Accuracy of typical AM radio station carrier for calibrating instrument?

[jrmymllr]

I got a pretty nice arbitrary function generator for free and it seems to not have any issues. It only has a standard crystal oscillator, not the optional crystal oven, but I'd still like to get an idea of the reference accuracy.

Years ago I remember an article in Radio Electronics, or maybe Popular Electronics, that would pick up the horizonal scan frequency of an NTSC TV and that could be used as a very accurate reference. Of course NTSC TV transmissions are gone (good riddance), but it got me thinking, could I use an AM radio station to calibrate the FG?

There's a Youtube video on this sort of thing, but it's a specific station in the UK and ignoring if I could even get it in the middle of the US I don't have a receiver that can pick this up.

What about a well established AM radio station? How accurate are they typically? This isn't important enough to me to buy a reference, but it would be cool if I could do this for free.

()

[mag_therm]

Better to use WWV if you can borrow a receiver .
I used a sound card audio spectrum analyser to calibate DDS to zero beat at 10 MHz, but that method only accurate down to within 10 Hz or so

[jrmymllr]

In the US, the FCC rule for AM stations is maximum +/- 20Hz deviation, which is probably worse than your FG.

That's good to know. I think it'll still be useful.

Let's say I use a station high on the dial at 1570kHz. That's 0.0127ppm. The OSC in my FG is only specified at 5ppm, and 5ppm per year aging but it was likely calibrated within the last year. I think this'll work.

Edited because I had the incorrect AM station PPM.

[Kleinstein]

How accurate the carrier frequency is depends on the station. AFAIK the long wave radio france had a very accurate carrier that was meant to be used as a frequency reference too. This probably does not help very much in the US.
The time services like WWV are pretty good bet to find an accurate carrier, though the signal may be relatively weak compared to a more local AM radio station.

[jrmymllr]

Let's say I use a station high on the dial at 1570kHz. That's 0.0127ppm. The OSC in my FG is only specified at 5ppm, and 5ppm per year aging but it was likely calibrated within the last year. I think this'll work.

You might want to check your math.

Hint: If the station is at 1MHz and is 20Hz high, how many ppm is that?

LOL you're right, I clearly have a PPM comprehension problem. In that case an AM station anywhere on the dial could be roughly 15-40 ppm off, so not great.

[Zoli]

I got a pretty nice arbitrary function generator for free and it seems to not have any issues. It only has a standard crystal oscillator, not the optional crystal oven, but I'd still like to get an idea of the reference accuracy.

Years ago I remember an article in Radio Electronics, or maybe Popular Electronics, that would pick up the horizonal scan frequency of an NTSC TV and that could be used as a very accurate reference. Of course NTSC TV transmissions are gone (good riddance), but it got me thinking, could I use an AM radio station to calibrate the FG?

There's a Youtube video on this sort of thing, but it's a specific station in the UK and ignoring if I could even get it in the middle of the US I don't have a receiver that can pick this up.

What about a well established AM radio station? How accurate are they typically? This isn't important enough to me to buy a reference, but it would be cool if I could do this for free.

()

If you really want to go with a radio, try a WWVB receiver(for EU: that's the local DCF77);
https://www.nist.gov/pml/time-and-frequency-division/time-distribution/radio-station-wwvb

[Dr. Frank]

I got a pretty nice arbitrary function generator for free and it seems to not have any issues. It only has a standard crystal oscillator, not the optional crystal oven, but I'd still like to get an idea of the reference accuracy.

Years ago I remember an article in Radio Electronics, or maybe Popular Electronics, that would pick up the horizonal scan frequency of an NTSC TV and that could be used as a very accurate reference. Of course NTSC TV transmissions are gone (good riddance), but it got me thinking, could I use an AM radio station to calibrate the FG?

There's a Youtube video on this sort of thing, but it's a specific station in the UK and ignoring if I could even get it in the middle of the US I don't have a receiver that can pick this up.

What about a well established AM radio station? How accurate are they typically? This isn't important enough to me to buy a reference, but it would be cool if I could do this for free.

In the 1970ties, I built my first UK based, Droitwich 200kHz receiver as a very stable and precise reference for adjustment of my DIY - TTL based counter.
Nowadays, the transmitter still exists, based on a Rb, but emits on 198kHz.
The signal of our PAL color TV system was quite inconvenient to use, and I also experimented with the 19kHz stereo pilot tone of our FM radio, which was also quite stable.

Later I built a DCF77 receiver, from a Dutch / German electronics magazine, 'elektor', which converted the 77.5kHz to a precise 10MHz reference signal, provided one averaged over several minutes or hours. This device still works very well (after 35 years or so).

For the last 25 years, I worked near the Mainflingen DCF77 transmitter, and our favorite lunch restaurant is only a few hundred meters away from the antennas.
So even the fish on Fridays is AM-modulated.

