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Author Topic: FM mode on traceiver will demodulate AM signals  (Read 1813 times)
W8MLS
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Posts: 11




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« on: August 02, 2013, 05:24:08 PM »

Hi,

Maybe a dumb question,  but I have noticed that I can listen to AM signals in FM mode.  Is this due to the capture effect?

73

Mick W8MLS
« Last Edit: August 02, 2013, 05:46:33 PM by KD8SJI » Logged
K7KBN
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Posts: 2754




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« Reply #1 on: August 02, 2013, 11:19:03 PM »

Slope detection.  http://www.radio-electronics.com/info/rf-technology-design/fm-reception/fm-slope-detector-discriminator.php
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73
Pat K7KBN
CWO4 USNR Ret.
AF6WL
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« Reply #2 on: August 03, 2013, 12:00:46 AM »


Slope detection allows demodulation of FM using filter slope and a AM demodulator.

AM demodulation in FM mode could be down to  AM-PM conversion as the limiting amplifiers that preceded the discriminator go in and out of compression.
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G3RZP
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« Reply #3 on: August 03, 2013, 12:49:31 AM »

If the limiting strip isn't hard limiting very well, that as well as AM-PM conversion, could explain it.
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TANAKASAN
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« Reply #4 on: August 03, 2013, 01:10:17 AM »

Way back before FM ICs became common there were two or three methods of detecting FM using a transformer and a pair of diodes. Here's a good example:

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/fmdet.gif

Now, given that you have a diode followed by a filter to eliminate the RF it's not surprising that some AM would leak through using this arrangement, it's just a crystal set with some of the components rearranged. Modern FM receivers detect FM using either a phase locked loop or a quadrature detector but mathematically they are doing exactly the same as the circuit above which is how you are able to listen to FM signals.

The capture effect is a totally different beast. If you have two FM signals on the same frequency then your FM receiver will almost always lock on to or 'capture' the strongest of the two signals. This can be a good thing if you're trying to receive a broadcast station over, for example, a local pirate radio. It's also why aircraft use AM because AM is NOT subject to the capture effect and if two aircraft talk at the same time the control tower will hear both signals.

Tanakasan
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WB6DGN
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« Reply #5 on: August 03, 2013, 01:52:57 AM »

Quote
It's also why aircraft use AM because AM is NOT subject to the capture effect and if two aircraft talk at the same time the control tower will hear both signals.

Actually, they may not be able to understand either signal but, at least, they'll know that there is more than one aircraft in their control zone and look for all of the traffic.  With FM, they may assume that the station they heard is the only aircraft present with disastrous results.
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W8MLS
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« Reply #6 on: August 03, 2013, 04:31:55 AM »

Thanks for the info guys.   When I read up on the capture effect it did not sound right but I did not find  anything else on my google search.   I appreciate the quick reply's.

73

Mick W8MLS
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AC5UP
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« Reply #7 on: August 03, 2013, 11:24:53 AM »

It's also why aircraft use AM because AM is NOT subject to the capture effect and if two aircraft talk at the same time the control tower will hear both signals.

Another reason is that FM has a limiting curve (quieting) at weak signal levels that won't 'hear' down to the noise floor like AM will. Disconnect the antenna from an AM receiver and you have quiet, do the same with an unsquelched FM receiver and you have a pantload of noise. When you're trying to locate the transponder on a downed aircraft AM will give you a signal further out and save time......
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KE3WD
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« Reply #8 on: August 03, 2013, 12:12:45 PM »

...It's also why aircraft use AM because AM is NOT subject to the capture effect and if two aircraft talk at the same time the control tower will hear both signals.

Tanakasan

In theory. 

However, in the realworld, while it may be possible to copy two simultaneous signals when planes are in the air and pretty well separated, on the ground and in close proximity at congested airports, the examples of these AM signals "stepping on" each other and yielding an unintelligible "intermod" sounding situation much as can happen in FM mode.  I kid you not, here's a youtube example where you can hear various planes steping on each other during a busy day at JFK: 

https://www.youtube.com/watch?v=eyO-bWGxWBU

The problem is very likely due to each transmitter being centered at a slightly different center frequency and when two transmit simultaneously, we hear the difference freq, which is always an audio frequency.  And it obliterates any intelligence on *either* signal. 

Best laid plans of mice and men...


73
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AC5UP
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« Reply #9 on: August 03, 2013, 12:24:13 PM »

One exception to this might be WWV and WWVH.

On a good day I can hear both on the 10 and 15 MHz frequencies and they play well together. The clock ticks line up and I can hear the time announcement twice with no hint of heterodyning. AM BCB stations will do the same and it's rare to hear anyone more than a few Hz off their allocation. But, the typical aircraft transceiver isn't WWV......... Not even close.
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WB6DGN
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« Reply #10 on: August 03, 2013, 09:07:39 PM »

Quote
The problem is very likely due to each transmitter being centered at a slightly different center frequency and when two transmit simultaneously, we hear the difference freq, which is always an audio frequency.  And it obliterates any intelligence on *either* signal.
Best laid plans of mice and men...

BUT, the most important part still works.  The controller (or ATC) still knows that there is more than one aircraft in the "zone" and watches for other evidence.  At that moment it is more important to know that than to know what was said.  Conflict resolution first!
Tom
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G3RZP
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« Reply #11 on: August 04, 2013, 12:57:54 AM »

If the FM receiver doesn't limit on internal or external noise with no signal, you can get a 'threshold extension' effect. This was used commercially on some non-GSO satellite  ground station receivers  to gain a few extra dB of S/N.
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