Looking for used frequency counter recommendations - Page 1
Forum section: electronics / testgear

author: unimorpheus
author_rank: Regular Contributor
timestamp: September 29, 2023, 03:59:05 pm
content: I have a Rigol DG4062 waveform generator with integrated frequency counter. I recently attempted to align the crystal based reference oscillator on two ham radios only to find out the oscillator output of about 20mV was way below the signal threshold of the DG4062. Along with other issues such a fake calculated digits beyond the first 7 I am finding this counter to be near worthless. What counter makes and models should I be looking out for on eBay if I wanted an oven controlled time base, at least 8-10 digit precision and high signal sensitivity? Budget maybe up to about 2K max so I know I'm looking for something older but well built. Thanks.

author: alm
author_rank: Super Contributor
timestamp: September 29, 2023, 04:35:41 pm
content: What frequency range are you talking about? Generally sensitivity goes down with frequency, so it makes a big difference if you need 20 mV at 10 MHz or 200 MHz. I like the HP/Agilent/Keysight 53181A/53131A/53132A series which gives 10 digits/second of resolution (12 digits/second on the 53132A, there's a thread on how to convert the 53131A to 53132A). But given your limited requirements I'm sure there will be a range of older HP counters that will do what you want.I like the HP/Agilent/Keysight 53131A/132A/53181A series, which gives 10 digits / second of resolution (12 digits/second for the 53132A, and there's a thread on this forum onhow to convert the 53131A to 53132A. The Philips PM668x/669x series (some of which later became Pendulum CNT-8x/9x) are also quite nice. But given your limited requirements, I doubt you need to spend that much. If a GPIB computer interface, or no computer interface is okay, and you don't require advanced features like high single shot duration resolution, statistics, etc, then pretty much any old HP/Philips/Racal-Dana/etc counter would work. For good resolution of lower frequencies without long gate times you want to look for a reciprocal / universal counter. For example the PM6671/6672 can do 8 digits of resolution with a gate time of 10 seconds. It's a decent counter, but it should be much cheaper than the HP 53xxx series, in my opinion.The advantage of picking a popular model from for example HP is that its quite plausible that someone designed a board to fit an aftermarket OCXO, likeGerry Sweeney's OCXO option for the 53131A. I'm not familiar with the older HP counters or other brands like Racal-Dana but I'm sure someone else can jump in.I'd suggest changing the title of the first post to something more meaningful like "Looking for used frequency counter recommendations".

author: nctnico
author_rank: Super Contributor
timestamp: September 29, 2023, 05:06:40 pm
content: For US$2k you can buy a nice counter for sure. I wouldn't buy something old that is overpriced. The Tektronix FCA3000 / Pendulum CNT90 could be a nice option and might be available on the market in near mint condition at your budget level. Somebody has created add-on modules to get the extra HF input on the FCA3000/CNT90 so you can get into GHz frequency counting at relative low cost. The Keysight counters have a more modern interface but what they lack is accuracy in the time interval mode and no automatic reversing the time interval measurements.

author: unimorpheus
author_rank: Regular Contributor
timestamp: September 29, 2023, 05:17:15 pm
content: Thanks everyone for the info. I don't think I would need high accuracy above 200 - 250MHz. The reference oscillator frequency I was attempting to measure was 22.625MHz doubled to 45.250Mhz. There are other secondary reference oscillators as well but they are generally within the same range. If I don't have to sped over 1K that would be great but I would rather buy quality than cheap out. That's how I got into the Rigol situation.

author: Grandchuck
author_rank: Super Contributor
timestamp: September 29, 2023, 06:21:42 pm
content: I like alms suggestions. The Agilent 53131A is sometimes available used at a reasonable price. You can add a prescaler and/or an oven controlled reference oscillator to units without those. I have an Agilent 53131A that I upgraded and then sold my other counters.

