Joe,
I agree with you basically. I think it's just the manor in which you
state the problem.
You touch on the real problem 14-16 bits of sampling vice having
more bits in the sample. It's not specifically the direct sampling
architecture. I think the radio companies just can't get 24 bit ADCs
fast enough to do direct sampling at commodity pricing which would allow
direct sampling with out the baggage. If I can do 24 bit sampling and
high and low pass filtering, I can simplify receiver design AND have a
an acceptable dynamic range. Multiple instances of DSP processing of the
24 bit samples will allow as many virtual receivers, slices, as
processor power will allow. My guess is we are on the verge of getting
to where we would like to be.
Flex is using a 16 bit ADC and I'm not sure what the 7300 is using,
but I infer it, too, is 16 bit. I do believe that the Elecraft
architectures are all based on 24 bit sampling. The Elecraft designs
should be tolerant to serious environmental abuse short of a local
thunderstorm.
Our comments reflect a rigorous tech discussion. But, there is
another issue that has not been discussed in anything I've read so far,
ultimate filter rejection. Flex, Kenwood, and Elecraft have filtering
that runs in the 100 db region, or a little better. I saw that the
IC-7300 is about 20 db worse. What that means if you are a CW operator,
serious or otherwise, and there is a pile up on a DX station, a contest,
or just a really busy weekend, you may have problems from strong
stations not too far away while you are trying to work an S1-3 station.
SSB operators have other problems like splatter and no amount of superb
design will fix that.
I'm pleased there is a vigorous discussion on the new radios. It
helps push the designers and the technology.
73,
Barry
K3NDM
------ Original Message ------
From: "Joe Subich, W4TV" <[email protected]>
To: [email protected]
Sent: 4/26/2016 10:10:32 PM an
Subject: Re: [Elecraft] Sherwood's receiver performance table updated
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
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