Why a frequency counter? - Page 1
Forum section: electronics / beginners

author: polishdude20
author_rank: Contributor
timestamp: July 09, 2014, 01:34:08 am
content: So my basic question is: why do we use frequency counters when an oscilloscope can tell you the frequency of a signal no problem?

author: Dave
author_rank: Super Contributor
timestamp: July 09, 2014, 01:48:12 am
content: Resolution, accuracy, simplicity.

author: EEVblog
author_rank: Unknown
timestamp: July 09, 2014, 02:07:37 am
content: Quote from: Dave on July 09, 2014, 01:48:12 am+ it might have a reciprocal mode for accurate low frequency measurement+ if it's a "Timer/Counter" it could have a handy counting mode and other stuff.And there are two types of scope counter, hardware and software based. Software based ones have poor resolution.

author: w2aew
author_rank: Super Contributor
timestamp: July 09, 2014, 02:22:50 am
content: Quote from: EEVblog on July 09, 2014, 02:07:37 amAll of the above, and...- cost. A frequency counter for microwave frequencies is a LOT cheaper than a scope for the same frequencies- additional functions like Phase, delay, totalize, etc.

author: TheRuler8510
author_rank: Regular Contributor
timestamp: July 09, 2014, 02:36:08 am
content: I picked up a vintage EIP microwave frequency counter (18GHz!) off eBay last year for $50! Amazing--I remember when they cost thousands of $$. Some of the vintage HP low frequency (<100MHz) counters go for as low as $20-$30.

author: MrsR
author_rank: Regular Contributor
timestamp: July 09, 2014, 04:48:37 am
content: Back in the 1970's if you were into making some money on the side an F.C. was a must to fix the multitude of CB's that were splattering the frequencies either side of the channel being used. It was best to keep quite about it as a lot of Ham radio guys hated the CB users.CROs over 20 Megs cost a fortune ThenI made my own F.C. from a circuit published in ELECTRONICS AUS.DICK SMITH made a kit for it and it was a lot prettier than mine but it still did the job. That was when he really wanted to supply reasonable priced components. He got a lot of people into Electronics with his kits and materials.If you can get a Ghz one now for Chicken Feed Grab it as you never know when it will come in handy.I think I still have it if so I will send it into DAVE's Mail Box you can see how bad hand drawn PCBs can be.ALL THE BESTRachael

author: David Hess
author_rank: Super Contributor
timestamp: July 09, 2014, 05:00:36 pm
content: Quote from: EEVblog on July 09, 2014, 02:07:37 amAnd if you have a Tektronix 2247A or I believe a 2236 or 2236A, the built in frequency counter is also a reciprocal counter so a fixed number of digits are returned for a given gate time independent of frequency.The really useful thing about this type of instrument is the ability to gate the frequency counter using the B sweep to make precision frequency and time measurements on a selected part of a waveform.There is also some overlap between oscilloscopes and frequency counters in the opposite direction. Some time/frequency counters have high impedance inputs with compensated attenuators, level adjustment, switchable coupling, and slope selection which you would normally expect in an oscilloscope. Besides being able to use inexpensive x10 passive probes, this allows them to make rise and fall time measurements that would normally require an oscilloscope.I have never quite understood why DSOs with long record lengths cannot make high resolution frequency and time measurements. My suspicion is that some modern DSOs which have "dedicated" high resolution frequency counter hardware are actually doing this in software taking advantage of a long record length rather than dedicated hardware.

author: sfiber
author_rank: Regular Contributor
timestamp: July 09, 2014, 05:12:03 pm
content: sometimes you dont want to see signal shape,because of you actually know what kind of signal is this.for example motherboards oscillators or somethings.And oscilloscopes are device that process image.If there is so many ripple and overshoot or triggering point problem you cant measure the signal.But if you use frequency counters you can do it clearly.Then finally;Cost.how much you should pay for 2.4 ghz oscilloscopes(I dont know,there is an oscilloscope with this spesific.).100 dollars for frequency counters.

