[Elecraft] Icom 7300 is available

[Joe Subich, W4TV]

On 3/1/2016 4:52 PM, Barry N1EU wrote:
> N5AC made some interesting comments on the topic:
> https://community.flexradio.com/flexradio/topics/adc-overload-myths-debunked

N5AC is "talking his book" and totally out of touch with reality.
While he claims the issue of ADC overload is blown out of proportion
and cites mathematical models to support his claim, it doesn't always
work that way in the real world.  Put another way, the real world is
not always "neat" with a bunch of similar level signals across a wide
spectrum.  Try copying a -130 dBm signal among multiple -30 to -50 dBm
(S9+20 to S9+40 dB) amateur signals on 160 with a dozen AM signals also
present at -30 dBm to 0 dBm at the receiver input.  See the spectrum
analyzer plot from K8AQC showing fifteen AM signals stronger than -40
dBm and *five* stronger than -10 dBm: 
<http://cliftonlaboratories.com/z10020_am_medium_wave_band_reject_filter.htm>. 
  I doubt N5AC can show
that a Flex 6300, 6500, or even a 6700 (without the optional second
SCU) can *simultaneously* provide -130 dBm sensitivity and +100 dB
2 KHz IMDDR3 on 160 meters while running a second slice receiver
(panadapter) on 80 or 40 meters under those conditions in tests
certified by independent experts - yet those are the very conditions
he claims mathematically can't happen.

NC0B (in personal e-mail) has reported significant symptoms of ADC
overload with a non-Flex direct sampling SDR with a 250 MHz ADC on 160
meters at his *rural* Colorado location.  ARRL Labs shows the same
trade off between sensitivity and IMDDR3 with the both the Flex 6300
and Flex 6700 in the April 2015 QST review 
<http://www.arrl.org/files/file/protected/Group/Members/ProductReview/PROD_REV_0415.pdf>. 
  The
review shows a substantial reduction in sensitivity with the preamp
turned off and a substantial fall-off in dynamic range with the preamp
turned on.  ARRL does not even publish dynamic range at 2 KHz with the
highest gain preamp even though their measurements are made with only
two signals - not a dozen or more close in signals at the same high
level.

Sherwood shows the tradeoff between sensitivity and dynamic range in
his "Receiver Test Data" table <http://www.sherweng.com/table.html>.
It is most clear in the 100 KHz Blocking values where the Flex 6700
shows "A/D Limit varies with Preamp Gain" ... in other words, when the
preamp is on (as required for the -135 dBm sensitivity), the Blocking
values fall apart (vs. > 150 dB 100 KHz blocking range for the K3S).

The issue of ADC overload can't be simply dismissed by hand waving as
N5AC and others would like.  The current technology is not robust
enough to simultaneously provide -145 dB MDS, 100 dB+ 2 KHz IMDDR3
and 130 dB+ Blocking Dynamic range as the case for the K3S, K3 and
KX3.

ADC technology - instantaneous peak voltage handling - needs to improve
another 10 to 20 dB before direct sampling can be said to be completely
reliable in all conditions.  It may "work fine" in many locations where
antenna selectivity, external bandpass filters and even propagation
protect the ADC from overload.  However, direct sampling does not work
well in the presence of multiple, very strong, close in signals as
shown by both ARRL and NC0B testing - no matter how much the proponents
of direct sampling would like to make you believe.

73,

   ... Joe, W4TV