May I add that another good feature of a direct sampling SDR is low IMD
in the passband. That is due to lack of crystal filter in the passband.
The result is that that output audio is cleaner especially when there
are several stations calling on the same frequency.
73, Igor UA9CDC
30.07.2017 21:46, Wayne Burdick пишет:
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]
______________________________________________________________
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]
