FM (Broadcast Band) Transmitter

[GeekGirl]

Hi all,

I am having trouble finding a decent FM Tx design (I used to have a great one in kit form from Altronics, but they have stopped doing it). I need something that is very stable (going to be used in a car) that I will then feed into a small power amp. I would prefer Digital tuning, not worried about having to use a micro and lcd / keypad.

Why do I need to do this ? My Partner has a Peugeot 308 2006 model, this does not have a line in or MP3 cd support, so we use an Ipod. Due to some reason this stereo does not like low powered Tx's, everyone I have tried has lots of static coming through (seems when the AGC goes to max it is very noisy).

The Deck can not be swapped as it also controls the heads up display (all the trip functions (eg fuel consumption, fuel remaining rtc))

I am happy to buy a kit. I have tried a Jaycar Kit based on the Rohm BA1404 Tx chip, but the tuning is not that good (1.5turns on a crap former and crap slug, making it very sensitive to movement.)

So I am looking for something more stable and able to maybe put a bit more power out or I will feed it into a small PA.

[GeekGirl]

I used to own about 20 of the Altronics kit, but during a clean up they were thrown out (I was not doing the work they were bought for (rally Australia) and it had moved from Perth.... DAMN MURPHY

[AnotherAusEE]

SiLabs make some digitally tunable (via SPI or I2C) single chip FM transmitter and receiver devices that require very few external components.

The receivers are great - never tried the transmitters.

The relevant part numbers are Si4710 and Si4711 (RDS version).

They only come in a 3 x 3mm QFN, though. Not sure if that would be a problem for your application.

https://www.silabs.com/products/audiovideo/fmtransmitters/Pages/Si471011.aspx

[joelby]

There's a great collection of practical RF design information available at http://hem.passagen.se/communication/meny.html . I don't think there's a stereo FM transmitter circuit, but there IS a 1W power amp kit for a commercial iPod transmitter! Also, plenty of other interesting things for people like me who are muddling their way through RF design after doing (most of) an overly theoretical engineering degree.

[Simon]

in the UK and I'm sure worldwide you can buy small transmitters meant for this, they take 2 AAA batteries or can be powered by USB

[GeekGirl]

in the UK and I'm sure worldwide you can buy small transmitters meant for this, they take 2 AAA batteries or can be powered by USB

Hi Simon,

As I said the commercial ones (Belkin etc) are not powerful enough, using the low output power, the S/N is so bad that singers develop lisps and the static is not good. Commercial radio stations are fine (even community radio stations are fine (some of the local community stations only use 30W Tx))

So I am looking for a chip that is a bit more stable (might use a commercial one after the post by joelby) then add a small PA to give me the needed signal (and lower the noise floor)

[Simon]

hm the one I bought was quite good never had these problems. but then its one of those things that has sunddenly become popular with many different manufacturers making units that have the same case.

[Murphy]

Why not use one of the modules that connect directly into the antenna wiring? Seems that would be a better option than having a big amp kicking around in the car.

[Brett]

Why not use one of the modules that connect directly into the antenna wiring? Seems that would be a better option than having a big amp kicking around in the car.

This is definitely the way to go... I was reading all of the replies to see if it was mentioned... all I can say now is "ditto".

With this you don't need digital tuning... just low to no drift and clean modulation.

[rossmoffett]

I cut a piece of wire to a quarter-wavelength of the frequency I use and soldered it to the cheesy PCB antenna on my FM transmitter.  My broadcast range more than quadrupled.. unless you don't want the wire hanging free.  In that case, as mentioned above, consider other antenna options.  Anything is better than those PCB loops or wavy lines.

Some models have a pot for gain control on the PCB, certainly not as much fun as a kit though.