Rodo wrote:

> How do you calculate the bit rate ? The stream of pulses (010101000) is not
> of equal period. There is a header where is 1ms on and  1 ms off. Then there
> is the data that is 2ms on and 1 ms off.

You MEASURE the bit rate. The transmitter data rate will vary according
to range [somewhat, due to slicer vagaries], battery level,
temperature, etc. The preamble (the "header") allows you to measure the
timing of the incoming signal.

A DSP is NOT necessary by any stretch of the imagination.

Is this where a dsp chip would be useful ? Like the dsPIC from microchip. 
The filter and decoding could be done in the same chip ?

How do you calculate the bit rate ? The stream of pulses (010101000) is not 
of equal period. There is a header where is 1ms on and  1 ms off. Then there 
is the data that is 2ms on and 1 ms off.

Thanks



"David R Brooks" <davebXXX@iinet.net.au> wrote in message 
news:45223258$0$8427$5a62ac22@per-qv1-newsreader-01.iinet.net.au...
> Rodo wrote:
>> Hi all,
>>
>> I've been experimenting with receiver circuits and I'm missing something. 
>> The LC receiver I have (one of Ming's) and the Microchip (433.92 MHz, 
>> rfRXD0420) seem to have noise at the output when they're not receiving a 
>> signal. I see the signal (on a scope) when I press a button in the 
>> respectively transmitter. But if I have this signal apply to the input of 
>> a micro, it looks like I'm going to decode something (wrong) all the time 
>> because of the noise. Shouldn't the output pin be low until the pulses 
>> are demodulated by the receiver ? So that a high or low transition could 
>> be used to trigger an IRQ and decode the signal ? As I said before... it 
>> looks like I'm missing something in between the RX module and the MPU.
>>
>> Could someone shed some light into my information gap please ?
>>
> Many of the low-cost RX modules have no filter on their outputs, so you 
> get noise when there's no signal. You need to provide a filter. How 
> elaborate a filter, is up to you. It also depends on the data format you 
> use. Typically, the highest frequency in the output will be half the bit 
> rate (ie a string of alternating 1's & 0's). That sets your filter cutoff.
> This won't guarantee to exclude noise, but it will set an upper limit on 
> the rate those interrupts can occur. So long as your CPU can handle that, 
> & you then validate the data by software, it will work. 

Rodo wrote:

> The LC receiver I have (one of Ming's) and the Microchip (433.92 MHz,
> rfRXD0420) seem to have noise at the output when they're not receiving a
> signal. I see the signal (on a scope) when I press a button in the

Exactly. While there is no signal, there is only noise. I'm assuming
these devices include a slicer of some kind. When RSSI drops down to
the noise floor, the slice threshold falls quite rapidly until the
slicer starts chewing on noise and wiggling its output randomly.

Rodo wrote:
> Hi all,
> 
> I've been experimenting with receiver circuits and I'm missing something. 
> The LC receiver I have (one of Ming's) and the Microchip (433.92 MHz, 
> rfRXD0420) seem to have noise at the output when they're not receiving a 
> signal. I see the signal (on a scope) when I press a button in the 
> respectively transmitter. But if I have this signal apply to the input of a 
> micro, it looks like I'm going to decode something (wrong) all the time 
> because of the noise. Shouldn't the output pin be low until the pulses are 
> demodulated by the receiver ? So that a high or low transition could be used 
> to trigger an IRQ and decode the signal ? As I said before... it looks like 
> I'm missing something in between the RX module and the MPU.
> 
> Could someone shed some light into my information gap please ?
> 
Many of the low-cost RX modules have no filter on their outputs, so you 
get noise when there's no signal. You need to provide a filter. How 
elaborate a filter, is up to you. It also depends on the data format you 
use. Typically, the highest frequency in the output will be half the bit 
rate (ie a string of alternating 1's & 0's). That sets your filter cutoff.
This won't guarantee to exclude noise, but it will set an upper limit on 
the rate those interrupts can occur. So long as your CPU can handle 
that, & you then validate the data by software, it will work.

Hi all,

I've been experimenting with receiver circuits and I'm missing something. 
The LC receiver I have (one of Ming's) and the Microchip (433.92 MHz, 
rfRXD0420) seem to have noise at the output when they're not receiving a 
signal. I see the signal (on a scope) when I press a button in the 
respectively transmitter. But if I have this signal apply to the input of a 
micro, it looks like I'm going to decode something (wrong) all the time 
because of the noise. Shouldn't the output pin be low until the pulses are 
demodulated by the receiver ? So that a high or low transition could be used 
to trigger an IRQ and decode the signal ? As I said before... it looks like 
I'm missing something in between the RX module and the MPU.

Could someone shed some light into my information gap please ?

Thanks