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Old 27th Jul 2022, 8:08 pm   #159
Radio Wrangler
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Location: Fife, Scotland, UK.
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Default Re: 6-gang FM stereo tuner heads

Close, but not there yet. The two signals which are intermodulating don't have to be close to the wanted signal

Let's say I have two strong signals on 95 and 100MHz. Their 3rd order products will be at 90 and 105MHz. If I'm trying to receive a third signal, on 90MHz, the intermod product can be smack on the centre of the channel I'm trying to receive.

In this case with the intermodulation-causing pair are 5MHz and 10MHz off of our tuned frequency. So RF selectivity (if we have some, not all radio receivers do) will help by attenuating the causes. However earlier stages in the RF get them stronger than the mixer does. Also the varactors in the first RF tuned section gets the full thing.

Now let's try something closer... if the problem signals are 90.0 and 90.2MHz representing typical channel frequencies, and we are listening to 90.4MHx, then the big signals are close enough to the tuned channel that there won't be much help from RF selectivity. The RF amp and the mixer get hit hard. One intermod product will again lie exactly on our wanted channel. It is the proximity of the interference causing signals that is of interest and relates to benefits from RF selectivity. The intermod product doesn't have to be spaced from our wanted channel, it can hit it dead centre.

For each channel I could tune to and for each value of spacing of intermod causers, there will be two pairs of causers which will hit the channel. The RF selectivity can't do the channel filtering job, so a few channels on either side of the wanted channel will be the worst for intermod performance, only when you get further out does RF selectivity start helping.

You can't handle or test all possibilities, so you have to sample several cases to indicate that the structure covers the possibilities.

This is a big problem for aviation radios. The ext band up from Band II FM broacast is where instrument landing system localisers live. If you ever fly anywhere, your life depends on this in bad visibility or for autolanding (Cat 3A and Cat 4) So there are pairs of FM broadcast channels which will plant an intermod on each localiser frequency. The aviation ILS receivers have to be remarkably well protected from intermodulating broadcast channels. Filters can't do it all. They help, but there has to be remarkable toughness in the receiver front-end design. I've spent the past several years on this! FM receivers are much easier in comparison, lives don't depend on them.

For many years there was a gentlemen's agreement not to populate the FM band above 100MHz to give a safety margin and give some space for filtering in aviation radios to start getting some attenuation in.

So now you know why older FM radios usually didn't tune above 100MHz.

But governments discovered that MegaHertz could be transformed into MegaBucks, and the part above 100MHz got sold off to commercial broadcasters. Plane owners were forced to scrap their radios and buy ones hardened against the problem.

Spectrum management agencies look at the localiser frequencies assigned to each aerodrome, and look at frequencies assigned to broadcasters in that area to try to minimise the risk. Minimise isn't quite the same as eliminate.

David
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