LF: Re: Dynamic range for FM link (was: Carriers)

To:	[email protected]
Subject:	LF: Re: Dynamic range for FM link (was: Carriers)
From:	[email protected]
Date:	Tue, 28 Aug 2001 13:32:53 EDT
Reply-to:	[email protected]
Sender:	<[email protected]>

Hi Larry, Rik, and the group,

As Rik's wrote, the dynamic range of some cheap FM transmitters is possibly
just in the range of 40dB or so. But IMO, a homemade "simple but true FM"
transmitter could do better.
The designers of these transmitters certainly do everything they can to keep
the costs down (good engineering work :-) . But the dynamic range can vary
greatly between different rigs, depending on the principle how and at what
frequency the FM is generated, and how much attention is paid to the AF path
before the signal is applied to the varicap (or whatever). There is a lot of
noisy but cheap AF amplifiers and preamps out there...

Using a simple "straight" design where the FM'ed oscillator is multiplied to
the final frequency should do better, at least for the distortions of the
modulation. For example, ( 27.11MHz+- 300 Hz) * 2 * 2 * 2 * 2 = 433.76 MHz
+- 5kHz.

The question is, how many dB's are required (see Larry's mail), and how much
effort must be put into the FM link vs the LF frontend...

Thanks Rik also for the idea of using a digital link instead of an analog one
(by direct mail). It doesn't necessarily have to be a high-speed packed link,
a simple serial 16-bit frame plus a startbit from a serial ADC (possibly the
LTC129x ) could possibly do the job, even without a microcontroller at the
remote RX site.

Now I'm off to think about all this for a little longer. Thanks to everyone
for their suggestions, and apologies to others for drifting into the HI-tech
corner again..

73 Wolf, DL4YHF.

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