Designing AM/FM receivers

[kybert]

Hi,

Im about to start designing a rough and dirty AM or FM reciever.

My Specs are as follows:

1. Must run @ 3V with v-low current requirment

2. Must be very small outline on pcb ( < 10mm sq.)

3. Will be used to receive digital data (on / off) @ <1K bits/sec (can be slower if req'd)

4. Output will be interfaced to a 3v micro-controller (PIC).

5. The transmitter can be either 418Mhz / 433Mhz.

6. A minimum range of 20m (internal) is required.




Depending upon the best Rx solution, the Tx will be choosen to fit later, the Transmitter will be an off-the-shelf solution to enable 'licence exempt' status.



Can somone point me in the right direction here, i have no idea where to start.

I've looked into Rx theory but everything appears to be geared to the commercial radio bands (88-108).

Should i use FM or AM ? I know FM has a better noise rejection, but will there be that much noise in the system?

<confused>

 

[stevez]

I'd be inclined to review the work of others to get a sense of the complexity and component cost of various approaches. In posting to this forum it does seem that you agree with this approach. I've seen, but never used, the 433 mHz modules - transmitter or reciever. It would seem that this could greatly simplified the task of building a data relay station.

Unless you are working with the modules described you may find that working at UHF is rather demanding in terms of skills - design and construction. Even at FM broadcast frequencies circuits can be very sensitive to all sort of things. If you've not done it - start with something simple at lower frequencies/low power to get a sense of what I mean.

You will find a lot of general RF transmit and receive information in amateur radio publications and literature. ARRL, American Radio Relay League or RSGB, Radio Society of Great Britain are two places to look. Those groups, among others, use all modes of communication.

Good luck.

 

[kybert]

I shall be using a Tx module for the project, but the Rx modules are all too large and complex for what im looking for.

I need somthing stupid-simple, i was looking at diode dector circuits for AM. Any thoughts?

 

[k7elp60]

In general the range of a transmitter/receiving system depends upon three things. 1. The output power of the transmitter. 2. The sensitivity of the receiver. 3. Minus the path losses.

For the frequencies you are thinking of using the transmitters are unlicensed in the U.S. as result the power output is limited to a certain amount.

To use a diode detector without some frequency selective circuits could be cause for all kinds of interference to the specific application. Even a simple tuned circuit with RF amplification and a diode dectector can improve the range of the receiver and prevent a lot of intereference to the operating frequency.

 

[kybert]

Hi,

I fully intend to add a small tuned RF amp to the front end of the detector, i just dont know how yet!

 

[Nigel Goodwin]

Hi,

I fully intend to add a small tuned RF amp to the front end of the detector, i just dont know how yet!

At the risk of being rather cruel, if you have to ask you're not likely to be capable of constructing a UHF radio system.

At UHF construction is EXTREMELY critical, coils are just short straight pieces of wire - no turns required. As you can imagine, this makes it very tricky!.

The commercial licence free receivers are already very small, probably as small as they can make them - using all surface mount components on a ceramic thick film substrate. It seems unlikely you could make anything smaller using home construction techniques.

The commercial modules get their small size by using fairly crude techniques, generally they are TRF sets rather than superhets.

 

[kybert]

I really dont like RF, just want to get this done quickly and move on to the rest of the hardware... Then the software.

I have been looking at some other options:

Data Sheets:
MAX1470 **broken link removed**
MAX1471 **broken link removed**
MAX1473 **broken link removed**
MAX7033 **broken link removed**

I've looked at the datasheets, and some of the devices are only 5mm X 5mm and have 32 pins... now that is small!

Won't be able to solder them by hand :lol: ... But they do sell a 28pin
TSSOP version too.


Using an all-in IC seems the best way to go, they even supply the PCB layout files.

What do you think? Im leaning towards the 1470 which can be made to work at 433.92MHz, runs at 3V and is a TSSOP.