http://elecraft.85.s1.nabble.com/K3S-SDR-superhet-hybrid-vs-entry-level-direct-sampling-radios-tp7632855p7632861.html
We’re quite familiar with this document. But blocking is blocking: a single strong signal in the passband of the RF band-pass filter can overrange the ADC, causing desense or spurious responses, thus requiring that gain be reduced ahead of it. Clever firmware can help, but it can’t overcome the basic limitation on ADC input voltage spec.
> On Jul 30, 2017, at 9:47 AM, Kevin der Kinderen <
[hidden email]> wrote:
>
> 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
> still a bit over my head but owning both the KX3 and the Flex-6500 I can
> say, I am never disappointed with either receiver in any environment. I
> kind of feel like the luckiest dog in the world to have both.
>
> I hope it is OK to post the link:
>
>
https://community.flexradio.com/flexradio/topics/adc-overload-myths-debunked>
> 73,
> Kev K4VD
>
>
>
> On Sun, Jul 30, 2017 at 12:27 PM, GaryK9GS <
[hidden email]> wrote:
>
>> 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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