[AMPS] Receiver performance

[Ian White, G3SEK]

Tom Rauch wrote:
>
>I'd want more than one bit toggling on noise. I don't know about 
>you, but I (like most CW DX ops) can copy signals below noise 
>level. That requires the receiver to not alter the noise.
>
>I have a certain "DSP rig" I won't name sitting right next to me now, 
>and the DSP is useless for signal processing unless the signal is 
>equal or above noise.

When doing a comparative review of several DSP audio filters, I found
that most - but significantly not all - units gave good CW filtering
performance right down to the lowest signal/noise ratios. I had been
expecting to hear "on or off" quantization effects at low levels, but
there was no sign of that because the noise added random dither to the
incoming signal and avoided the threshold effects. The "noise reduction"
feature was universally useless at low signal levels, as you might
expect.

Note - the weak-signal tests were at VHF/UHF where noise really is
random, as opposed to the ground-up and reconstituted QRM/QRN that
passes for "noise" on HF, which might have different statistical
properties.

In direct comparisons that included well respected analog filters,
several DSP audio filters held up very well, including the Timewave DSP-
9, 9+, 59 (this was pre-599) and the MFJ-784B. Others had low-level
noise of their own making - so if you're interested in weak-signal,
always try before you buy.  


73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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