[Laser] minimum frequency PSK31

[Art]

James and Paolo,

I have the following comment..........


>>Another question I have is, if there really is a practical reason to narrow
>>the bandwidth of the signal from the optical detector, does that 
>>mean that a 500  Hz tone is better for PSK31 than a 1KHz 
>>tone?  Carrying on that logic,
>
>I think you've mixed up two concepts: tone center frequency and 
>signal bandwidth.
>
>Without interfering sources, so with a white background noise, if 
>the receiver has a definite and constant BW (say 250 Hz) centered on 
>your tone, there are no improvements operating at 300 Hz or 2576 Hz: 
>bandwidth at the receiving end will always be the same.


If you use the same receiver and pick out a relatively thin slice of 
spectrum to monitor for your psk31 (or whatever) signal, you will 
have the same s/n ratio whether the psk31 signal is at 300 Hz or at 
2567 Hz...so long as both signals are within the passband of the receiver.

However, I'd like to also add this......

If I have a PSK31 transmitter operating at 300 Hz center frequency 
and a psk31 transmitter operating at 600 Hz AND........

If I use the same receiver hardware and change only the feedback 
determining resistor to modify the passband of the receiver...........

I can obtain a 6 db advantage by operating at 300 Hz vs operating at 600 Hz.

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In more detail.........

System 1 is a dc to 400 Hz bandwidth receiver listening to a PSK31 
signal at 300 Hz.

System 2 is a dc to 800 Hz receiver listening to a PSK31 signal at 600 Hz.

The hardware in the receivers is identical, expect the feedback 
resistor in the current to voltage converter is half the value of the 
system 1 unit, which is WHY system 2 has a wider passband.

One might expect system 1 to be able to detect a signal that is 3 db 
weaker than system 2 because it has one half the bandwidth. But, this 
is NOT what we end up with!

In reality, we get a 6db advantage, not a 3 db advantage. So, system 
1 can detect a 6 db weaker signal that system 2 can.

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In conclusion, if the receiver is the same and both signals are 
within the passband of the receiver, then there is no benefit to be 
obtained by operating at a higher or lower frequency.

However, if the receivers are optimized for a particular frequency, 
the receiver that uses the narrower overall passband will have a much 
greater ability to detect weaker signals than conventional wisdom 
might suggest.

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Taken to extreme, you might expect a 10 times narrower bandwidth 
receiver to have a 10 db advantage because the passband is 10X 
narrower. In fact, the receiver with a 10X narrower bandwidth has a 
20 db advantage, not a 10 db advantage.

----------------------------------------------------------

Hope this helps.

Art



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