FRG7 Mod

[Wendymott]

Hi Peeps. While making the FM Modulator for my 2M multimode transceiver, I realised I couldnt test the quality of the modulation with what I have here. I have the FRG7, but in its original form has no FM facility. My modulator is at 10.81 Mhz. So I decided to dust off a TBA120U and try that.
The greatest difficulty was getting the tank coil to the correct frequency, but to cut a long story short "mercifully" the eventual value was 150 uH .
There is plenty of room in the FRG7, so it was just a case of hooking it up. I fitted a AF select relay onboard. Connecting the filter to the last I.F secondary before the detector device, slightly reduces sensitivity, however in my case its no real hardship.
Eventually I may need a FM deviation meter, but until then... "ittl do"

[G6Tanuki]

If you're setting the deviation of a FM transmitter, you don't need anything more than a simple SSB/CW receiver and a bit of O-level maths [do they still teach Bessel in GCSE? I fear not].

Given a good 1KHz-and-a-bit signal-source [there's plenty of downloadable freeware software-packages for Sindoes/Mac/Linux computers and apps for your Android/Apple phone] you feed the signal into the input of the transceiver then wind the deviation up from zero while listening to the output on your SSB/CW radio and watching the nulls.


https://www.qsl.net/g3oou/fmdeviation.html

[Terry_VK5TM]

Had a look at that link and am now completely confused (then again, maths anything over addition and subtraction was always a mystery to me ).

[Cruisin Marine]

Originally Posted by G6Tanuki

If you're setting the deviation of a FM transmitter, you don't need anything more than a simple SSB/CW receiver and a bit of O-level maths [do they still teach Bessel in GCSE? I fear not].

Given a good 1KHz-and-a-bit signal-source [there's plenty of downloadable freeware software-packages for Sindoes/Mac/Linux computers and apps for your Android/Apple phone] you feed the signal into the input of the transceiver then wind the deviation up from zero while listening to the output on your SSB/CW radio and watching the nulls.


https://www.qsl.net/g3oou/fmdeviation.html

The effect of the Bessel function and the null seen on a Speccy analyser are one of those magical things to see in the real world. Adjustments are extremely fine though.
I think the rough procedure is mentioned in maybe older ARRL manuals

[Techman]

Another way to do it on these old receivers without messing with the front panel is to use the 'AM' switch position. The receiver will mainly be used for HF SSB listening anyway, with any AM only listening being acceptable in the AM/ANL position, so the straight AM position can be rewired for the FM conversion.

[Radio Wrangler]

Originally Posted by Terry_VK5TM

Had a look at that link and am now completely confused (then again, maths anything over addition and subtraction was always a mystery to me ).

Ah, now this is neat. Maths it may be but it's so neat you have to admire it.

The process is referred to as "Bessel Nulls"

If I take a carrier and FM it, I will usually get a large number of sidebands, all at the modulation frequency spacing around the carrier. It's a lot more complicated than AM. You would need a narrow receiver to tune to individual sidebands in order to resolve them and see them as individuals... or a spectrum analyser with narrow enough filters to resolve them.



Now, if I vary the deviation setting on my signal generator, the sidebands all stay where they are, frequency-wise, but they change in amplitude, but not all together. Some go up, some go down, some pass through nulls... hell! even the carrier itself nulls at some points.

THe nice bit is that the ratios of deviation to modulating frequencies where these nulls happen can be calculated. So, as long as your modulating frequency is accurate, there are certain deviation values where you can be pretty certain of the deviation value.

Sneaky, huh?


To understand what's going on, you need to know about the phasor diagram representation of modulation. A carrier can be represented by an arrow spinning around its blunt end. Spinning at the carrier frequency. The height of its pointy end represents the instantaneous voltage, and by spinning it traces out a sinewave in time, at the carrier frequency.

To amplitude modulate it, we need to be able to change the arrow lenght, without mucking about with its direction, that has to keep spinning at the sme old rate.

Someone did the maths for one sinusoid multiplied by another one and came out with an equation which ALSO fitted three sinusoids. One at the carrier frequency, one at the carrier plus the modulation frequencies, and one at carrier minus the modulation frequencies.

Look at the signal on a scope and you see AM. Look with a spectrum analyser, and you see the carrier with the sidebands. Both views are truthful and just alternative viewpoints of equal validity.

Now, for FM, we want to rotate the arrow so it swings a bit ahead o itself to a bit behind. The amplitude/length must not change while we do this or we'd get AM as well.

The difficulty is that the pointy end of our arrow mus swing precisely along a circular arc.

What we did with AM shows that signal components are additive and while they can do straight line things, curves are a whole different world. You can't make a perfect curve, but you can approximate to one. So you have one pair of sidebands, not in phase like the AM ones, but 90 degrees shifted. So they add wag, not length... well they do affect the length because an arc isn't straight. So we add a pair of AM sidebands at twice Fmodto try to cancel most of the unwanted AM. But this affects the wag, so we add another pair of wagging-phased sidebands at three times Fmod, and that needs amplitude correction, and that needs wag correction and so on.

We wind up with a series of sidebands making a sort of iterative approximation tending to the right place on the arc. If we allowed infinite number of sidebands, and infinite bandwidth to fit them in, it would be perfect.

All mathematical, all seeming abstract, but it touches on the real world of soldering irons in a couple of ways:

1) If you can't give an FM signal infinite bandwidth, you have to live with some distortion.

2) That bit about sidebands being at 90 degrees to what you'd expect for AM means that if you mix (multiply) an FM signal with itself shifted through 90 degrees, you get a nice FM demodulator. It's called the quadrature detector and features in a zillion FM receivers near you.

The addition of a lot of (usually) progressively smaller vectors to make a staircase approximating to a circular arc can be done with a mathematical series, the Bessel function. Ns the bessel functions describe the amoplitudes of our different sidebands. Bessel functions pass through zero several times, and reverse phase as they do so. The zero crossings are the 'nulls' and are superb ways to calibrate signal generators and deviation meters. All yoiu need is an accurate audio generator and something to let you see the individual sidebands and the carrier.


It's a whole world you wouldn't suspect existed, but you can do useful tricks there.

Squaring the circle? or circling the square?

David

[Wendymott]

Bloomin heck guys..... I didnt realise I would need a PHD to read the replies .. I just put it out there as a mod for the Frog7. But thanks anyways. If i get good reports... alls well.. if not... tweek it.. .

[orbanp1]

Hi Wendy,

Nice work!

Here is a good article about using the TBA120 ICs in NBFM applications:
https://www.g3ynh.info/circuits/TBA120.html

Interesting is your choice of L-C values for the phase-shift network.
According to calculations those values resonate at about 600kHz.
For that inductance to resonate at 455kHz you would need a cap about 815pF.
Is there so much parasitic, internal capacitance there?

Regards, Peter

[Wendymott]

Hi Peter. I originally used a Toko 421 coil that had been used for a 455Khz I.f No additional cap. It refused point blank to operate, after lots of winds and rewinds, I used some fixed RF chokes... finishing up at 150uH... and a 470pf.. Incidentally I had removed the input filter and fed the chip from my Anritsu generator. So thats where it finished up....I had the TBA120U in stock, plus the MC3357, which is a dual conversion IC. But I decided the MC3357 was overkill, I already use the MC in my new 2 M multimode radio project. OK about the article..... I already have that and I based my version on that data sheet, but thanks

[Cruisin Marine]

Wendy, watch this extremely simple way to estimate the deviation level - it is so simple
https://youtu.be/Nw3_w1HrlEs

[Wendymott]

Thanks Cruisin.... Saved it for later