Anyhow, I'd recommend to simply get a GPS time receiver, although all these 'historic' sources are quite exciting to explore.

Frank

[jrmymllr]

Between time service stations (the WWVs) and the beacon stations which are documented to have atomically tied time bases you're probably pretty well set for short wave.

But also don't forget that using a mere 1 Hz reference with or without high short term precision but compared over a longer term is commonly done e.g. many GPS receivers output 1-PPS logic signal pulses at approximately the GPS synchronized time (+/- NNN ns sometimes with a triangle wave offset from reality if they're based on the closest edge of a NN MHz LO as opposed to a PLL tuned / interpolated VCXO).  But over the long term (days, weeks,...) anything locked to such a periodically corrected time base will be very accurately comparable to find even the smallest progressive net drift in timing due to a static frequency error.

Also some GPS units have the ability to reprogram the PPS output or augment it and generate synchronized NN kHz or maybe higher time base outputs.

I have found some really cheap (<$20) options for this, so that's interesting.  CORRECTION: Even less than $5 on Aliexpress. Correction again: they only provide the time, not a frequency output. Dang.

[David Hess]

Network TV broadcasts used to be a good way to get a frequency reference because they were phase locked to the master station during the network broadcast.  I do not think FM or AM broadcast stations were ever that good.

WWV has the problem that skip off of the ionosphere causes Doppler shift as the altitude of the ionosphere changes.  It is eary to watch.  There have been a few untuned receiver designs for recovering the WWV carrier.

[Conrad Hoffman]

What frequency is the oscillator? Before I got a GPS reference I set my 10 MHz counter oscillator by putting a couple inches of wire on it for an antenna, then using a portable shortwave in the same room to tune in WWV at 10 Mhz. Easy to hear them beat together if you get the levels sort of matched.

[jrmymllr]

What frequency is the oscillator? Before I got a GPS reference I set my 10 MHz counter oscillator by putting a couple inches of wire on it for an antenna, then using a portable shortwave in the same room to tune in WWV at 10 Mhz. Easy to hear them beat together if you get the levels sort of matched.

It's 10MHz. I don't have any shortwave equipment though as that's never been my thing.

[J-R]

Quite a few DMMs will give you usable results to sanity check your function gen.  Same for a typical scope.  Beyond that, there are lots of counters to be found online, new, old or DIY kits.
Don't you want a GPSDO?  I personally have the Leo Bodnar unit.  It has an internal TCXO.

[bd139]

+1 for the bodnar reference.

I had one of these for a bit: http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=301

[edpalmer42]

Call your local AM stations and talk to the head engineer.  Explain what you're doing and ask them about their equipment.  You might find one that's friendly or maybe wants to brag about his equipment.

Ed

[jrmymllr]

Quite a few DMMs will give you usable results to sanity check your function gen.  Same for a typical scope.  Beyond that, there are lots of counters to be found online, new, old or DIY kits.
Don't you want a GPSDO?  I personally have the Leo Bodnar unit.  It has an internal TCXO.

I haven't connected it to my scope yet, but I'm guessing it can't be that far off. For my purposes, ultimate accuracy isn't important. I just thought it would be cool to compare it to something very accurate and nearly free to try.

[jrmymllr]

Call your local AM stations and talk to the head engineer.  Explain what you're doing and ask them about their equipment.  You might find one that's friendly or maybe wants to brag about his equipment.

Ed

Interesting idea. I might try that.

[metrologist]

Supposedly, NERC (North American Electric Reliability Corporation) limits the mains frequency excursion from the nominal value of 60Hz to under 0.083%. Now you can solve how many ppm that is.

http://techlib.com/electronics/atomic.html
http://techlib.com/electronics/atomic2.html

[jrmymllr]

Supposedly, NERC (North American Electric Reliability Corporation) limits the mains frequency excursion from the nominal value of 60Hz to under 0.083%. Now you can solve how many ppm that is.

http://techlib.com/electronics/atomic.html
http://techlib.com/electronics/atomic2.html

I thought about that, but while it's great for long term timekeeping, it isn't great for calibration. If my calculations are correct, it's 833ppm.

[mawyatt]

Supposedly, NERC (North American Electric Reliability Corporation) limits the mains frequency excursion from the nominal value of 60Hz to under 0.083%. Now you can solve how many ppm that is.

http://techlib.com/electronics/atomic.html
http://techlib.com/electronics/atomic2.html

I thought about that, but while it's great for long term timekeeping, it isn't great for calibration. If my calculations are correct, it's 833ppm.

The power grid slows down as the load demand increases, and attempts to "catch up" time wise in the very early morning when load is minimum. The general idea is to have the 24 hours time as close as possible to ideal 1 day, this helps keep the  clocks in "tune" long term, or 60*60*60*24 or exactly 5,184,000 cycles per day.

So not that good a short term time nor frequency reference.

