[Elecraft] K3: KRX3 question

[Bill W4ZV]

David Cutter wrote:
> 
> Bill
> 
> I can't recall seeing that figure published before; where would I look for
> that figure?
> 
> David
> G3UNA 
>> snip
> 
> 
>> The purpose of the roofing filter is mainly to reduce strong
>> (approximately
>> S9+30) signals from over-driving the ADC in the DSP.  If they do that,
>> then
>> the DSP determines the final selectivity over its ~100 dB dynamic range.
>> 
>> 73,  Bill
> 
> 

David I'm not sure which figure you meant, so I'll try to cover all bases:

1.  S9+30 limit for over-driving ADC.  This can be deduced by the setpoint
of the Hardware AGC Mod quoted from Elecraft's mod page below:

"This modification raises the hardware AGC threshold from about S9+5 to
about S9+25 to +30. The new threshold still protects the DSP's
analog-to-digital converter, while providing better receive performance when
wider crystal filters are used."

2.  100 dB dynamic range of DSP.  I'm not going to search for the exact
source but this is often quoted by manufacturers as the practical limit of
current DSPs using 24-bit ADCs.  Theoretically it should be higher based
only on 24-bit ADC resolution but practically it's limited by other factors. 
All hybird heterodyne/DSP rigs such as Orion, K3, FT-2000/9000 and
IC-7700/7800 have about the same limit.  I suppose if we get 32-bit ADCs
this limit could increase.

3.  DSP Shape Factor formula from KK7P:

http://www.mail-archive.com/elecraft@mailman.qth.net/msg53271.html

Hopefully I covered the figure you were questioning.

73,  Bill



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