[Laser] minimum frequency PSK31

[Glenn Thomas]

Hmmm... lots of confusion here.

Reducing the bandwidth does not by itself do anything to gain - they 
are not intrinsically related. The signal power remains the same if 
you do nothing else to it. What does improve with reduced bandwidth 
is the signal to noise ratio (SNR). The tradeoff is that you can only 
improve SNR by reducing bandwidth up to a point. After that, further 
reducing the bandwidth takes away signal as well as noise.

The use of PSK31 on a laser link provides a ready made system where 
the receiver bandwidth, set to ~31 Hz via filtering done in the PSK31 
software, is matched to the signal characteristics. Any wider 
bandwidth would degrade the SNR by allowing more noise and any 
narrower bandwidth would degrade the signal by cutting off sidebands.

I suspect that a comparison of a PSK31 signal at 300 Hz and a PSK31 
signal at 600 Hz will show little difference. The receive bandwidth 
will be the same in both cases  because it is achieved in the PSK31 
software. An unsupported assumption I'm making here is that the noise 
power at 300 Hz and at 600 Hz is the same or nearly so.

BTW, changing the feedback of an opamp changes both the gain and the 
bandwidth. That's an implementation effect that is because of the way 
that opamps work. Other circuits behave differently. For example a 
crystal filter can dramatically reduce bandwidth (thus improving SNR) 
while at the same time reducing gain.

73 de Glenn WB6W

At 06:50 AM 11/15/2006, you wrote:


>>In a message dated 11/13/2006 9:50:33 AM Central Standard Time,
>>KY1K at verizon.net writes:
>>
>>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.
>
>
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