Making FM from a Signal Generator and a Function Generator

[RedShoeRider]

Good Morning, All:

I'm sure there's a good way....ok, I'm not sure there's a good way of doing this, but that's what askin' is for!

I recently got  my  hands on an Anritsu MG3670B Signal Generator for cheap. Lucky, that....300khz to 2.1ghz range, nice screen, but it's a Digital Modulation Signal Generator, so it can't make an FM or AM signal on its own. If I wanted to do GSM, sure, it can do that, but that's far outside of my normal scope of work.

Sitting next to it on the bench is a Tektronix 5010 function generator, good for DC to 20mhz.

So, the way I see it, I have a carrier generator (the Anritsu) and I have a signal generator (the Tek).....but what's the easiest /most effective way of externally combining them so I have an FM test signal?

Thanks, all!
-Red

[scatha]

Given the sig gen has analog I/Q inputs (0->2MHz) it should be pretty easy to fudge AM, just alter the amplitude on either of the channels. FM might be a bit more of a challenge as the function generator only produces a passband FM output, and you need a way of altering the instantaneous phase of the complex sig gen input (maybe use a cheapo DDS with quadrature outputs instead?).

I'm just going by the device datasheets so I could be missing something obvious.

[Howardlong]

Take a look at this...

http://whiteboard.ping.se/SDR/IQ

If you're new to quadrature modulation, I would get it to generate a simple SSB carrier first, this is just a sin and cos of equal amplitude, i.e., two sine waves 90 degrees out of phase. I don't know your IQ signal generator myself, but this could be as easy as plugging the unit directly into your computer's stereo soundcard Line Out. Use either with your own software, or with one of the freeware IQ signal generators.

FWIW Rick Lyons has an excellent book on DSP called Understanding Digital Signal Processing, in my opinion the best on the subject by some way as it avoids pages and pages self-serving academic ego massaging equations (there, got that off my chest). The chapter on quadrature signals is largely reproduced here: http://www.dspguru.com/sites/dspguru/files/QuadSignals.pdf . Be patient with it, and be prepared to read it a few times before it sinks in!

[RedShoeRider]

Wow.... I think I might have to read that more than a few times!

I'm very new to modulation in general; while I understand waves in very general terms, the ideas/physics/mathematics behind modulation are really new to me. I had set out to get a signal generator that as capable of AM/FM internally, but the one I got was just not pass-up-able. Besides, learning something new is never a bad thing, so now I'm going to learn!

Thank you both for  the input!
-Red




 

[Howardlong]

You are lucky that you have something to try it out with, doing that stuff purely as a mental exercise is hard.

If you have an all mode receiver or an SDR, you can hear your generated stuff too. I'm a bit old fashioned: although I have spec ans, I'm just as likely to use a radio to listen for a signal particularly if I know its ball park frequency.

[RedShoeRider]

As a purely mental exercise, I don't know if I'd even try. For the most part, I'm far more into hands-on engineering than just the theory. Though, of course, without the theory, the practical doesn't work. Anyway....

I do have a nearly all-mode receiver in the form of a "police" scanner. Covers just about everything from 25mhz to....well, better than 800. That would be fun to listen in to the signal; thanks for the suggestion!

[pa3bca]

These are absolutely worth watching if you want to learn about (the basics of) IQ signals and IQ modulation:


and

[RedShoeRider]

Wow. Is it bad that the only IQ I was familiar with this morning had to do with my (now lacking) intelligence? 

I think I set everyone a bit off track though, as my original question was worded less-than-well. I should have emphasized that I want to combine the signals externally, so that even though the Anritsu can do all sorts of IQ modulation, I'm just using it to put out a clean, unmodulated, wave for my carrier, and use the Tek to generate the information. My goal is to have a, say, 400mhz carrier with a 500hz tone on it that I could pipe into a handheld jobsite transceiver (a walkie-talkie, if you will) to work on the receiver.
It's easy to find schematics to make a breadboard 100mhz carrier and input a given tone, but I can't find something that would let me use the huge range that the Anritsu can generate as my carrier. 

Thank you all!

[Howardlong]

As a purely mental exercise, I don't know if I'd even try. For the most part, I'm far more into hands-on engineering than just the theory. Though, of course, without the theory, the practical doesn't work. Anyway....

I do have a nearly all-mode receiver in the form of a "police" scanner. Covers just about everything from 25mhz to....well, better than 800. That would be fun to listen in to the signal; thanks for the suggestion!

FWIW, by all mode, I mean something that can resolve SSB. If you can get one where you can switch off the AGC that's even better. Generally police scanners are FM and/or AM only. An SDR with a decent LO would work, be aware that the RTL SDRs are cheap for a reason, their frequency stability is not very stable for detecting narrow band signals, but they do work.

[RedShoeRider]

Ah! Ok, I read it as multifrequency, not multimode.
I don't have one of those, or at least not yet. Thanks for the correction in my thinking.

[RedShoeRider]

Good Evening!

Now that I've had a bit of time to play and learn, I'm even more confused!

I did exactly what you suggested: took the output from the function generator (sine wave, 200mv p-p, 100hz) and put it to the external I/Q input (I used the I), set the sig gen to 100.3mhz at -50db, and plugged it into my little radio's antenna on my desk. Setting the radio's frequency for 100.3, I got a nice, clean, 100hz tone though the radio! Changing the frequency to 200, then 300hz....just like that, it worked.

Neat, I think. So, I took the output and hooked it into the scope. What I saw.....


Here's what I don't get: why on earth did that work on my little FM Radio? That looks like an AM signal to me, not FM. To make sure I wasn't doing something completely stupid, I tried it again using my weather radio up in the 162mhz range....and that worked, too. So it looks like AM, but the FM receivers know what to do with it? By putting the external signal on the I, I should just change the momentary amplitude of the signal, not the frequency. Unless my ears are making stuff up, I hear it clear as a bell in FM.

Thanks for any direction you can give me!
-Red

Given the sig gen has analog I/Q inputs (0->2MHz) it should be pretty easy to fudge AM, just alter the amplitude on either of the channels. FM might be a bit more of a challenge as the function generator only produces a passband FM output, and you need a way of altering the instantaneous phase of the complex sig gen input (maybe use a cheapo DDS with quadrature outputs instead?).

I'm just going by the device datasheets so I could be missing something obvious.

[w2aew]

By just applying an input to I like you've done, you simply created a phase modulated signal. You may want to review my videos (linked earlier) to understand why that is. In this case, it is simply varying between 0 and 180 degrees. As it does, the amplitude cross through zero, which is why you have an amplitude variation as well. And of course, a phase modulated signal can be detected by a frequency discrimator as well.

[RedShoeRider]

Ah, I feel like a heel for not understanding that before I posted my question. After watching your videos a few times and re-reading some of the links posted earlier, I get what I missed that morning. Thanks for your patience with me; I have a LOT to still learn. Fun learning, though. Beats trying to learn French in High School!

Cheers!