[Elecraft] Re: K3 vs HPSDR Mercury

[Steve Ireland]

G'day 

 

I have been using an Elecraft K3 for about a month now, which is the best
performing superhetrodyne/analogue transceiver (with a great DSP back-end)
that I have ever used.  Once I got the filtering and DSP adjusted right, in
terms of pulling weak signals out of noise it was clearly better than my
previous FT-1000.  

 

I also have a working HPSDR Mercury digital down conversion (DDC) SDR
receiver (the Mercury receiver board plus the Ozy communications interface)
which use with the PowerSDR software.  For those not familiar with SDRs, the
DDC type effectively convert radio signals from analog to digital at the
antenna socket, using an analog to digital converter. 

 

Mercury has a Blocking Dynamic Range of 119dB, which is basically
independent of frequency spacing

 

When it comes to pulling weak signals out of noise, on the basis of A/B
testing that I've done, weak signals almost lost in noise are easier to
understand and less tiring to listen to (more of the latter in a minute) on
Mercury than on the K3. 

 

I am not an engineer, but a scientific journalist by profession and a CW and
contest operator with some 37 years 'on the clock' as a radio amateur. I
have been using SDRs for about three years (and am the co-author of the SDR
column in the Radio Society of Great Britain's RadCom with my friend Phil,
VK6APH who is an engineer and one of the main designers of the HPSDR). I
clearly have a personal interest in SDR, but until I started using them was
extremely skeptical about their performance.

 

My understanding why the HPSDR sounds better to me than the K3 (and others
who have witnessed the comparative tests) relates to the crystal filtering
that the latter - and other analogue HF receivers - use.  

 

As many on this reflector will know, when noise pulses/spikes pass through a
crystal filter, the phase response of the filter changes, depending on the
noise frequency.  However, when noise pulses/spikes pass through an ADC with
a linear response - such as the LTC2208 that Mercury uses - the phase
response stays the same, because the ADC treats them in a linear manner.

 

What happens in practice is that on the DDC SDR any 'noise' actually sounds
mellow and easy-on-the-ear (!), in a manner that has to be heard to be
believed.  In the case of an analogue radio like the K3, it seems that even
one stage of crystal filtering is enough to cause a phase response to noise
that eventually irritates/tires the user (certainly in my case) and makes
them want to switch the radio off.

 

I love using the K3 and find it an absolutely brilliant radio, but am
looking forward to putting together a HPSDR transceiver to take advantage of
the way a DDC SDR receiver deals with noise.  Whether top ten contesters
find DDC SDRs to their taste will be interesting to see, but as a 160m CW
DXer with over 220 countries confirmed, as far as I am concerned they are
the way ahead.

 

Incidentally, I think both the Flex SDR-1000 and FLEX-5000 are QSD-type
SDRs, rather than DDCs.

 

Vy 73

 

Steve Ireland, VK6VZ

 

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