Transmitter and Receiver frequency in a rf module

[sadpony]

I am working on project using 433 rf module. HT12E and HT12D are used as encoder and decoder IC respectively. I read the datasheet and had few doubts. Why is the frequency of decoder 50 times that of encoder. Can I change the value of oscillator resistors to in drease the speed of transmission. Currently I am using 1.750M ohms at Tx and 33K ohms at Rx. Please do help.
Thanks in advance.

 

[chuckey]

As the decoder and encoder is asynchronous. When the incoming bit is a '1', the decoder does not know if its a '1' or a '111111'. So if it samples the incoming bits at a high rate, then it can find out how long a '1' or a '0' last for (in terms of of its own clock pulses), i.e. 27 '1's then 11 '0's is actually '110' and then lock up its clock to the incoming bitstream.
Frank

 

[sadpony]

Hey chuckey thanks a lot. This is what I understood. If suppose frequency of decoder is 3 times frequency of encoder, now if decoder receives 1's for three consecutive clock pulses than it would consider it to be a single 1.Similarly if 6 1's are received, than decoder will consider them to be 2 1's. Please tell me if I am right and correct me if wrong. I tried changing the Tx and Rx resistor to increase the rate of transmission, but couldn't get valid transmission. If decoder has high speed only to understand 1's, than it should also operate at high frequency different value of resistor.
Thanks in advance.

 

[chuckey]

Remember its non synchronous, so in your example, the decoder clock is 3 X the encoder clock, but in ten minutes time it might be 2.9 X the encoder clock, so it will decode only 2 '1's, what will it make of this. using a high ratio, means that '1's can be dropped slowly as the frequency of the encoder/decoder drift and the chip can make sense of it and re-adjust its frequency to minimise this effect. Or do a statistical analysis on what the actual puls shoulld be.
Frank