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FM Transmitters

This is a discussion of today's available transmitters and receivers. It is moved here because the email ping-pong was getting long. Below are some exerpts with very good technical information from Ichabod.

I have a couple of devices operating at 1.2GHz. Essentially taking banana jack inputs (L+R+video) at baseband (less than 80MHz) modulate
it up to RF. A corresponding receiver reverses the process for banana jack outputs. They operate off of 12Vdc at maximum 200mA (RX) and 400mA (TX). Capacity of each AA is about 1800mA*hr. AAA would be 1200mA I would estimate. Proportional to volume.

So besides a cordless telephone and a microwave operating, are there any other household devices that you can think of or have seen that interferes? They use the FM transmitter devices at the workout places. They set a TV channel to transmit to a specific FM channel and there is a receiver at every exercise machine. Pretty good use of the technology, I thought. I wonder if we can rig up the same setup at home (cheaply) for watching TV.

Everything interferes. FCC opened up this band for consumers so every
manufacturer of widgets climbed on the bandwagon. Power levels by
regulation have to be low to minimize screwing thy neighbor, but says
nothing about screwing thyself. I know my cordless gets staticky when
the u-wave oven kicks on.

Parts at 1.2GHz RF aren't readily available at RadioShack. Besides, the system is heterodyne FM so filtering, noise rejection, power supply/conditioning, etc make it a fairly complex endeavor. The old days of making a 2-transistor AM-receiver from a Heathkit have long passed. Integration is the key. For that, you need a fab, CAD tools, analog experience, oscillscope, spectrum analyzers, ... Try to design ANYthing in your garage using discreets nowadays and you quickly hit a brick wall with component availability.


February 17, 2004 (10:45)

You are quite right on the thought of INTEGRATION. Building from scratch is no longer a feasable or desirable approach. In this case, it is good because ready built components are available. The components aren't as in abundance as part of a PC are, but they are more so now than say 20 years ago. I think the hard part of the task is in finding the components that we need.

Take our concrete example of the wireless use at 24 hour Nautilus. Each TV is tuned to a TV channel and outputs to a wireless transmitter. An FM capable headset can be bought (probably pretty cheaply) to act as a receiver. Those are pretty common and are marketted as "jogger's headphones". The question now is: do we know of commonly sold transmitter component?

February 17, 2004 (11:14)

Radio Shack stopped printing catalogs after 1999. Now it's

The best bet at home made TX/RX is to take (borrow/steal) an existing working device and figure where/how to tap into the audio signal path ... long before it makes it up to IF 10.7MHz (455kHz for obsolete AM). Beyond IF don't even bother, the filtering/tuning requirements will make it virtually impossible to change what's there. Can't even measure it w/out a bank of $20,000 equipment.

Regarding previous post, errata: the transmitter/receiver I mentioned operate on 900MHz and 2.4GHz (I have two different sets). This makes more sense since it is the same spectrum cordless phones used.

February 17, 2004 (12:55)

We have mentioned that it's not reasonable to create your own TX/RX device. I brought up a possible source for RX devices in the previous message. For TX, Fry's has a "Beam It" for $30 that runs on AA or car cigarette adapter. The input is a microphone/headphone jack.

So do you think this combination would work?

PS: You should try out the notify email feature of the discussion board. If you type in the person's email, it tells them that they have something waiting here.

Hoang Do
February 18, 2004 (10:28)

For a little while there (over six years ago now) I had one of those cheesy Fry's transmitters which kicked out signal on an FM frequency (around 90MHz I think) selected via a little analog dial. It was so distorted that music sounded better through some of the crappiest MP3 data rates. If I remember correctly the transmission was very directional and actually affected the sound quality (some frequencies are "directed" more than others by a particular antenna?) and I always heard a hiss similar to AM.

I had intended on providing a laptop to car stereo transmission mechanism similar to the old car-trunk CD changers (not normal back then to have aux inputs on car stereos), but the sound turned out of such lousy quality I lost interest then lost track of the transmitter. I'd suggest the CD changer transmitter route instead because of superior sound quality, though I'm pretty sure the signal gets onto the stereo's antenna jack through a series connection with the original antenna plug -- not wireless. You'd still have to find an FM signal amplifier and a useful omnidirectional antenna, and I haven't ever tried finding either of these.

February 18, 2004 (11:23)

Not sure what you're trying to pull off. FM (88~108MHz) should be more within reach since there are tons of equipment around.

If all you want is to intercept TV audio from the 24 hr Nautilus, why not just grab an FM stereo headphone off the shelf ... like all the other Spandex clad hausfraus?

If you want to beam audio from point A to point B, then the $30 Fry's equipment and a standard FM receiver or above mentioned headphone should do. Don't know what audio bandwidth Fry's stuff puts out. I'd be surprised if it were the full 20~20kHz audio spectrum. Most featherweight headphones have little response below 1 kHz.

February 18, 2004 (18:04)

FM radios have poor reception when placed near computer equipment. The switching power supplies in these equipment run at 50~300kHz. These are nasty looking square waves (not sinusoids) with harsh L*di/dt spikes on leading edges. They corrupt the spectrum for half a dozen octaves with their harmonics. Shielding this stuff in a metal cage (PC case) helps, but realize that competing radio signals floating through air are measured in microvolts.

... which is why the 900MHz and even more so the 2.4GHz stuff can coexist in a modern home/office. FM doesn't stand a chance.

February 18, 2004 (18:13)

Here is Doc Searles talking about the same topic:

July 02, 2005 (09:04)

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