author: edpalmer42
author_rank: Super Contributor
timestamp: September 29, 2023, 07:42:44 pm
content: A few other counters that are worth considerinig are the Stanford Research Systems SR620 and the Fluke PM6681. The HP 5370B is also worth considering, but it's older and much bigger than the other two. They all can measure frequency, of course, but they really shine when measuring time intervals. They can measure intervals as short as 25 ps, 50 ps, and 20 ps, respectively.These values are much better than the 53132 which is more like 150 ps, although the 53132 is a newer design which may be a consideration. Note that all these counters can report better numbers by averaging multiple measurements, but that will obscure jitter. You'll have to decide whether that's an issue or not.Time Interval measurements are typically used to precisely match the frequencies of two high high-quality oscillators. I've always found that the adjustment is much easier with time intervals rather than frequency.The Racal Dana 1991/1992 models are popular (I have a 1992), but I can't recommend them. The key switches will usually fail and need replacement, which is a nuisance, and they use a semi-custom IC which seems to be prone to failure with old age and it's impossible to find a replacement.Ed

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 29, 2023, 10:01:07 pm
content: Tell me if I am wrong here but the accuracy of the Rigol can be significantly improved by adding a solid 10 MHz external freq source. Something like a GPSDO.I do a lot of work on Ham radios and the frequency tolerances are not that great,If you want to see if a weak signal is Identical to a Known Freq, you can compare then both on a scope using two channels and align appropriately.This would circumvent the the higher signal level needed for the Rigol used as a counter.In other words you can use a good oscillator as an ext reference on the Rigol and then compare the Rigol output with your sampled signal source on a scope. Since you are dealing with relatively low frequencies the scope should not be a problem.Perhaps a Leo Bodnar GPSDO Frequency Source would be practical.I do not have any Rigol Equipment so tell me if I am in error here. Also is the recomendation of the Bodnar Unit a good one? I do not have one but built A 10 MHZ GPSDO from some junk I collected.

author: BillyO
author_rank: Super Contributor
timestamp: September 30, 2023, 01:01:26 am
content: Single purpose hardware frequency counters are relatively simple devices and are as accurate as their frequency reference. Your Rigol's "counter" mode is just a tagged on software feature and is therefore at the mercy of complex software. Buy a hardware counter that can either take an external reference or can be easily modified to take an external reference, an example is the Data Precision 5845 which can be modified in about 30 minutes. Mine cost me about $25. And buy one of those $100 GPSDOs. Use it as the reference for your counter. $125 later you will have something that cannot be beaten for accuracy. No fake digits. It's all in real hardware, and a reference oscillator good the 10-11or better.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 30, 2023, 11:58:43 am
content: You can indeed find older freq counters that work just fine and will accept an external time base.But one of your problems is the small voltage signal you wish to measure. Many counters do not measure minute signals.Besides that counters only measure the biggest signal they area exposed to and there may be off the air signals in the radio that may confound your efforts.

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2023, 12:07:31 pm
content: Sensitivity is a good point. This is part of the specification for all decent frequency counters. Often it varies by frequency as well (higher frequency -> worst sensitivity). Measuring a 20mV signal is not a job any frequency counter can do.

author: alm
author_rank: Super Contributor
timestamp: September 30, 2023, 12:26:42 pm
content: Quote from: Wallace Gasiewicz on September 30, 2023, 11:58:43 am20 mV is not extremely small either for a general-purpose counter. All the counters I mentioned in my post have a sensitivity of < 20 mV or better below 100 MHz. (PM6672 < 15 mV up to 75 MHz I think). Shielding may certainly be a concern at this level, especially since the counter inputs are often high impedance for these low frequencies.