author: katzohki
author_rank: Frequent Contributor
timestamp: July 09, 2014, 05:19:02 pm
content: Quote from: TheRuler8510 on July 09, 2014, 02:36:08 amGot a model number to look out for?

author: David Hess
author_rank: Super Contributor
timestamp: July 09, 2014, 07:07:54 pm
content: Quote from: w2aew on July 09, 2014, 02:22:50 amIronically the fastest oscilloscope I have access to is an order of magnitude faster than my fastest good frequency counter which is only about 1.2 GHz. The oscilloscope is many orders of magnitude less accurate though.

author: Dave
author_rank: Super Contributor
timestamp: July 09, 2014, 07:10:51 pm
content: Quote from: katzohki on July 09, 2014, 05:19:02 pmHP 5314A, 5315A, 5381A, 5382A, 5383AAdd them to your saved searches and wait for a good auction to pop up. Best of luck to you.

author: Neilm
author_rank: Super Contributor
timestamp: July 09, 2014, 07:51:15 pm
content: The biggest issue I have had with frequency counters is that I quite often want to measure the frequency of a signal in a burst. Generally, they tend to give me the frequencies of the burst rather than the waveform.

author: jpb
author_rank: Super Contributor
timestamp: July 09, 2014, 07:59:54 pm
content: Quote from: Neilm on July 09, 2014, 07:51:15 pmI think the burst microwave mode of the Agilent 53230A will do this - but it is not cheap of course!

author: David Hess
author_rank: Super Contributor
timestamp: July 09, 2014, 08:46:08 pm
content: Quote from: Dave on July 09, 2014, 07:10:51 pmThe 5316A is a 5315A with a GPIB port and optional 1 GHz C input. The 5314A is a 5315A minus some functions and I do not think it has reciprocal counting capability.I really like the 5315A as a basic universal counter.

author: David Hess
author_rank: Super Contributor
timestamp: July 09, 2014, 08:47:43 pm
content: Quote from: Neilm on July 09, 2014, 07:51:15 pmThe oscilloscopes I mentioned can do this because the frequency counter may be gated by the B sweep to cover only the burst.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 09, 2014, 09:57:57 pm
content: Don't forget that many frequency counters can also count events and make time measurements.

