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superious emission

C

CaptBob92

Has anyone run into a problem with a BE Fm transmitter ( newer model maybe three years old tops ) that creates a spurious carrier, modulation and all, at 600 khz (give or take) above and below the fundamental frequency? This is operating at the lower end of the fm band. The spurious carriers are weak but can be heard 30 miles away and much stronger with 20 miles or less of the transmitter..

There are no other transmitters operating in the vacinity.

Recently, another fm station near the top of the fm band with an antenna some 20 miles away from the first began broadcasting the same thing i.e a spurious signal modulation and all 600 hz above and below too.

I'm thinking something is getting into the tank circuts of these two stations but it seems unusual that 1) there would be a strong signal around 600 khz near both stations ( there arn't any am broadcasters on 600 khz within 150 miles ) or 2) that both station's transmitters are generating the same problem. ASny ideas Theorys?
 
Don't discount the audio processor/stereo generator or even your exciter putting out crud in that range, often due to an aging power supply. In my RDS work, I've seen it more than I would have liked.

A modern-era analyzer that can see down into the composite spectrum can tell you a lot.
 
CaptBob92 said:
Has anyone run into a problem with a BE Fm transmitter ( newer model maybe three years old tops ) that creates a spurious carrier, modulation and all, at 600 khz (give or take) above and below the fundamental frequency? This is operating at the lower end of the fm band. The spurious carriers are weak but can be heard 30 miles away and much stronger with 20 miles or less of the transmitter..

There are no other transmitters operating in the vacinity.

Recently, another fm station near the top of the fm band with an antenna some 20 miles away from the first began broadcasting the same thing i.e a spurious signal modulation and all 600 hz above and below too.

I'm thinking something is getting into the tank circuts of these two stations but it seems unusual that 1) there would be a strong signal around 600 khz near both stations ( there arn't any am broadcasters on 600 khz within 150 miles ) or 2) that both station's transmitters are generating the same problem. ASny ideas Theorys?
Click to expand...

BE transmitters are pretty much clean. I've had very good results with the BE FM 250's. They usually don't put out any spurs. I would first check to see if the Processor/Stereo Generator could be having some issues with some electrolytic caps drying out. If you can, try another processor to test if the spurs are coming from the processor. It does happen on occasion. If the spurs are continuing, try the checking the exciter. If at all possible, try to isolate the exciter on a dummy load or go directly to the antenna, bypassing the PA. If no spurs, then more than likely it could be in first stage or in the final. Let us know what happens. Good luck.


Peter Q. George (K1XRB)
Whitman, Massachusetts
 
I had this exact thing happen with a brand new BE...turned out to be the RDS encoder...so make sure you examine everything before the chain.
 
That spur is as illegal as a pirate station (or in this case, two pirate stations). It causes worse interference than IBUZ.
 
I am not associated with the station and know only that they are using new BE equipment which I too have found to be very reliable.

At first I thought there might be something at the same site that was operating around 600 khz and was getting into the tank but when the same thing occured at an entirely different station miles away I became puzzled. What's the odds of a spur on another station 600 khz up and down too? Especially when 600 khz is way far away from VHF and probably wouldn't get in through the antenna.

Now I'm wondering if this is a strong signal closer to some multiple of 600khz and getting in through as a harmonic.
 
There's a station I am aware of that's had an ongoing battle with this exact issue, only it's +/- 800khz. And the spur has been heard a good 20 miles away at times, although it varies. The engineer is aware of it & may have already fixed it, but it is a BE (I want to say a 30KW).

If there's an AM co-located at the same site & it's near 600khz, it's pretty easy for the 600khz to enter the composite input. I had this issue 33 years ago with an AM on 1010 & FM on 101.5. At 100.5 & 102.5 (during the day when the AM was on), there was a spur that was audible perhaps 3 miles. Double shielded coax between the stereo generator & composite input knocked it down to almost nothing. Never could totally remove it.
 
For future reference, the correct spelling of this malady is Spurious...often referred to a "spur".
 
Is there a powerful AM radio station near by on 600Kc by chance? Sometimes when an AM station is running high power near another FM it will cause the two signals to mix in mid air generating two new signals, one up 600Kc and one down 600Kc from the FMs frequency of operation.
Simple way to check this is just to turn on an AM radio and see if there are any powerful signals at 600 on the dial.
If no station is there then I would take the radio around the transmitting equipment and audio equipment and see if something is generating a signal down there before it reaches the audio inputs of the TX.
 
Are you checking out this problem on just one receiver? Or are you using multiple receivers? It wouldn't surprise me if the problem is locally generated.
 
No, there are no stations within 100 miles of the sites near 600khz. There is a station on 1200khz but it is not anywhere near either site.
 
You really need to check this with the ultimate geek radio that has a screen, the trusty spectrum analyzer. Check the sample port of the TX closest to the output, then check by using an antenna. Go from there.
 
If you happen to have a PI FIM 71 in the area you can use, try and see if you can hear it. If you do use a spectrum analyzer, be sure to remove the fundemental frequency before you look for spurs. That's a very common step for people to skip, and you'll get false-positive reading from overloading the spectrum analzyer input. http://scott-inc.com/html/fmnotch.htm
http://www.eagle-1st.com/plist.nsf/B422!OpenForm
 
Seriously, the comments about processing and RDS encoders are very valid. They need to pull off EVERYTHING that might be modulating that transmitter and see if the spur goes away.

I had a composite STL that caused the same problem at +- 600KHZ.
 
Didn't this drill show up here already? Short the RDS input to the exciter. If it goes away, fix the RDS generator. If not, Short the exciter input. If the spur is still there, fix the exciter. If not, short the STL input. Still there, fix the STL. If not, find out what's making the source grow the spur, or find out where the RF is getting into the audio.
 
50ohmload said:
I had the same issue with a newer BE transmitter, and the problem was due to the RDS encoder.
Click to expand...

I had a similar issue when a BE transmitter with an RDS encoder which obliterated our signal, exactly one MHz away. It wasn't the encoder though. It seems it was the BNC cable that connected it. Or so said the station's engineer (who is very good at what he does).
 
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