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I just want to check the calibration of my frequency counter, so anything constant under 250 MHz would work(?)

endolith
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    You can get some pretty good OCXO oscillators (new and used) that are designed for cell base station applications, however they're not "atomic" standards. There may still be rubidium used standards available. There are time enthusiasts who are very much up on what is available. – Spehro Pefhany Nov 27 '23 at 17:19
  • @SpehroPefhany Well the whole point is to measure the accuracy of crystal oscillators, so I don't think using another crystal is valid? – endolith Nov 27 '23 at 17:23
  • Contacting a time reference via the Internet will be high jitter zero drift. Streaming a digitized "test tone" will be only as good as the clock in the DAC you feed the digital data to, i.e. no advantage over a locally-played waveform. – Ben Voigt Nov 27 '23 at 17:23
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    @endolith You didn't give any specifications.. but clearly some crystals are much, much better than others. – Spehro Pefhany Nov 27 '23 at 17:27
  • @SpehroPefhany Oh, OK, I guess that would be a valid answer then. ±1 ppb is better than anything I will be measuring. – endolith Nov 27 '23 at 18:17
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    @endolith Careful though- that's stability. Initial accuracy is nowhere near that good. – Spehro Pefhany Nov 27 '23 at 18:40
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    Many AM & FM broadcasters calibrate their carrier frequency using GPS. For a casual check of your frequency counter's accuracy, you might try measuring a few local strong signals (carrier frequency). In NewYork, there should be a big choice. WWVB, WWV, WWVH are not an option for you for various reasons. – glen_geek Nov 27 '23 at 18:49
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    How many digits does your frequent counter have? Does it have a reference output? You can measure (average) frequency of an oscillator to within 1 ppb with a $20 eBay GPS module, a 20 MHz scope and 15 minutes. A precision of < 1 microsecond over 1000 seconds. – tomnexus Jan 04 '24 at 07:45

3 Answers3

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GPS signals have good long term stability but not short-term?

The calibration of your crystal is also mainly about the long-term stability. The short term frequency wander and jitter will be more due to the noise in whatever circuit you have attached than to the crystal itself.

And I don't know of any way to get it out of a phone.

You can buy a GPS receiver with a 1 pps (pulse per second) output. Under $200 if my internet search is to be believed.

The Photon
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You can use a temperature compensated crystal oscillator (TCXO) and get 1 or 2 ppm accuracy for under $5 up to very expensive. TCXOs do drift, perhaps no more than 1ppm per year, so you may want an adjustable TCXO if accuracy is important. However, you'll need an accurate source to calibrate the oscillator.

If you need good precision at low cost, you can buy GPS based timing systems that have a disciplined oscillator, often a 10MHz output with 0.001 ppm accuracy. It usually takes a few minutes for the oscillator to reach specified accuracy after satellite lock. If you do an Internet search, you can find kits for perhaps $100. We have a couple of these GPS disciplined oscillators at work to verify frequency accuracy of our products.

qrk
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  • "However, you'll need an accurate source to calibrate the oscillator" That's the part I'm looking for, though... – endolith Nov 28 '23 at 16:01
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    @endolith That's why I suggest using a GPS disciplined oscillator which can have accuracy down to 0.001 ppm. – qrk Nov 28 '23 at 16:28
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but I don't have a shortwave receiver

60 kHz is really easy; just a ferrite rod antenna and tuning capacitor followed by an op-amp amplifier followed by a comparator. I built one back in about 1982 and the signal was big enough to see on an oscilloscope without amplification. I added a simple transistor amplifier that also envelope detected the amplitude modulated data and it could be seen on a chart recorder. Of course you don't need to go down the detection route.

But if you are not in a strong reception area you might struggle (and that's where you would add more sophistication to enhance the signal from the tuned ferrite rod). I'm thinking of a band-pass filter stage using a BJT and collector tank circuit then, feed to the op-amp.

You can even buy something from ebay that you can use: -

enter image description here

That gets you the all-important ferrite rod and, with a bit of luck might also get you the appropriate RF front end circuit that can feed your op-amp and comparator.

Here's a circuit of an MSF/NPL 60 kHz signal receiver that works in the UK: -

enter image description here

But, the important bit is that it uses 60 kHz. So, although you don't need to detect the data, it can be used to receive the carrier of any 60 kHz transmission.

There is also this interesting WWVB design and, I have included a schematic section showing the antenna amplifier: -

enter image description here

It happens to be phantom powered but, that's a trivial modification if you wanted to power it from a separate supply.

Andy aka
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  • I don't have a ferrite rod antenna either. – endolith Nov 27 '23 at 17:12
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    See my edit (I anticipated that one). – Andy aka Nov 27 '23 at 17:13
  • Oh I see, that eBay thing is outputting a digital signal that represents wall clock time, and not a 60 kHz tone? – endolith Nov 27 '23 at 18:00
  • @endolith it is but, all you are realistically buying is the ferrite rod antenna for a rock-bottom ebay price and, quite possibly, dumping the rest of the circuit. I don't usually recommend buying from ebay but, this is an occasion that is likely to be a decent risk. – Andy aka Nov 27 '23 at 18:13
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    endolith: NewYork U.S.A. is in fringe area for WWVB at 60 kHz. Andy's MSF receiver is a regen receiver that cannot injection-lock to WWVB's BPSK-modulated carrier. In any case, weak signals in a fringe area are very difficult to injection-lock with this kind of receiver. Am unfamiliar with Rugby's (U.K.) MSF modulation -perhaps it could injection-lock. – glen_geek Nov 27 '23 at 18:30
  • @glen_geek you may be right if the 60 kHz carried is modulated using suppressed carrier techniques however, I'm only really suggesting the use of a ferrite rod antenna as a way of picking up the signal. The ebay item claims to be a WWVB receiver so, the front-end antenna will work. – Andy aka Nov 27 '23 at 18:36
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    Andy: yes, I'll confirm that these ferrite rods can provide a 60 kHz signal from WWVB, even in my fringe-area region. NewYork is fringe too (for WWVB). The noise level in urban-fringe is most often MUCH larger than WWVB's carrier, so extremely narrow-bandwidth filtering is needed. I also have that E-bay receiver here - it often produces OK output, but there's no way to output 60 kHz. – glen_geek Nov 27 '23 at 19:01
  • @glen_geek then the ebay item is useful for its 60 kHz antenna (plus tuning capacitors that will need to be added). Hey, for an EE, this stuff is simple. Thanks for contributing. – Andy aka Nov 27 '23 at 19:09
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    WWVB is not quite as "really easy" as it used to be, since they added the BPSK time signal (periodic phase reversals). It's not the end of the world, but it makes it trickier to work with for frequency applications. One approach is to frequency-double it up to 120kHz, so that the 180° phase shifts become 360° phase shifts and almost disappear. – hobbs Nov 27 '23 at 19:37
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    @hobbs do you think that these ebay circuits use a squaring stage to double the frequency? – Andy aka Nov 27 '23 at 19:58
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    @Andyaka I don't know what those boards do, it's not even clear from the listings what they output, and I don't have one :) – hobbs Nov 28 '23 at 23:28
  • OK. I can't be sure either. – Andy aka Nov 28 '23 at 23:34