author: w2aew
author_rank: Super Contributor
timestamp: July 09, 2014, 10:33:18 pm
content: Quote from: jpb on July 09, 2014, 07:59:54 pmYes, there are several that will measure the frequency in an RF burst - Agilent, Tektronix, Pendulum, etc. to name a few.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: July 09, 2014, 11:02:18 pm
content: Quote from: Neilm on July 09, 2014, 07:51:15 pmThinking about it in the frequency domain, it makes more sense. You have more signal energy in a band of frequencies around the measured signal, than the time-varying nature of the modulation. This gets to the "video bandwidth" spec of log-amps/log-detectors. The basic detector circuit will sniff out a power level, but a higher performance detector circuit will output a changing-level signal, which would be the actual signal whose frequency you want to "count."To the original question, "why a frequency counter?"The answer comes back as a question, "What is your application?"Suppose I am debugging a microcontroller project, or FPGA project - something digital which involves clocks. A scope is perfectly fine to tell me that I'm getting 25MHz, or 10MHz. Any off-the-shelf digital scope today will tell you an answer within 3 total digits of accuracy (i.e. 25.0 MHz, 145 MHz, 1.02 MHz etc). Consider how it actually calculates it:1 GSPS = 1ns per sample10.000000MHz = period of 100nsThere are 100 counts per period.Now, suppose the current horizontal settings is set such that we get 10,000 samples at 1 GSPS (1us/div). This integrates 100 cycles of 10MHz. 5000 samples are "high" and 5000 samples are "low."What if we actually have 10,000,010 Hz (1ppm)? The period is now is 99.9999000001ns. Our sample rate is still 1ns/sample - the scope is blind to this ever so tiny change in period.Consider the same 10,000 samples (such as what you get with a TDS3000 series scope). The scope will see (with 4 digits of precision) 100.0001 cycles of 10,000,010 Hz. In other words, it rounds out that value and happily report "10 MHz."A frequency counter is what you grab when youreally need to know the smallest detailsof your signal source.Look at many off-the-shelf counters, and you'll notice they have a time-base in the tenths of seconds, or seconds, or even tens of seconds - and then keep in mind "counter."Lets suppose that we have a really big counter, and it only counts while in the mode "integrate." This mode lasts precisely 100ms (0.1s). The signal of interest is used as the clock of this counter. How many times does 10,000,010 Hz happen in 0.1s?0.1 / 99.9999000001ns = 1000001Does that number look familiar? Our time base is 0.1s, which is the period of 10Hz. The ability to measure runs out around the 10 Hz mark. We have 1,000,001 "counts" of 10Hz precision.What if we integrate for 1s instead of 0.1s?1 / 99.9999000001ns = 10,000,010Again, 1S is the period of 1 Hz, and now we have high confidence in our answer down to the 10s of Hz (there is a degree of uncertainty in the single-Hz values) as we have "counts" of 1HzNow why does this matter?There are many applications where you have to be at a very precise frequency for a particular operation. Consider, easiest of the examples, tuning into a VHF radio channel. A transmitter you built is set to 146.200 MHz. You used a cheap, off the shelf, 10.000 MHz crystal oscillator from a part supplier to use for the frequency synthesis. You key up, and a friend who is helping you validate the design informs you that he couldn't hear you at all on that channel. You also couldn't hear him.What gives?Look at how you get the 146.200 MHz oscillator. Your divide-by-N synthesizer takes your 10MHz reference, and divides it down to 100kHz (/100). It takes the output of the VCO and divides it by 1462. The VCO is effectively 14.62 times your reference oscillator. How good in that oscillator? Well you pull up the datasheet, and find that the oscillator is rated to ±100ppm. This means the reference clock is as high as 10.001 MHz, and as low as 9.999 MHz) What is theactualfrequency of your LO? You have an uncertainty: Nominal will be 146.2 MHz, the worst case-low is 146.185380 MHz, and the worst case high is 146.214620 MHz (a spread of 29kHz!). On VHF, channels are typically spaced at 5kHz, which means you're actually transmitting on (almost) any one of ±3 channels.If you measure the LO on a scope, it'll tell you 146.2 MHz with some large error of uncertainty for the 100s of kHz, and nothing below it.Put a frequency counter on it, with a 1 second time-base, and you would find that you're at 146.208201 MHz. Now you can "do something" about it.Think of the scope as a $50 meter as far as frequency precision goes, and a good frequency counter as a $1000 uber meter with 6 digits of display.

author: TheRuler8510
author_rank: Regular Contributor
timestamp: July 10, 2014, 12:02:05 am
content: Quote from: David Hess on July 09, 2014, 08:46:08 pmHere's the cheapest one currently on eBay, the Hp 5314A for $20.http://www.ebay.com/itm/HP-5314A-/181442323677?pt=LH_DefaultDomain_0&hash=item2a3ece24dd

author: Unknown
author_rank: Unknown
timestamp: July 10, 2014, 01:21:37 am
content: Quote from: TheRuler8510 on July 10, 2014, 12:02:05 amThe budget Tektronix counters from their rebadging days, like the CFC/CDC/CMC series, are also hidden gems with roughly the same specs as the 5314A/5315A. The CDC250 (175MHz) & CMC251 (1.3GHz) in particular can be good buys, since they come with a 1ppm TCXO as standard.I picked up my CDC250 for AU$50 - I've seen them go for as low as ~AU$20 on eBay, but I've also seen people asking AU$300 for them. YMMV.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2014, 01:38:42 am
content: I am never quite sure what their rebadged bench top counters support and of course they have little (none?) service documentation.The Tektronix TM500 and TM5000 series counters are nice though but getting long in tooth. The later ones support reciprocal counting. I like the older ones in that series because they have such good signal conditioning.