Using INTOSC 32Mhz as RF Radio Control and Data Communication

[rayzrocket]

I'm attempting to generate some simple & low cost ($2 + PIC) RF radio control and data comm' by taking advantage of the 32Mhz INTOSC found on some of the PIC's (16F1826 is on the bench now). It would be nice to get 3kbps.

I'm trying a few circuits(op-amp & RLC filters) now without success by outputting the clock (32Mhz) on a pin.

Am I crazy to think this is possible?

Any ideas or anyone interested?

[mackrackit]

Some things to consider if you have not.

A quarter wave length antenna for 32Mhz is around 7 feet long.

But if I am not mistaken you will only be able to output 1/4 of that.
8Mhz = 29 foot antenna

[Ioannis]

I think yes, that you are ...

The OSC power is too litle to make a good transmission, plus at 32MHz you need a long long antenna.

Ioannis

[rayzrocket]

OSCCON = %11110000
'32Mhz intosc use
CLKRCON = %11010000
'Turn on ref clock output BE SURE TO DISABLE "CLOCK OUT" IN MEPROGRAMMER
With these statements, I see the 32Mhz on the 'clkout' pin. On a 5v pwr supply, the waveform looks like a sawtooth ~1volt peak to peak that is offset by 2v from zero.

Most low cost radio control toys are 27Mhz or 40Mhz. I tested one that is 27Mhz using Tx antenna length of just 4inches and receiver antenna length of 8inches; the toy car worked for over 40feet.

[mackrackit]

Most low cost radio control toys are 27Mhz or 40Mhz. I tested one that is 27Mhz using Tx antenna length of just 4inches and receiver antenna length of 8inches; the toy car worked for over 40feet.

And they have a loading coil before the antenna. Giving an electrical length of well over 4 inches. A straight wire for an antenna will need to be the lengths given above.

I will suggest you pick up a book about antennas and wave propagation. Then start on a project like this.

Here is a good place to start
http://www.arrl.org/

[Ioannis]

See post #3 about power.

Also if you use a loaded coil with short antenna, much of the RF power will be spent in the coil and mismatches.

With picowatt I think you can go a few mm away...

Unless you are going to amplify the signal. Maybe a MAR-1 or more in series can help.

Look here for data: http://www.minicircuits.com/pdfs/MAR-1+.pdf

Ioannis

[rayzrocket]

Thanks.
I tried a few simple (perhaps stupid) experiment setups such as...
Transmitters tried:
a. 8" wire soldered straight to the clk out pin (with a 5v supply to PIC, this pin shows 1.5v p-p 32mhz waveform that is 2v offset above zero)
b. tried modulating the full 5v using RF transistors and amps unsuccessfully (likely due to impedance issues of clk pin feed).
c. successfully modulated the full 5v supply onto a wire(antenna) using a Schmitt trigger, triggered by clk(clock) out pin, and powered by 5v supply, unfortunately my received signal strength did not go up as this idiot(me) expected.

Receiver: ~5" wire feeding a bandpass filter (series R & L then parallel to gnd R & L) that worked great, feeding a RF amp (Avago ADA-4543).

The above gave an output of 45mv p-p centered on zero before the RF amp, and about 190mv p-p after the amp. This was at a distance of about 6" apart. My application needs about 36" (~1meter) apart maximum (for a rotating equipment sensor).

I modultated the 32mhz clk output either (both ways worked great):
a. through a NPN transistor where the data pin from PIC(controls if 32mhz is on/off) feeding the base directly(no resistor), collector connected to clk out pin, and emitter hooked to a 8" wire(antenna).
b. through software by turning on or off the Clock output on the clk out pin using the CLKRCON register and placing pausues statements inbetween. To send data out this way, one could(no start and/or stop bits shown here):
Data var BYTE
code....(setting a value to Data)
For x= 0 to 7
if data.x=0 then
CLKRCON = %00010000
'Turn off ref clock output BE SURE TO DISABLE "CLOCK OUT" IN MEPROGRAMMER
pauseus=300
if data.x=1 then
CLKRCON = %11010000
'Turn on ref clock output BE SURE TO DISABLE "CLOCK OUT" IN MEPROGRAMMER
pauseus=300
next x
...
I need to work on the receiver a bit more, just need to amplify it a bit more to ~50% of receiver supply voltage, then I can use a Schmitt trigger to send it to the +V rail so the PIC can see good data coming into a receive (SERIN2) pin.
...
If anyone has good ideas, pls pass them along.