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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