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Author Topic: AM Broadcast/LW/MW frequency standards  (Read 2539 times)
G3RZP
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Posts: 8142




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« Reply #15 on: November 14, 2017, 09:46:24 AM »

There are BBC transmissions on 198 kHz at five sites in the UK. The main one is Droitwich, south west of Birmingham, (500kW): then there's two in London - 4 watts covering the inside of the Rotherhithe Tunnel, and under 1 watt covering the Blackwall tunnel (both tunnels under the River Thames), one at Westerglen  (50kW), roughly half way between Edinburg and Glasgow, and one at Burghead (50kW), on the Moray Firth in north east Scotland. The sites are all owned by a company called Arquiva: frequency is controlled by a rubidium standard.

Droitwich uses a multiple conductor  'T' antenna strung between two 700 foot masts 510 feet apart. You get a good view of it from the train from Birmingham to Bristol.

I doubt you'll get good programme quality from any of them in the US, though! But especially at sunrise and sunset, you may well get a reasonable signal from Droitwich in a narrow bandwidth. They do carry some phase encoded time information.

How about LORAN C on 100kHz?
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N3DT
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Posts: 1267




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« Reply #16 on: November 14, 2017, 01:41:14 PM »

I doubt that I'm receiving 198KHz from EU. Whatever it is, an anomaly of the RX or not, it's pretty consistent 2Hz below 198K and very stable, no modulation that I can tell. I measured WWVB and CHU this morning before the ionosphere got into the picture and started scattering things, both of them measured within 0.001 Hz of their freq over an hour or more, average. I did try an AM digital local station, WMAL from DC and it's all over the place, it's even worse than my BFO. I may try a few AM broadcast stations and see if any of them seem stable. As far as LORAN, last time I remember trying to get info on them, they're off the air, I could be wrong but I think the GPS replaced them.

There's also a 2M beacon down in central VA I can get and it's locked to the guys GPS also. I haven't tried that one, but it's got a horrible chirp on the CW signal and doesn't produce a good trace on SL. You think there would be something out of the Naval Observatory in DC, but I guess not. I'm sure they've got some special timepiece there.

I still haven't tried my Rb osc, but I've been warming it up for 2 days now. It looks like the osc locked anyhow.

I may try building a shielded tuned loop for 60KHz.

I remember reading in SpectrumLab about using the 1 pps out of a GPS to lock the sound card, but never got into it that much, plus my GPS puts out an every other second pulse. But that's also not going to help me on VLF or even HF.
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N3DT
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Posts: 1267




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« Reply #17 on: November 16, 2017, 04:00:26 PM »

I still can't find any LF through HF signal that's anywhere as stable as either my Rb osc, GPSDO or my BFO which drifts ±10mHz with the fan on/off. Since I can't find a signal as stable, I guess I can't measure anything on HF to the resolution I can actually measure. I've got SpectrumLab down to decimate by 24 and FFT input at 37K with stable output measurements down to .0001 Hz. Anything more than that takes the computer too long to process. I spent all day yesterday figuring out the Plotter function in SL. That's a project but I'm getting so I understand it and also exporting text data I can put in Excel. Talk about a learning curve.
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WA3SKN
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Posts: 6523




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« Reply #18 on: November 18, 2017, 08:00:22 AM »

I suspect your GPSDO is far more accurate than either WWV or WWVB, and much more accurate than any AM or FM broadcast station... but you should verify this with NIST, as they are the time keepers.
And if you are curious about FMTs, check with ARRL to know their frequency references.
AM broadcast was 20 Hz tolerance and FM broadcast was either 2000 or 3000 Hz, not the best reference nowadays.
73s.

-Mike.
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N3DT
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Posts: 1267




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« Reply #19 on: November 18, 2017, 07:03:15 PM »

I did determine today that my Rb osc is ~.005 Hz below my GPSDO. Now I'm a guy with 2 clocks, which one is right? I'm going to assume the GPS one is pretty close and over time it seems to agree with what I can measure. Yeah, I've not found an AM station that doesn't drift all over, worse than my BFO. Thanks!
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WA3SKN
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Posts: 6523




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« Reply #20 on: November 25, 2017, 12:16:38 PM »

The big plus here is that you are only about 70 miles away from NIST!  Less than 90 minutes drive.
Just contact someone there and plan a trip! ( DO call first).
73s.

-Mike.
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AA4HA
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Posts: 2384




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« Reply #21 on: November 25, 2017, 05:27:40 PM »

I guess the underlying question is; "what are you trying to do with an accurate frequency reference?"

I have an Efratom LRPO-101 that is acting as a 10 MHz reference for my BB60C spec analyzer if I need to calibrate the oscillator on a microwave radio (work stuff).

Most ham gear is around 1 ppm, unless you buy a TXO (precision oscillator so most radios are 10E-6
My spec analyzer (0.1 ppm) is good to 10E-7
A rubidium "atomic clock" oscillator is good to around 10E-9 (1 ppb)
A GPS disciplined oscillator (often a rubidium oscillator) can be good to around 10E-12

I have an HP 3586B sold to me by someone who was a member of the "Time-Nuts" list and would of been good for frequency stuff. I used it a few times but am afraid of leaving it attached to an outside antenna for too long for fear of getting zapped by static. Those time-nuts people are.... well, nuts. Often they are trying to determine frequency down to the millihertz in some contest.

There is one local FM broadcast transmitter that is around 3 KHz off frequency and also clearly had something wrong with their transmitter (it was as if they were only transmitting with their exciter). I tried to be helpful by sending an email to the station engineer and to the email address on their FCC license only to have both bounce back as un-deliverable.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f
VA7CPC
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Posts: 2806




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« Reply #22 on: November 25, 2017, 07:53:48 PM »

FWIW --

The OP is talking about measuring to _milliHertz_ accuracy.  At HF (say 10 MHz) that one's part in 10-10.  A signal one mHz off, would drift one cycle in 1,000 seconds = one cycle in 17 minutes.

If he needs accuracy like that, using a 60 _kHz_ reference signal doesn't make much sense.  One part in 10-10 drift would be one cycle in (roughly) 47 _hours_ .

IMHO, that's too long a measurement time to be practical.  (Yes, I know that you don't need a full cycle -- but this is "back-of-the-envelope" thinking.)

.     Charles




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N3DT
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Posts: 1267




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« Reply #23 on: November 28, 2017, 02:29:09 PM »

Thanks for the replies. The reason I'm doing this is because maybe I can. That's all. I just like playing with super precise things. I should have been a QC inspector in another life. I've pretty much determined that at HF it's about useless to measure anything within a reasonable time to any accuracy better than a couple Hz. One can guess at the ionospheric Doppler shift, but that doesn't interest me. Last thing I did was look at some of the new digital TV stations, they have a video and an ATSC carrier which seem to be very stable. But I can't use the sound card and SpectrumLab with the Service Monitor. The SM does have a 1 Hz receiver filter readout and since it's locked to the GPSDO, I get consistent within 1Hz results with it, much better than any of the HF or AM stuff. I think I did mention that I did compare the Nortel NTBW50AA GPSDO with the Efratom LPRO-101 Rb after a few days warm up and they seem to agree within about 3mHz. But now here I am like the man with 2 clocks, which one is right. I'm going to assume the GPS one is the better. But 3mHz doesn't mean anything to the TS2000.

I'm not sure what going to NIST in G'burg would do, they probably don't want me wasting their time. I'm sure there are some labs around the Beltway that will be glad to take my money to measure my Rb LPRO-101 withing 1mHz and verified back to NIST.
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