I know that the UT61E can measure upto 25MHz at 3.3V logic level; never tried with lower level or higher frequencies, ymmv I guesscertainly it's not replacing a real counter

author: iMo
author_rank: Super Contributor
timestamp: January 26, 2021, 08:23:55 pm
content: In order to measure "100mV" like voltage levels at 75MHz you would need an RF voltmeter, or, at least a 200MHz oscilloscope, or a module with, for example, AD8307 chip.Also you have to understand at which impedance you want to measure such a voltage (because of the impedance matching).To "detect" whether there is "signal of around ~100mV" a simple detector with 2 Ge diodes may work as well. For "more precise" indication you would need to calibrate the diode detector, however..https://www.qsl.net/g3oou/simplerfdetectors.html

author: joeqsmith
author_rank: Super Contributor
timestamp: January 27, 2021, 01:23:37 pm
content: My old home made RF probe for my meter.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 30, 2021, 08:05:09 pm
content: A purchased detector. Showing 100MHz fundamental, detector, detector + lowpass. Lower signals would need to be characterized but can easily measure below 100mV.

author: iMo
author_rank: Super Contributor
timestamp: January 30, 2021, 08:23:16 pm
content: There is a trick described in an HP's whitepaper (I saw it long time back) - a small forward current via the detector diode (afaik a couple of uA) will improve significantly the detector's linearity at ~mV voltages..

author: joeqsmith
author_rank: Super Contributor
timestamp: January 30, 2021, 08:45:24 pm
content: The two diode arrangement should also help. Maybe use four matched diodes. Two to form the bias and two for the detector. A couple of small 1.5V AAAs would run it for a very long time. May be a good exorcise.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 31, 2021, 12:35:29 am
content: 4 X 4148s. Basically using one diode drop above and below the reference to bias the other two. A less than impressive thought.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 03, 2021, 12:58:35 pm
content: BAT54S mounted on PCB w. two caps. Sweeping from 5 to 100mVpp. I have no idea on the units of the OP's 100mV request, what range they needed to work over, accuracy.... so, I'm just playing around. Maybe a coin cell with two of these diode packages would improve it.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 04, 2021, 12:54:38 pm
content: Using a couple of BAT54S, noise is worse but it looks like it may be more linear. Sorry was too lazy to run a least squares. I also ran a sweep from 5mV up to a 1Vpp.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 04, 2021, 12:59:34 pm
content: Adding a little gain, then rectifying the signal may help linearity. In this case, the amplifiers response is not flat enough.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 04, 2021, 04:03:17 pm
content: Using a flatter amplifier the errors were not as good as I expected. I wondered if this was the generator running near it's lower limit so I added a 40dB attenuator to the output and then increased the amplitude. Still sweeping from 5mV to 100mV. Looking much better.I wanted to see how much the generator contributed to our original single BAT54S error. Looks like a wash.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 04, 2021, 04:16:01 pm
content: This is looking at the purchased amplifier/detector/filter sweeping 1mV to 75mV. This particular amplifier is a Mini-Circuits ZFL-1000LN. The filter is a Mini-Circuits SLP-1.9. The detector was made for someone and has only an internal number.

author: MF-jockey
author_rank: Regular Contributor
timestamp: February 06, 2021, 01:44:01 pm
content: Hi,have checked some multimeters with the sine wave generator regarding the limit frequency.If someone is interested, the result can be seen in the table.The max. Frequency is with no visible decreasing amplitude at my oscilloscope (may be - 2%).

author: radiolistener
author_rank: Super Contributor
timestamp: February 07, 2021, 02:46:04 am
content: Quote from: MF-jockey on February 06, 2021, 01:44:01 pmwhat do you mean when you wrote "max AC frequency"?Any DMM works with any frequency, but it's error depends on the frequency.What deviation error you're used to define max frequency?if you used 10% error to define max frequency, you can add Brymen BM867S to the list.Tested with 2 Vpp sine on AC measurement mode.At 114 kHz BM867S shows 10% higher voltage than real.

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2021, 02:58:52 am
content: Quote from: yalect on January 26, 2021, 03:24:25 pmNot easily. There are RF detector probes (as mentioned by others here) that are used to convert the RF to a DC voltage at some ratio, but they need to be calibrated because that ratio changes with input level and frequency (to a certain extent). RF voltmeters and power meters that use various detector types all have a complex calibration curve that allows them to be somewhat accurate. However, it involves measuring the output of the detector with a large variety of calibrated inputs and generating a bunch of correction constants, often a dozen or more. So there's no easy way to bootstrap this--without the ability to accurately measure RF voltage, you can't make an accurate RF voltage detector.

author: iMo
author_rank: Super Contributor
timestamp: February 08, 2021, 09:29:53 am
content: @joeqsmith: how did you wire the detector with the 3V coin cell (a schematics)?

author: joeqsmith
author_rank: Super Contributor
timestamp: February 08, 2021, 12:30:06 pm
content: Quote from: imo on February 08, 2021, 09:29:53 amSee attached

author: Electro Fan
author_rank: Super Contributor
timestamp: February 10, 2021, 05:32:25 am
content: measures +20dBm to -50dBm from 1Hz to 6GHz @ 50 ohmshttp://vi.raptor.ebaydesc.com/ws/eBayISAPI.dll?ViewItemDescV4&item=113852031623&category=181978&pm=1&ds=0&t=1587630709000&ver=0&cspheader=1https://www.ebay.com/itm/FA-2-1Hz-6GHz-Frequency-Counter-Frequency-Meter-Statistical-Function-11-bits-sec/123894014490?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20180213162448%26meid%3D7814487bd31f411aa0a9083b6be58904%26pid%3D100930%26rk%3D1%26rkt%3D12%26sd%3D113852031623%26itm%3D123894014490%26pmt%3D1%26noa%3D1%26pg%3D2332490%26algv%3DDefaultOrganic%26brand%3DUnbranded&_trksid=p2332490.c100930.m5375