Local ATL AM Station Frequency Offsets

[radio.engineer]

I never tried looking at this until recently, but I looked at the AM carrier frequency errors of stations in the Atlanta area that I could pick up with sufficient signal strength. I did this with a high precision R&S spectrum analyzer that was locked to a high stability, GPS locked 10MHz reference. Here is a sampling of what I found:

WSB 750 kHz is actually 0.7 Hz low = 749.9993 kHz (one of the stations in town that is nearly dead on frequency)

WCNN 680 kHz is actually 2.5 Hz high = 680.0025 kHz

WDWD 590 kHz is actually 33 Hz high = 590.0330 kHz

WMLB 1690 kHz is actually 85 Hz low = 1689.9150 kHz (the station in town that has the highest offset)

Usually nobody cares about these trivial frequency offsets (except the FCC around performance test time), but it does have a dramatic effect at nighttime when stations bleed into the area through skywave. The result can be rapid fading if the offset is great enough with the interfering station(s).

WMLB was the station that had the highest frequency offset that I measured and its frequency would slowly wander during the course of the day.

 

[littlejohn]

(In an Artie voice0 VERRRRRRRY INTERRRRRRESTING. wHat was your methodology?

 

[taylorengineer]

The maximum frequency tolerance for AM broadcast stations is +/- 20 Hz.
I suspect your measurements because both stations have almost new transmitters and most equipment today is very stable - it would be very unusual(not impossible)to find a modern transmitter (AM) off carrier by 85 cycles.....

 

[DashRiprock]

Do 100+ temperatures affect stability of the transmitters?

 

[CJ_Jackson]

WMLB has one of the newer transmitters in the market, a 3DX-50. It should be within a couple hz of dead-on freq or it'll go out of lock and shut off. I have an outside frequency measuring service check all five of my sites every month, and I get five nice postcards with the measurements. I do this to be absolutely sure things are in tolerance. By the way, I've heard the occasional am-er thats off enough to make things messy at night on a channel, heard one last month late afternoon when I was driving around the upper Ohio area coming back from Ontario that had to be a couple hundred hz off. It was nailing the primary station, 1050 CHUM (which was using a cool old Top 40 jingle package by the way).

 

[radio.engineer]

I checked the measurement accuracy of the spectrum analyzer (+ GPS locked frequency reference) I used against WWV's 15.000 MHz signal and the frequency I measured is right on.

The WMLB frequency has drifted a bit since this morning at now it is only -65 Hz from its assigned center of 1690 kHz. So, it could be it is out of lock with GPS? The cooling system could be struggling? New transmitters could certainly be off frequency if their reference is off or unlocked.

I checked some more local AM signals this afternoon and measured...

WCFO 1160 kHz is actually 2.5 Hz low = 1159.9975 kHz
WNIV 970 kHz is actually 33 Hz high = 970.0330 kHz
WAFS 1190 kHz is actually 3.5 Hz high = 1190.0035 kHz
WLTA 1400 kHz is actually 2.5 Hz low = 1399.9975 kHz

 

[littlejohn]

Again, could you share your methodology for these measurements?

Thanks.

 

[taylorengineer]

WMLB, 1690, actually uses a DX-10. I think CJ is mixing 1690 with 1160.(1160, WCFO has the 3-DX-50.)
I took care of the DX10 at 1690 until Weber bought it. I can assure you the building has redundant A/C units and the xmitter was always within a cycle or two of being dead on.
I, along with Littlejohn, would like to know more about your technique. Accusing stations of illegal operation in a public forum is pretty serious.....'splain yourself pleeze!

 

[the_widows_son]

Yes, share I'd like to know if my Belar monitor is lying to me, is the factory test data I have is wrong or are you a charlatan?

 

[littlejohn]

I don't think 'charlatan' is necessarily indicated here, but we'd certainly like to know how the gent is making his measurements. Given a resolution bandwidth of something on the order of 100Hz (the last R&S I messed with), I don't think I'd particularly trust a simple screen reading of the frequency of the displayed signal. Given their hi - stab oscillator, I might consider beating the tracking gen against the signal of interest... but then you'd have to measure the beat note accurately, and if it's under 10Hz, ain't gonna be easy. I'd much rather see a traceable counter to measure the thing. Or, take my own counter and feed it the 10MHz for the master clock off my GPS time system. If I had Doung And The Boys after me, I'd use a traceable counter. Or just have the service do a special measutrement, and supply them that along with the last few months normal measurements. Not that the R&S - or Agilant's, or the Anritsu one aren't accurate in their time base, rather that's not the type of measurement they're designed to make which leads me to wonder how accurately they will make one, which leads to a request for the methodology.