Three RF Signals Combined to form New RF Sig...?

[Hawaiiankong]

I work for the Air Force on air traffic control equipment. Right now we're experiencing a problem with three 10-watt transmitters (132.025, 135.6, and 140.975) that, when transmitted simultaneously, cause that signal to be received on any receiver tuned to 126.65.

I'm not an engineer, but I have been working with these systems for a long time, and I am very frustrated by this. The best theory I can come up with is frequency heterodyning (but not sure), and the only FH formulas that I can find can only combine two frequencies to form something completely out of left field.

Can somebody tell me what might be happening here?

 

[MacGyverS2000]

Two frequencies create a third... that new signal can then combine with others. You have three signals, so it's not as simple as just two.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Hawaiiankong]

Thanks for the reply. Any idea how I would go about calculating that out?

 

[VEBill]

Basically, make a table of everything vs. everything and compute the deltas:

126.650 132.025 135.600 140.975
126.650 5.375 8.950 14.325
132.025 3.575 8.950
135.600 5.375
140.975

You can see 8.950 and 5.375 pop out as commom products.

And this extremely simplified "analysis" (barely) doesn't even include image frequencies and L.O.s.

I'll need a coffee before I can interpret into plain English.

Normally we let the E3 guys run their complicated software to do this stuff.

 

[VEBill]

Okay...

140.975 MHz beats (mixes) with 132.025 MHz to produce a mixing product at 8.950 MHz. This then mixes with 135.600 MHz to land exactly on 126.650.

The basic root of the problem is fundamental overload, almost certainly in the receiver, that goes non-linear and creates mixing products.

The solution might be to move the receiver antenna further away, or try to track down the non-linearities. Or add notch filters (large cans at these VHF frequencies).

 

[VEBill]

By the way (excuse the repeated posting), but you've got another route.

140.975 MHz beats (mixes) with 135.600 MHz to produce a mixing product at 5.375 MHz. This then mixes with 132.025 MHz to land exactly on 126.650.

Double trouble.

 

[Higgler]

Do you need to solve this problem? Or just explain it?

To solve it, I assume you have 3 differnt transmitters, each having their own antenna, and they are spaced apart from each other by a few feet or more. Or is this three transmitters using the same antenna?


Either way, it would be informative to look at a Spectrum Analyzer attached to a wideband antenna and see if the signals are pure in the airwaves.

If they are pure, then you can fix it in the receiver with a filter if needed.

kch

 

[Hawaiiankong]

I'm thrilled that not only are you guys tracking with my problem, but that you are able to solve it!

VE1BLL, pardon my ignorance, but what do you mean by compute the deltas? I've tried the Heterodyning formula (.5cos(f1-f2)-.5cos(f1+f2)) and wasn't able to get the numbers in your table. Could you please explain a little more how you got that table of numbers?

As for our work center's solution, we have already begun the process of getting one or two of the frequencies changed.

Thanks!

 

[MacGyverS2000]

Forget the cosines, forget the extra math, just use simple addition/subtraction. Run through the tables of frequencies... pick two frequencies and if they aren't equal, find their summation and difference. Add those new values to the table and start again. You'll quickly find your frequency of interest popping up.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

~World's Shortest Answer Contest~

Runner up:
Q: "...what do you mean by compute the deltas?"
A: -

And in First Place:
Q: "How did you end the story?"
A: .

;-)

 

[Hawaiiankong]

Sorry for the late reply but it makes sense to me now (I do have the propensity to over-complicate things). Thank you!