[Elecraft] DSP - 4 questions

[Bill Coleman]

On 5/7/03 6:13 PM, Lyle Johnson at wa7gxd@fidalgo.net wrote:

>> 2 - Cab the DSP unit be disengaged at will so any DSP noise is
>> not constantly present.
>
>Again, there is no 'DSP" noise that has been reported.
>
>But, you can place it in "bypass" mode which puts the DSP chip to sleep, and
>turns off the analog-to-digital and digital-to-analog converters, so the
>unit acts as if it weren't present.  Power consumption drops to about 10 mA
>in this mode.

Ok, this comes down to a question I had today. 

One of the nice features of the KAF2 is the low-pass filter. It really 
helps to eliminate some high-frequency audio hiss that is present in the 
K2. In fact, it is likely the only feature of the KAF2 that I use.

Now, with the KDSP2, there's no such analog filter. Looking at the 
schematics, it appears the audio goes directly through the A/D D/A chip. 
I don't see any way that it is bypassed, unless that happens internally.

Other than a single pole R/C filter, there doesn't appear to be any other 
low-pass operations to eliminate the hiss, even if the A/D D/A can be 
bypassed.

So, this implies that the low-pass operation takes place in the DSP 
itself. The bad point about that is that the digitizing operation 
introduces quantization noise, which limits the dynamic range of the 
audio. Unless peak signals are hitting the maximum limits of the A/D 
converter, this dynamic range is going to be substantially less than the 
90 dB specified. 


While I like the features of the DSP, I'm worried that having the A/D 
conversion in line all the time will slightly raise the noise floor of 
the rig, especially for weak signals. And I don't like the idea of 
disabling the filter, since that will re-introduce the audio hiss.





Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901