×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

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

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

(OP)
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?

### RE: Three RF Signals Combined to form New RF Sig...?

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

### RE: Three RF Signals Combined to form New RF Sig...?

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

### RE: Three RF Signals Combined to form New RF Sig...?

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.

### RE: Three RF Signals Combined to form New RF Sig...?

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).

### RE: Three RF Signals Combined to form New RF Sig...?

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.

### RE: Three RF Signals Combined to form New RF Sig...?

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

### RE: Three RF Signals Combined to form New RF Sig...?

(OP)
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!

### RE: Three RF Signals Combined to form New RF Sig...?

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

### RE: Three RF Signals Combined to form New RF Sig...?

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

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

### RE: Three RF Signals Combined to form New RF Sig...?

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

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!