http://elecraft.85.s1.nabble.com/K3S-SDR-superhet-hybrid-vs-entry-level-direct-sampling-radios-tp7632855p7632859.html
Concerning Flex, one of the engineers posted a pretty detailed explanation
of how they handle strong signals in a wide band environment. This stuff is
say, I am never disappointed with either receiver in any environment. I
> Thanks for sending Wayne. To take this discussion a step further, is
> the Flex series of radios in the same category as the 7300?
>
>
> 73,
> Gary K9GS
> -------- Original message --------From: Wayne Burdick <
[hidden email]>
> Date: 7/30/17 11:14 AM (GMT-06:00) To: Elecraft Reflector <
>
[hidden email]> Subject: [Elecraft] K3S (SDR/superhet hybrid)
> vs. entry-level
> direct-sampling radios
> Hi all,
>
> Over the past two days I’ve been deluged with requests for our K3S vs.
> IC7300 comparison chart. This stems from a posting that mentioned the
> latter rig, and my subsequent offer to send out the chart, which we
> normally use for in-house training purposes.
>
> What’s become clear from all the questions and comments is that there’s a
> need for clarification on receiver architecture.
>
> The K3S, an SDR/superhet hybrid, includes narrowband protection of its
> A-to-D converters in the form of roofing filters (crystal filters). This is
> fundamentally different from the approach taken by direct-sampling radios,
> which have only very broadband filtering ahead of their ADCs. Typically,
> front-end band-pass filters are 0.5 to 4 MHz wide. Their ADCs will be
> impacted by all strong signals in this range, alone and in summation.
>
> When the ADC over-ranges in this “pure” SDR architecture, the radio
> usually reduces its gain automatically by either turning off preamps or
> adding attenuation. This increases the noise figure, often resulting in the
> loss of weak signals, both audibly and in the panadapter. Another word for
> it is “desense.” You might hear a pulsing sound as the noise floor goes up
> and down in response to a particular signal, or the sensitivity may degrade
> for many seconds at a time while the firmware waits for levels to drop at
> the input to the ADC.
>
> Such over-ranging is almost unheard of in the K3S, for multiple reasons.
> First, we use very strong mixers and gain stages. Second, in many cases
> there’s no need to turn the preamp on at all. (Example: the K3S’s
> preamp-off sensitivity is typically -135 dB, while the preamp-off
> sensitivity on a typical direct-sampling SDR is -115 to -125 dB.) The third
> reason for the K3S’s strong-signal performance is its crystal roofing
> filters. These protect the ADC from not only wide-spaced signals, but also
> from signals very close by. As K3 and K3S owners will attest, you can have
> a huge signal just a few hundred Hz away and not even know it’s there --
> unless that station’s transmit phase noise is blanketing the band anyway.
> (There’s no defense against an unclean or clicky transmit signal.)
>
> I’ve updated the referenced comparison chart to clarify this important
> difference. Our webmaster will put it up next week.
>
> Meanwhile, I’ll continue to send it on request (email me directly). Feel
> free to share the information with anyone interested in the topic.
>
> 73,
> Wayne
> N6KR
>
>
>
>
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