On Sat, 7 Feb 2015, Ian Jackson wrote:

In message , Brian Reay writes
On 07/02/15 17:20, Jeff wrote:

Also, AM detection is probably more tolerant of mistuning than FM.
There's also the reason why Air Traffic Control use AM and not FM - ie
lack of capture effect.


I suspect it is more historical Ian. Aircraft VHF sets are not VFO
controlled, these days there will be PLL but in the past they were
crystal controlled.


It is not the mis-tuning that is the reason, it is the ability to hear 2
stations when they transmit simultaneously, at least under some
conditions. Also it makes it easier to have a ground station transmit on
the same channel simultaneously from several different locations with
offset frequencies, which would be more difficult with FM.

The 'capture' effect of FM is rather limited with NBFM. While you may not
be able to understand if two transmissions are present (just as you may not
on AM) you can often tell if there are.

To really gain (or perhaps not in this application) from the capture
effect, you don't really what NBFM.

The capture effect was mentioned as one of the reasons for UK CB being FM
but it was rather a dubious one, certainly a 'make weight' in the RA's
argument.

I thought that the RAs insistence on FM was that the constant signal envelope
level was less likely to interfere with 'things' (apart from a click at start
and end of a transmission).

As I mentioned earlier, that was certainly one reason narrow band FM was
suggested for the HF bands decades ago. AM would get rectified by first
stages in audio ampliers, and the neighbors would be able to identify the
voice. None of that with FM.

But I remember tuning CB here in Canada in the early seventies, nad much
of the time, at least in the summer, it was a mass of heterodynes. Come
to think of it, since that was with a shortwave receiver, I wonder what it
was like on a channelized CB receiver?

The capture effect has always been attributed to FM, but in reality, it's
the limiters that bring on the capture effect. You can't have limiters
with AM, since that would wipe out the modulation. But if an FM receiver
had no limiters, where does the capture effect come from? The limiter
makes sure that a relatively modest difference between signal levels means
one will be on top. That said, I can remember instances of hearing two FM
signals at the same time, presumably they were pretty much identical
signal strength at the receiver.

On the other hand, maybe CB sets where FM is used don't have good
limiters. I finally found an SSB CB set a year or two ago, and once I
found information about it, discovered that the IF filter is relatively
wide. I was expecting a nice narrow SSB filter (which is why I'd hoped
for years go fined one), but instead it was sort of mediocre bandwidth,
wide enough for AM, and "narrow enough" for SSB. So they saved on the
flter. The odd part is, a good audio filter will make sure the
transmitted signal is narrow (if the actual bandwidth of a voice isn't
good enough), the IF filter only needs to knock off the unwanted sideband.
And I suppose on receiver, the channelized nature of CB means a wider
filter doesn't matter, the next channel up is far enough away so a wider
filter won't let in interference.

Michael