2nd Channel Interference Question.

[Info-warrior]

In another thread about transmitters, I mentioned that I think that my BW TX4's buzzing/chirping sound when in stereo might be related to the switching power supply. After doing some research, I think I'm on the right track.

However, I was curious to see if the operating frequency had any effect; so I moved it up to the middle of the band. The buzz/chirp remainded, but I heard something interesting and wonder if the pros could explain this. Here's the arrangement, station on 99.1, station on 99.9, my tx at 20mW into a short antenna on 99.5. Reception of 99.1 unaffected, tuning to 99.9 on the receiver and I hear 99.1. That's not the type of interference I would have expected. Would this be considered front end overload, some type of intermodulation, something seriously wrong with the transmitter?

It's surprises like this that reinforce why the FCC and other regulatory agencies conduct studies before granting a license on a given frequency. I've tried searching the net before asking, but the info I come up with on 2nd channel interference relates to the type of interference I would have expected (bleedover).

Any commentary is appreciated.

 

[Bill981]

This may not be 2nd channel, but an image. For example, if you have a strong station 10.6 MHz. or 10.8 MHz away (the standard 1st IF frequency in a receiver is 10.7 MHz.)
88.3 + 10.7 = 99.0
88.5 + 10.7 = 99.2
89.1 + 10.7 = 99.8
89.3 + 10.7 = 100.0

Although images are usually (IF*2) away, which puts any FM image outside the band. It could still mix with the local oscillator, though.

Or it may be a mixing product:

99.1 + .4 = 99.5
99.9 - .4 = 99.5

400 kHz may be a 2nd IF frequency in your receiver.

I'll bet the phenomenon goes away when you remove the 99.5.

In any event, it's the receiver that's the problem, not the transmitter and the Commission doesn't regulate receivers. They do, however, try to keep stations with IF separations far enough apart so as not to create the mixing product. Some stations that are IF separated may have a requirement to reduce power if interference results.

So it's not just 1st, 2nd and 3rd adjacent channels, but also IF separation (that is more rare) that is considered.

If you look in the FCC Rules http://louise.hallikainen.org/FCC/FccRules/2010/73/207/ you'll see what I mean.

 

[Info-warrior]

Thanks for the detailed reply. This is the kind of stuff that keeps me interested in radio. Despite being out of the business for about five years, I was a run of the mill board-op, I can't get the radio out of me.