Unwanted tone RX by comm radio

[corbinace]

I am getting a strange tone in my comm. radio on one frequency. I have a Terra TX 760D.

The problem has at least two possible clues.

First for several years I have intermittantly gotten a dual tone when coming into my home area monitoring 123.0, It is kind of an EE-AH sound resembling a donkey. (now stop laughing) It is on a maybe 60 second interval and a very strong signal. I had a guy with some radio monitoring equipment try to hear it a couple of hours after landing one day and we could not find it.

Next clue. I just got an APRS tracker, Micro-Trak RTG. When this unit transmits on 144.35, I get a similar tone. It is different in pitch and not quite as noticeable a difference in front and back as the other tone but still very similar. I can tune .02 up or down and the APRS tone is gone.

This is not an antennas, cables or power supply too close issue as I ran the unit off of my pickup 30 feet away and listened to the airplane via the headset I had placed on the cowl.

Next I listened on 123.0 with my handheld while the APRS transmitted and no tone is heard on the Garmin handheld.

Is this maybe some kind of ?multiples of frequency? issue that I can maybe filter out some way?

Is this more likely a problem with the radio or the airframe and wiring?

If it is the radio, is it possibly repairable or should I just write it off and get a good radio?

I stand ready to learn, thanks for your thoughts on this issue.

Tim.

 

[PaigeHoffart]

Image Frequency

I'd be willing to bet that you're listening to the image of 144.39. You can Google superheterodyne for a more in depth explanation, but basically modern radio receivers convert the signal they're receiving to a fixed Intermediate Frequency (IF) using a local oscillator and mixer before demodulating the signal.

The math work out like this:
Local Oscillator Freq: 133.7
-Received Frequency: 123.0
------------------------------------------------------
= Intermediate Freq: 10.7 (a very standard frequency)

Received Frequency: 144.39
- Local Oscillator Freq: 133.7 (the same one)
------------------------------------------------------
= Intermediate Freq: 10.69 (close enough to cause a problem)

The other radio that doesn't have the problem either has a much better front end filter, or it has a different IF (more likely).

There probably isn't a whole lot that can be done if the 144.39 signal is relatively strong.

FWIW,
Paige

 

[corbinace]

Thank you Paige,

I will go and do the google search you mentioned. I am pretty new to this RF stuff but it is very interesting to me.

OK, I am back from doing a bit of learning.

Would it stand to reason that I may be able to hear it on 133.7 as well? I will check that area of frequencies tomorrow.

Tim.

 

[PaigeHoffart]

There will only be two image frequencies for 144.39 using a 10.7 MHz IF:
122.99 (133.69 LO), and 165.79 (155.09 LO). If you tune up 133.69, you should get a mixing product near DC, which shouldn't make it through the IF section of the radio. Having said that, there is a possibility that it could overload the mixer and desensitize the receiver, interfering with reception on that frequency. You might also be able to hear the LO on a 2M receiver (144.4 if you tune 133.7)

Hope that helps.

Paige