Mike,

Thanks, I hadn't realised that the data slicer averaged over such a
short time interval. I'd assumed it was at least 8 to 10 bits and
likely to be more.  Looks like I'd definately get an improvement if I
went to manchester type encoding.

Do you have a ref. for the Maxim app note?

RP


On 13 May 2010 09:42, Michael Watterson <mike@radioway.org> wrote:
> Richard Prosser wrote:
>>
>> Hi Mike,
>> No, I'm just using standard ASCII encoded charecters. It was initially
>> intended as a test bench to try out different encoding schemes and
>> eroor detection/correction etc. But it turned out adequate for a
>> monitoring project and I left it at that.
>> I did find I had to send a reasonably long preamble to get the data
>> slicer setup proerly and that the quoted 10kb/sec ? (9600 in my case)
>> was unreliable.(IIRC it was 10k for the transmitter and 5k for the
>> receiver anyway). =A0I think I ended up using 2400. I've still got a
>> spare transmitter somewhere that I intend to use to expand the system
>> so I might have a look at improving things as you suggest.
>
> The performance is only decent if the DC average is zero, within a short
> time period < x3 clock rate.
> Maxim has a decent application note explaining the dataslicer theory and
> how you need to change the various Cs and Rs to match your clock rate
> and why it has to be manchester encoded (least time for zero DC
> average). Data rate is of course 1/2 clock rate.
>
> the data slicer can work with as few as 2 preamble bits, but 5 is
> better, if you have all the time constants right.
>
> You can use a zero crossing detector instead of a Data Slicer, and you
> need no preamble, but it's very much more sensitive to noise.
>
> The transmitter doesn't care about the datarate as it's just a SAW
> stabilised =A0Colpitts oscillator and with decent UHF transistor will turn
> on/off >20kbps. The "data" in is usually just bias resistor to base of
> transistor.
>
>
>
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