Best,

[jonpaul]

The Leo Bodnar GPS synth is app control, USB, including GPS antenna output 400 hZ...800 MHz

http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=301&zenid=8539ffcac9e452787f959a7a8e4b2017

 £ 110, 165

Jon

[J-R]

Can't see how the radio or grid frequencies would be of any use.  Not enough range, precision or accuracy.   Even excluding the fact we're in a metrology thread...

[jrmymllr]

Call your local AM stations and talk to the head engineer.  Explain what you're doing and ask them about their equipment.  You might find one that's friendly or maybe wants to brag about his equipment.

Ed

I sent a local station a message but I'm sure they'll have no idea what I'm talking about. But I tried this on that same station and was able to zero beat it at 1Hz over the FG claimed setting. So I know that's not definitive for anything by any means but thought that was interesting. I might try others for the fun of it.

And that was the first time I tried the zero-beat method on anything.

[jrmymllr]

Can't see how the radio or grid frequencies would be of any use.  Not enough range, precision or accuracy.   Even excluding the fact we're in a metrology thread...

Isn't it valid to compare against the WWVB carrier, for example?

[David Hess]

Isn't it valid to compare against the WWVB carrier, for example?

Or any of the WWV transmissions could be used, but any signal which relies on ionosphere skip is subject to a Doppler frequency shift limiting accuracy.

[Conrad Hoffman]

https://www.govinfo.gov/content/pkg/CFR-2010-title47-vol4/pdf/CFR-2010-title47-vol4-sec73-1545.pdf

AM/FM not as good as I thought. I've got the Bodnar unit too, and it just takes all the effort out of frequency calibration. You can set the output to just about any frequency and be done with it. Worth every penny.

[Wallace Gasiewicz]

I just built a GPSDO out of junk and a nice aluminum box. It also has a voltage controlled ovenized oscillator. After all the assembly, it seems to work well. I can only see maybe 1 HZ change in One GHz harmonic.
I really do not know if my other test equip is accurate but all my TE capable of counting seems to think the homebrew unit works.
After all the work involved, I think I would have been better off sending Leo Bodnar a check...his unit seems to be quite nice.
But the project kept me off the streets.....

[J-R]

Yes, the Leo Bodnar units are a slight luxury.  Being able to change frequencies is a great feature.  Support was responsive.  I wish the software was a little better, but no big deal.

[Maurizio1957]

http://www.isaacasimov.it/ik5qlo/correlation.htm
see the "reference receiver" part
Maurizio

[tkamiya]

WWVB signal is frequency shift modulated, so it isn't suitable as a calibration source.  WWV signals do have a carrier, so it's much better choice.  With wide and cheap availability of GPSDO, I don't think anything else would make much sense.... especially if your function generator is a beginning of setting up a personal lab.

[Wallace Gasiewicz]

When using a HP 3746 Selective level receiver, if I use WWV and tune the OCXO for as close as I can get, I really can tune the internal OXCO to within one Hz.
I know this is not what you metrology guys are used to, but it does work.
However the Hp unit is a measuring rxvr and has a 55 Hx filter and can measure signal strength very well, so you can actually use the signal strength to align the Xtal.

Most common freq generators do not have anythimg resembling a metrology grade standard and probably aligning them with the stated freq of several AM stations would be as close as you can really get. I think the internal xtal ref would move enough by itself to confound any serious attempt at calibration to anything closer than a few Hx at 10 MHz.

[David Hess]

WWVB signal is frequency shift modulated, so it isn't suitable as a calibration source.  WWV signals do have a carrier, so it's much better choice.  With wide and cheap availability of GPSDO, I don't think anything else would make much sense.... especially if your function generator is a beginning of setting up a personal lab.

60 kHz WWVB continues its legacy amplitude modulation, and has added phase shift keying.  The phase shift keying is not compatible with legacy receivers that tracked carrier phase, but nothing prevents a modern receiver design from recovering the carrier, presumably using a Costas loop.

[Conrad Hoffman]

You can (I think) fix those old WWBV time receivers by frequency doubling the input signal and bypassing one of the later /2 stages.

[Wallace Gasiewicz]

On my HP 3746, WWVB reads as it should, 60 KHz. I think if you beat the freq generator with a AM radio tuned to WWVB you can get close. Probably will sound funny though.
That is, if your AM radio can receive the WWVB signal, which might be low and variable depending on time of day and weather etc.....

[Circlotron]

I have a Rigol DG1022Z function generator and just now I zero beat it against the national broadcaster on 774kHz in Melbourne. After a half hour warmup for the function generator you could hear the radio dip to a minimum every 8.2 seconds. That’s a difference of 0.122Hz or about 0.157ppm. Not the gold standard perhaps, but better than a wet finger in the breeze.

Of course, using this method, both signals could be off by the same amount and you would never know, but that would be quite a coincidence.