[Elecraft] Sherwood's receiver performance table updated

[Joe Subich, W4TV]

On 4/26/2016 8:08 PM, Kevin Stover wrote:
> Everybody is ignoring W4TV's 800lb Gorilla, ADC overload.

I can't take credit for that ... the warning comes from some of the
best RF designers in the world.  Direct sampling receivers are very
good in controlled situations.  Unfortunately, those who are used to
analog superhetrodyne receivers do not recognize the symptoms of ADC
overload/overflow in many cases.

The overflow can show up as an occasional click or pop if the number
and strength of signals continues to increase conditions go down hill
very rapidly.  The best 16 bit ADC based SDRs can and do show very
significant overload issues when run without effective front end
bandpass filtering in high RF environments (160 meters near AM BC
stations, 40 meters in Europe, 160 meters in New England during a
contest, etc.).

One can apply filtering (which Icom seems to have done in the 7300),
apply attenuation (which Flex seems to have done in the 6000 series)
or both.  Filtering limits the number and width of "slice receivers"
while attenuation kills weak signal performance.

I find Icom's implementation interesting in that they are using the
technology as a single band, single channel receiver (they could have
added Dual RX within the same band at practically no hardware cost) - an 
application where, with tight bandpass filtering, direct sampling
may be ready to compete as indicated by the fact that Sherwood's tests
place the 7300's above some "good" conventional rigs like the Ten-Tec
Eagle, Kenwood TS-590SG and TS-990 in MDS, LO Phase noise and IMD DR.

However, for the ultimate multi-channel receiver, ADC technology is
still 4 to 6 bits short as witnessed by the need for 12 to 20 dB of
attenuation in the Flex 6000 or the 0 to 30 dB electronically stepped
attenuators in some of the other products like the Anan, Hermes, etc.
Sherwood's results hint at this when comparing the MDS with the
preamp on and preamp off ... even with its preamp on the Flex 6700
MDS is the same as the K3S with *no preamp*!

73,

    ... Joe, W4TV