Re: How much power can I receive from a FM station
- From: Jeff Liebermann <jeffl@xxxxxxxxxx>
- Date: Sat, 10 Nov 2007 23:53:45 -0800
Eeyore <rabbitsfriendsandrelations@xxxxxxxxxxx> hath wroth:
Allen wrote:
I am designing a FM radio receiver. Can anyone tell me show much power
can I receive from a FM station that is in a acceptable range? I need
that value to decide the gain of all the stages. Thanks.
You need *variable gain* to cope with the wide range of signals you're likely to
receive. The gain required for any given station is determined by the AGC
(automatic gain control) circuitry.
Wrong. FM broadcast receivers do not have AGC circuitry (unless
you're trying to keep the group delay through the receiver constant).
FM receivers run with the front end gains at roughly maximum gain,
followed by IF stages running as limiters. There is no amplitude
component to the signal so there is no need to keep the signal in the
linear part of some mythical AM detector.
It seems your question should be "what's a typical minimum signal that's worth
trying to receive" and it's normally given in microvolts not watts.
FM receivers can be designed that use very little power. They can
drive something like a piezo earphone, which also is very low power.
You don't need watts.
If this is for ordinary commercial FM 88-108 MHz, I'd look at some typical
receiver specs to get a feel for it.
Blundering through various car stereo specs, I see 10-13dBf for
"usable sensitivity". 14-18dBf for 50dB quieting. 1dBf is about
0.274uV into 75 ohms. 15dBf would seem to be a good minimum operating
point, which is about 8.7uV.
I don't see what the receiver sensitivity has to do with this. The
problem is to deliver enough power to run the radio. It would be a
fair assumption that the signal level necessary to supply this power,
is far higher than the basic receiver sensitivity.
Graham
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Jeff Liebermann jeffl@xxxxxxxxxx
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Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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