FM Deviation Meter Project?

[Chris55000]

(Mods, please move if needed!)

Can anyone point me in the direction of a reasonably easy to build F.M. deviation meter project that can be built without needing to wind special inductors or obtain awkward–frequency crystals please?

I am trying to put together a good FM Stereo Alignment Generator that can be built out of published projects or ready–built modules, and I've got most things covered except the deviation meter!

I'm looking to build something with 440 Hz left and 440 Hz right internal or external modulation, that can provide "L", "R", "L + R" , "L – R", "L & R" plus Line, and a means of measuring the FM deviation and displaying it on a moving coil meter!

I've got the Audio Oscillators, F.M. Stereo Encoder, F.M. Transmitter all sorted out, I just need a Deviation Meter block now!

Chris Williams

[G6Tanuki]

Do you actually need a deviation meter??

There is a well established bit of maths which shows that at certain predictable and reliable deviations the carrier nulls to zero. And so these nulls and multiples can easily be identified with a relatively normal receiver.

That was how we adjusted deviation on our transmitters back in the 80s. A bit of A-level maths saved the cost of special test gear.

[Cruisin Marine]

You can get Racal, Dymar, Marconi, Radiometer etc. Dev. meters cheaper than you could build one for.
The Racal one's are even auto tune.

[Chris55000]

Not at the moment you can't, except by paying absolutely silly prices for things in America, not to mention the postage and import charges!

Chris Williams

[jimmc101]

How about using a cheap SDR?
e.g.https://www.qsl.net/kp4md/freqdev.htm

Jim

[Sinewave]

Spectrum analyser and a directional coupler?

[Cruisin Marine]

Originally Posted by Chris55000

Not at the moment you can't, except by paying absolutely silly prices for things in America, not to mention the postage and import charges!

Chris Williams

Yes Chris, you are quite right! How prices have inflated- you could get a Racal 9008 or similar for about £60 not long ago.
I guess your cheapest/ easiest bet is an FM tuner where you can add a dev meter on board.

[Radio Wrangler]

Read HP journal article for the HP 8901A modulation meter. It's a bit of a beast, but the article shows neat IF demods for both AM and FM.

David

[orbanp1]

Originally Posted by Radio Wrangler

Read HP journal article for the HP 8901A modulation meter. It's a bit of a beast, but the article shows neat IF demods for both AM and FM.

David

Hi David,

Which HPJ is that in?
I only found HPJ index up to '77. Are there indexes for the later HPJ issues anywhere on the net?

Thanks, Peter

[Terry_VK5TM]

Post #4 here https://www.vintage-radio.net/forum/....php?p=1155658

[Oldmadham]

The few times I needed a dev Mon privately, I had the luck to be able to use a Dev Mon at work--most of the time, a massive panel mount Marconi device.

I can only remember one occasion where we had to use the Bessel zero method, & that was when checking the calibration of just such an instrument.

We were given a Drake R7 to look at the first IF in the Dev Mon, but came to grief on that, because the Drake didn't cover that high.
We thought we were stumped, but found that the R7 did cover the 100kHz second IF, & the method worked well.

Unfortunately, over many hamfests, I have only seen one Marconi Dev Mon.

A few of the package type RF Comms test sets have popped up, but they are usually expensive.

[Radio Wrangler]

Typing H P Journal 8901A into google got it first hit.

https://vtda.org/pubs/HP_Journal/HP_Journal_1979-11.pdf

The 8903 audio analyser (Low distortion osc, distortion meter SINAD etc) is the companion box to the 8901. 8901B is a big improvement on 8901A for power measurement. 8903B is a slight improvement on 8903A


David

[Radio Wrangler]

Another source of deviation meters is in one-box-testers for mobile radios.

I picked up a Schlumberger automatic one for£100 3 years ago, intending to give it to the local amateur radio club. Turned out they weren't much interested, so it got given to the Museum of Communications where it seems to have been interpreted as a donation to the collection rather than equipment for the workshop. These things can be bargains giving an RF power meter, sig gen, audio gen, distortion/SINAD meter and sometimes a spectrum analyser. These things evolved into cellphone testers and got rather specialised, but the HP models kept their analogue AM/FM measurements through many generations. All the cellphone stuff is centre stage and it's easy to not even notice it can still do AM/FM stuff.

David

[Cruisin Marine]

You could build an FM Stereo RX fairly easily and cheaply. All you would need is a CA3089/3189 and an MC1310, with a Dual gate Mosfet Front end and a LO fed in to g2.

[Radio Wrangler]

As a modulation meter, you'd want to design the discriminator to be somewhat wider than you'd use for radio reception, otherwise it will distort the calibration of your sig gens at the deviations you'd be using to test receivers.

The bandwidth of IF filters will be another limitation.

David

[Cruisin Marine]

Originally Posted by Radio Wrangler

As a modulation meter, you'd want to design the discriminator to be somewhat wider than you'd use for radio reception, otherwise it will distort the calibration of your sig gens at the deviations you'd be using to test receivers.

The bandwidth of IF filters will be another limitation.

David

Yes of course, another important factor is to use a peak reading meter circuit for deviation levels especially on dynamic deviation levels.
I would hope a CA3089 with a normal coil and no IF filters would suffice as it can pass pull FM stereo bandwidth/multiplex signal required normally. even with IF filters in place, If it can't, it can be modified.

[orbanp1]

Thanks for Terry and David for giving a pointer to the relevant HP-Journal!

Regards, Peter