Simple RF Transmit and Receive

[dhouston]

Could you please tel me what is the reason for using the CD Pin signal detection, why doesn't it work if I only monitor the RF input?

With the FSK modules, a logic 1 is represented by one frequency and a logic 0 is represented by a slightly different frequency. the Carrier Detect (CD) output indicates when either frequency (or any frequency close to them) is present and that's a fairly strong indication that a signal is present. Just monitoring the RF input (which really should be labelled Antenna Input) will give you a lot of ultralow level random noise as it is before any tuned circuits - it's equivalent to throwing away the receiver and just attaching the antenna to a PIC input. There are two data output pins - both are demodulated so they are in the audio frequency range. The AF pin is an analog output. Its amplitude can be usually used as an indication of received signal strength. The RXD pin is a digital output of the logical ONES and ZEROS. Use CD to tell you when to monitor RXD.

My web page is down right now due to a mixup with a new hosting service I've moved to. As soon as it's back up (later today, hopefully), you can see how to use your soundcard like an oscilloscope to record the audio frequency output signals as wave files. It's much easier to grasp the basics when you can see the signals output by the receiver.

EDIT: My page is back up. The second section - Wireless Application Notes may help you.

[koossa]

Thank you very much Dave, I really appreciate your help!!

[dhouston]

The FM transmitter/receiver pair you're working with are rather costly for this type application, especially if you are planning on several sensors. You can reduce the cost by using ASK modules like those I list on my web page (or those designed/sold by Ioannis, one of the regulars here) and using a simple protocol like the one I've posted an example of in the Code Examples forum.

• http://www.picbasic.co.uk/forum/showthread.php?t=6261

[koossa]

Thx Dave!

I was actually looking for a cheaper option, but did not know what to look for.

[koossa]

What is a typical application for a FSK chip?

[koossa]

I have updated my circuit to monitor the CD pin on the Receiver
If high, I turn on an LED, but the LED switch on about every 10 seconds while the transmitter is still switched off?

[dhouston]

I have updated my circuit to monitor the CD pin on the Receiver
If high, I turn on an LED, but the LED switch on about every 10 seconds while the transmitter is still switched off?

I'm not sure what you're saying. If you are using the recommended circuit (p6 of the PDF), the /CD signal goes low when there is a signal.

[dhouston]

What is a typical application for a FSK chip?

FSK is needed when higher data rates are desired. The pair you're using can go as high as 160kbps while ASK modules are usually limited to 4-8kbps. In simple terms, ASK is like AM radio which is subject to interference while FSK is like FM which has better noise rejection.