Posted by
Bill Frantz on
May 30, 2019; 12:18am
URL: http://elecraft.85.s1.nabble.com/3-technical-questions-tp7652460p7652468.html
Hi Skip. Let me take a stab at some of these, and thanks to
Alan, Jeff, and W2XJ for furthering my technical education about ADCs.
(1) With direct sampling, if you are operating in a contest, you
can keep an eye on other bands, like 10M in the current
conditions, while operating on bands which have propagation.
It's just another window on the screen. A superhet will severely
narrow the bandwidth in the IF stages, so you need a separate
superhet for each band (ala the 2nd receiver in the K3).
(2) As far as I can tell, the K4HD should perform as well as a
K3, or perhaps even better. Using the formula Alan posted, if
I'm tuning a 4KHz FT8 band on a K4 with its 122 MHz sample rate,
I will get a 44.8 dB improvement in S/N from the over sampling.
I would get 47.6 dB for a 2.1 KHz SSB bandwidth and 60.8 dB for
a 100 Hz CW bandwidth. (These are ranges I normally use in my
operations.) Add the 77 to 85 dB range for a real 16 bit ADC
with real support circuitry (these numbers are admittedly a
WAG), and you get some pretty respectable performance.
You also win big with the K4HD in the same way as with the K3.
The IF filters greatly suppress signals not in the passband,
such as that CW station in the tent next to your RTTY station,
(which describes the WVARA QRP field day setup).
(3) The details of the K3 ADC are beyond my current level of
knowledge, but the documentation lists it as 24 bits, as does
W2XJ's post.
73 Bill AE6JV
On 5/29/19 at 4:01 PM,
[hidden email] (Fred Jensen) wrote:
>I understand what direct sampling is and how it works. Why is
>it preferable [or whatever other positive term you might want
>to use] to the superhet design of the K3(s)? I'm curious what
>sampling the entire spectrum from 0.1 MHz to 54 MHz buys me?
>
>If I understand the "Superhet" option for the K4 correctly
>[problematical at best], it provides RF and mixer stages to an
>IF [~8 MHz??] which is then directly sampled by the base K4.
>Since that IF bandwidth can be adjusted by xtal filters and
>will be MUCH narrower than 0.1 - 54 MHz it will also be far
>less susceptible to overload from strong signals not in that
>passband. Is this what's going on?
>
>For reasons I cannot remember now, I thought the K3(s)
>oversampled the 15 KHz 2nd IF with a delta-sigma 1-bit ADC and
>decimated to the equivalent of a 22-bit word. Some of the
>thredlets here have now caused me to reconsider that belief.
>Have I got it wrong?
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