[sdiy] FM Transmitter IC

Rob Spencer rob at gmsn.co.uk
Mon Apr 25 20:16:40 CEST 2016


Thanks for the info guys. I’m really short of space which is why I’m trying to keep everything as neat as possible. I need to fit everything into roughly the size of an AA battery, including power. 

It’s for a small wireless controller which needs to send an acoustic signal from a small sensor to a receiver in the same room.

Rob Spencer
gmsn.co.uk
07590 267835

From:  Synth-diy on behalf of Tom Corbitt
Date:  Monday, 25 April 2016 17:22
To:  Neil Johnson
Cc:  synthdiy diy
Subject:  Re: [sdiy] FM Transmitter IC


In terms of old chips, if you can find some from a decent source you can always use the venerable BA1404; it was "the" goto pirate radio chip back in the 90's

Here in the US they were used in the Ramsey FM10 which was the goto armchair anarchist pirate radio station. They later updated to kits with digital tuning like the FM-30 which ditched the 1404 for a PLL based system. 

I highly doubt you'll find any modern device that doesn't require an additional cpu but if you're going to add a digital display and a tuner button circuit  I feel like adding a simple mega or like cpu would make the design of that part of the circuit so much much easier (not to mention you really don't want to roll your own rf modulator anyway) 

Si Labs makes some widely supported chips, the Si471X series, which you could drive with a cheap micro and still have plenty of gpio to mux a 7 segment display and switches. 

Better yet, unlike the old chips you also get RDS and a bunch of power info so you could have your device look at the world and figure out the best place for it to transmit before it started.(and the Si part also has two GPIO lines you can use) 

Great example design with code here:
https://learn.adafruit.com/adafruit-si4713-fm-radio-transmitter-with-rds-rdbs-support?view=all

I'm a huge fan of simple and reusable and doing things the "old" way (I built and ran a pirate radio station using the ba1404 while at college in the early 1990's) but the Tc and part/power variances made for quite a juggling act. If I'd had access to the Si4713 back then I would have used it in a heartbeat...

That would have allowed me to just waste all my time on better portable antenna designs (the other rf black hole of time)

It seems small but even 250uV @ 3 meters was a decent amount of campus coverage from the right height

Tom C


On Mon, Apr 25, 2016 at 10:18 AM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
Apparently leagal as long as the power is very low:

https://en.wikipedia.org/wiki/FM_transmitter_%28personal_device%29

Looks like nanoWatts.  100uV is plenty for local reception -  a
receiver should be happy with a couple of microVolts.

Neil
--
http://www.njohnson.co.uk
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