> GaryK9GS <[email protected]> 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?
Flex radios do use a direct-sampling architecture (with no roofing filters). The degree to which they’re subject to ADC over-range, and what they do about it in firmware, has been the subject of ongoing debate. It may be the case that, generally, the more you pay for a direct-sampling radio, the better the A-to-D converters, and the higher the ADC overrange threshold. That said, when measured with preamp off, the Flex 6700 has about 7 dB less IMD dynamic range than the K3S (see note Y in Sherwood’s chart), and 20 dB less blocking dynamic range. Both are direct consequences of not having narrowband roofing filters. On the other hand, the Flex radios have a very broad panadapter display for those who require it. 73, Wayne N6KR > > > 73, > Gary K9GS > -------- Original message --------From: Wayne Burdick <[email protected]> > Date: 7/30/17 11:14 AM (GMT-06:00) To: Elecraft Reflector > <[email protected]> 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 > > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
