[Elecraft] Comparison: K3 and SDR 5000A

[Lyle Johnson]

Hello Bill!

A few additional things to note:

WHAT THE ADC CONTENDS WITH

1a) In the K3 architecture, the 24-bit analog to digital converter (ADC) 
is exposed only to signals that make it through the roofing filter. This 
is typically less than a 3 kHz slice of spectrum, and may only be a 
couple hundred Hz.

2a) In a quadrature sampling detector (or integrating sampling detector) 
architecture, used by the SDR-1000/5000 series, the ADC is exposed to 
all the signals in the detector passband, typically more than 200 kHz 
wide even if the displayed passband is less.

SYNTHESIZER CONSIDERATIONS ARE ABOUT MORE THAN JUST PHASE NOISE

1b) In the K3, the synthesizer generates a single, clean signal for 
mixer injection.  Subsequent oscillators/mixers (including the software 
oscillators inside the DSP) are only dealing with band-limited signals 
due to the roofing filter.

2b) In the typical QSD/ISD system, a quadrature signal is generated by a 
DDS.  DDSes can have very low phase noise, but typically have numerous 
spurs.  While mitigation techniques exist, the fact remains that there 
are usually several spurs only 70 or 80 dB down inside a given 200 kHz 
wide passband.  These spurs can mix with other signals.

IT TAKES TIME TO PROCESS SIGNALS

3a) Radios based on embedded DSP, like the K3, can take advantage of 
processing techniques on a sample-by-sample basis.  This may translate 
to reduced latency (the time it takes for a signal at the antenna to 
appear in the speaker or headphones).

3b) Receivers relying on PCs for DSP deal with data in blocks, rather 
than sample-by-sample.  This results in significant latency - often 100 
to 200 milliseconds or even more -- regardless of the speed of the PC's CPU!

I hooked up an SDR-14 (using a dual core 3 GHz PC in block mode 
processing for its DSP) to the IF output of my K3 (which uses 
sample-by-sample processing in its embedded DSP) and listened to the 
same signal.  The audio from the SDR-14 appeared in the speakers *much* 
later than the audio from the K3.

In some cases, this may not be important.  In other cases (QSK CW comes 
to mind), latency can be very important.

Enjoy!

Lyle KK7P