author: BillyO
author_rank: Super Contributor
timestamp: September 30, 2023, 01:23:32 pm
content: Quote from: alm on September 30, 2023, 12:26:42 pmAgreed, that Data Precision 5845 and my HP 5302A have both been verified to work down to below 10mV. Although the HP is only specified to 25mV. That said I don't think I tested them at the OPs required frequency.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 30, 2023, 01:47:07 pm
content: Thank you for clearing up the sensitivity issue. I have not used these expensive freq counters, just run of the mill older HP stuff.I have en entirely different suggestion. For the money you are willing to spend perhaps purchase a good spectrum analyzer. This will give you a good frequency readout and has multiple other uses in radio repair.Another suggestion is a Service Monitorlike the HP 8920-7835 Family of Service Monitors. I use an HP 8935 a lot repairing Ham radios. I do not use a freq counter much at all any more.This way you get a spectrum, signal generator that can be audio modulated and a really good freq counter. It also has a swept freq gen for the SA.It is sensitive enough to pull off the air signals well and it is sensitive enough to use a 10X scope probe to probe for signals on a board.Audio generator for use with mic input, very useful especially in evaluating SSB signals. Mine has two audio generators. You can put two tones into the mic for some adjusts or use one for modulating the RF for testing Receive and one for the Mic input.You can AM or FM modulate your signal.They also usually have an input for an external reference.The scope function is not really very useful however.I find the Service Monitor almost all you need for servicing most radios.https://groups.io/g/HP8924-Family-Spectrum-Analyzers/messages

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2023, 02:14:02 pm
content: The downside of using a typical spectrum analyser I see is that it has 50 Ohm inputs only. So you will be loading a signal if you connect it directly and using a standard 10x probe won't work. You'd need a passive Lo-Z probe or an active AC probe (DIY models can be bought cheaply) to get the signal into the spectrum analyser.Besides that I'm not sure you get more than 4 to 6 digits of resolution. I think this should be enough for the OP's purposes as there will likely be drift over time as well.OTOH the dynamic range and other use cases are plenty for a spectrum analyser. I'm not into radio at all but even I use my spectrum analyser a lot more compared to my frequency counter. Mainly because the oscilloscopes I use can tell me the frequency of a signal with enough precission in most cases.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 30, 2023, 02:26:20 pm
content: I use a standard 10-X probe all the time with my HP 8935. Seems to work fine. I realize the magnitude of the signal is not easily quantifiable however. I have readings down to one Hz, so easily 9 digits, since the primary bandwidth is one GHz. 8 digits below 100 MHz.I service radios all the time. The 20 mV level signal is not sometime I see very often. It seems very low for a reference oscillator. Perhaps a mention of the radio involved with perhaps a schematic of the circuit in question?My oscilloscopes give me a freq but my Agilent 54820 scope is not terribly accurate in freq readout. And it does not have an Ext ref input.Any freq counter is only as accurate as its reference generator and the ones in scopes seem to have a lot to be desired if you wish to have a really accurate freq.The same goes for spectrum analyzers. freq is dependent on what reference they use. Many older SA had very good OCXO.

author: bdunham7
author_rank: Super Contributor
timestamp: September 30, 2023, 02:44:14 pm
content: Quote from: unimorpheus on September 29, 2023, 03:59:05 pmI have an HP5316B with options 003 and 004 (prescaler and OCXO) that is rated for 15mV sensitivity out to 650MHz. It is 8 digit but can be pushed to 9 using longer gate times and letting it overflow. The main input channel is 1M and works well out beyond its rating to 160MHz or so in overflow, although probably at a somewhat reduced sensitivity. The prescaler input is 50R and works from 50MHz up to 1.3GHz. IDK if those specs work for you.You can find them on eBay for a few hundred dollars, so you'll want to buy the nicest one you can find if you go this route. If you want the Option 003/Channel C prescaler, it has to be installed already. If you want Option 004/OCXO, you can easily make your own using a board designed by a fellow EEVBlogger--this is what I did. Or you can use an external reference.

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: September 30, 2023, 03:01:02 pm
content: I like my HP 5386A. It works great and has up to 11 digit resolution. I use it with a external 10Mhz reference and could not ask for anything more.

author: unimorpheus
author_rank: Regular Contributor
timestamp: October 02, 2023, 05:58:57 am
content: Thank you all for the suggestions. I ended up going a much cheaper route as show in the pic just to get this done. A cheap Chinese GPSDO reference and MMIC amp was all I needed, for this job anyway. Still want to upgrade the counter but this does give me a lot more to consider. I think there is a lot of retired gear coming out of telcom that can be repurposed and give us amateurs / hobbyist much more capability than we could have ever hoped for.