RF communication (433MHz)

Hello...

We have 4 boards with exactly the same components (except the xtal) (PIC16LF648A, RXQ2 RF module) and it`s been communicating really fine.

We went out on a distance test to make sure that RXQ2 module could achieve the range of 200m, as specified on the data sheet. Well, it DID work for the four boards.

However, boards A and B, uses a 2MHz crystal oscillator (pic external osc).
Boards C and D uses a 4MHz crystal.

Why? Because we need to save power, as it is battery powered. Slower speed means less power consuming.

So, the question is: WHY the boards A and B achieved a range up to 320m and boards C and D achieved a range up to 240m?

The antenna is all the same length, the boards are identical, the serial comm. between RXQ2 and PIC are all 9600, 8, N 1, the firmware is equal and so on...

If anybody could give me a reasonable answer to it, I will be really glad.

(forgot to tell you: The batteries were brand new, 16500mAh, with top current of 400mA. The TX current of RXQ2 gets at most a peak of 16mA)

Thanks in advance.

Sylvio

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Why? Because we need to save power, as it is battery powered. Slower speed means less power consuming.

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I think that might be the answer to your question. The faster one is using more power so less is available to the transmitter.

It does not take much of a voltage drop to make a big difference.
It is all about wattage.

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I think that might be the answer to your question. The faster one is using more power so less is available to the transmitter.

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As I editted:

The batteries were brand new, 16500mAh, with top current of 400mA. The TX current of RXQ2 gets at most a peak of 16mA)

The delta current between crystals is about 0,1mA. It's too small to cause an effect using this powerfull battery, don't you agree?

Thanks!

Have you tried to exchange the modules (only modules, not the antennas or the PIC driving part) and see if there is any difference?

Ioannis

No, I haven't yet..

I'll do the exchanging and let you know the answer.

Thanks again.

Sirvo,

Are you sure all the modules are transmitting at the same frequency? The higher the frequency the higher the transmitted power, but also the higher the frequency the higher the attenuacion. Longer wavelenth signals travel farther than shorter wavelength ones.

Robert

Hello Robert,

The firmware uploaded to pic is the same for every device. Therefore, all the RF frequency are the same. In this case, 433.1MHz (RXQ2 allows the range 430.0 to 440.0MHz, 200kHz step).

About the RXQ2 exchanging test, I have done that today. And the answer is: Still the same weird thing. The board with 2MHz crystal gets further than the 4Mhz one....

So, the RXQ2 is not the problem.. then, what is it???

[ ]'s

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then, what is it???

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It's the 4MHz oscillator in your receiver circuit, which introduces more noise, which reduces receiver sensitivity, which = shorter range...;o)

After your results I too agree with Bruce. What can you do? Just use metal screening of the 4MHz osc to reduce harmonics transmission reaching RF receiver input.

Also changing frequency may help but my guess is not very much.

Ioannis

Bruce, I'll take that explanation then.. I also agree with that.

Thank you all!

[ ]'s

Sylvio,

You might be able to filter out some noise, and help extend range, by adding a few small capacitors in your receiver circuit.

Start with 0.01uF to 0.1uF, and place them as close as possible to the Vcc and ground pins on the receiver.

Also make sure you have similar decoupling caps between the PIC Vcc and ground pins. Also as close as possible to these pins. If the oscillator isn't located too close to the receiver, this should help.

Hi Bruce,

There are 3 decoupling caps on the board.

1 of 100nF in the receiver power supply.
1 of 100nF in pic's power supply.
1 of 10uF on the battery source.

Maybe I should try to change this values to find out a optimal situation.

Thank you.