[Elecraft] Sherwood Receiver Test Data - K3S Added Today
Barry LaZar
k3ndm at comcast.net
Tue Nov 10 21:38:40 EST 2015
Steve,
Not having the schematic of the Flex isn't as important as knowing
they are using a 16 bit A/D. In a perfect world, the rule of thumb is
6db/bit, 96 db may be accounted for. However, an A/D that can cover all
the way to 2 meters has to be very fast > 296 msp, if you believe
Nyquist. If you apply hardware AGC in any form, it is reflected across
the entire spectrum. And to add further insult, high speed A/Ds have
noise figures great enough to mask ambient noise at mid and above HF.
That means you need to add a low noise amplifier ahead of the A/D with
enough gain to overcome system noise, and that will start to bite into
those 16 bits by a function of the gain of the amplifier. You can do
some creative things like use tapered gain amplifiers that have a
reverse taper so that max gain is at the high end of the spectrum;
Typically signals and noise are greater at the lower end of HF even if
SNR appear to be greater at the high end.
The use of a 24 bit A/D allows for using a simpler approach and yet
maintain a high dynamic range. It also disallows using A/Ds that can
cover the entire spectrum as they don't yet exist.
Unlike math and physics, EEs are more artists than scientists. I
suspect the discussions at Elecraft over the architectures would have
been very interesting. There are advantages and disadvantages to direct
sampling and to the hetrodyne SDR's. Which is better? It's a function of
how you want to handle the requirement set which may impact price. Why
do I call engineers artists? The sciences are absolute and engineers
have to craft a solution that accounts for the compromises, and few
engineers always agree on the very same solution. One may like red and
another may like blue so the creation comes out as a work of art. The
bottom line is that the chosen implementations of Elecraft radios are
Gee Whiz and exceed what the casual user needs and appears to be the
best for contesting. That last bit is my opinion and I'm sticking with
it.
Just for the record, I use a KX3. It is lower on Sherwood's
rankings, but I think it is the best overall radio out there. It
satisfies a greater number of my requirements. And, that is how I
colored my solution. ;-)
73,
Barry
K3NDM
------ Original Message ------
From: "Steve Ellington" <steven4lq at gmail.com>
To: k6dgw at foothill.net
Cc: "Elecraft Reflector" <elecraft at mailman.qth.net>
Sent: 11/10/2015 8:51:16 PM
Subject: Re: [Elecraft] Sherwood Receiver Test Data - K3S Added Today
>Part of the problem here is that Flex won't release the schematic to
>the
>6000 series so who knows?
>I suspect that the AGC-T (Agc threshold) control actually adjust the
>input
>level to the ADC....somehow.
>This control is manual and must be fiddled with per-band as conditions
>change. Strong signals will sound distorted and you must manually
>compensate often.
>N4LQ
>
>On Tue, Nov 10, 2015 at 8:42 PM, Fred Jensen <k6dgw at foothill.net>
>wrote:
>
>> OK. Remembering that I'm the one who disappointed Mom and Dad when I
>>got
>> A's in Math and B's and C's in EE, and finally transferred to the
>>Math Dept
>> ...
>>
>> I didn't want to imply it was a problem, it was just a question.
>>First
>> off, I believe a direct-sampling SDR is one that essentially does
>>nothing
>> to the entire RF envelope being received except maybe bandpass limit
>>it to
>> the ham band of interest. No guarantee that's correct, and if it
>>isn't,
>> you might as well delete this now.
>>
>> But if it is what direct-sample means, and since the RF envelope is
>>both +
>> and -, in an 8-bit ADC, 127 would be zero, 255 [all 1's] would be the
>> maximum along with all 0's for the negative parts. If the RF
>>envelope is
>> allowed to go above the value that digitizes to all 1's, it will
>>still
>> digitize to all 1's and it's excursions above all 1's are lost [i.e.
>> clipped]. In my experience as a ham, this is rarely if ever a good
>>thing.
>>
>> OTOH, you want as much dynamic range as possible, so you want the
>> strongest input to the ADC be at the all-1's level so the parts of
>>the
>> envelope below that level will digitize to something other than zero
>>[127
>> in my 8-bit example]. Any signals below that level will digitize to
>>127
>> and you'll never hear them.
>>
>> If the gain of the RF stage(s), and I'm assuming there is at least
>>one RF
>> stage, is such that the maximum of the RF envelope is below the
>>clipping
>> point, then it seems to me that signals you might have heard won't be
>>heard
>> because they never got digitized.
>>
>> My question was [and is], do direct sampling receivers employ some
>>sort of
>> AGC to keep the max RF envelope at the clipping point? If the answer
>>is
>> "yes", I have a second question in the wings waiting to be asked.
>>
>> Mom and Dad never recovered from their mathematician's defection from
>>EE,
>> despite having been a wireless addict since age 12.
>>
>> 73,
>>
>> Fred K6DGW
>> - Northern California Contest Club
>> - CU in the Cal QSO Party 1-2 Oct 2016
>> - www.cqp.org
>>
>>
>> On 11/10/2015 5:06 PM, Barry LaZar wrote:
>>
>>> Fred,
>>> When you have a 24 bit A/D and not looking at the entire
>>>spectrum,
>>> this should not be as big a problem as you imply, if gain
>>>distribution
>>> is correct. 24 bits should yield a great enough dynamic range to
>>>cover
>>> greater than ~ 95% of time; that last ~5% covers your friend next
>>>door
>>> with a KW or that thunderstorm over head. 24 bits also allows for
>>>some
>>> amplification to overcome the down stream noise figure that may be
>>> greater than ambient.
>>> On the other hand, the Flex uses a really high speed A/D, but
>>>it is
>>> only 16 bits, if memory serves. That architecture is far more
>>>sensitive
>>> to gain distribution. Those who have suggested that the Flex may
>>>have a
>>> problem with all the bits going to 1 in a stress environment may be
>>>very
>>> correct. IMHO: With the state of the A/D art as it is, I believe
>>> Elecraft has the better practical architecture. As soon as low
>>>noise,
>>> high speed A/Ds become available with greater than 16 bits, my
>>>opinion
>>> may change.
>>>
>>> 73,
>>> Barry
>>> K3NDM
>>>
>>>
>>> ------ Original Message ------
>>> From: "Fred Jensen" <k6dgw at foothill.net>
>>> To: elecraft at mailman.qth.net
>>> Sent: 11/10/2015 7:32:56 PM
>>> Subject: Re: [Elecraft] Sherwood Receiver Test Data - K3S Added
>>>Today
>>>
>>> Question from a mathematics major [whose parents wanted him to be a
>>>EE]:
>>>>
>>>> In a direct-sampling receiver, how do you control the input level
>>>>to
>>>> the ADC to achieve maximum available dynamic range without clipping
>>>>at
>>>> the ADC?
>>>>
>>>> 73,
>>>>
>>>> Fred K6DGW
>>>> - Northern California Contest Club
>>>> - CU in the Cal QSO Party 1-2 Oct 2016
>>>> - www.cqp.org
>>>>
>>>> On 11/10/2015 3:58 PM, Steve Ellington wrote:
>>>>
>>>>> Looks like the K3S should be above the Flex because the Flex
>>>>>achieved
>>>>> 2DB
>>>>> higher due to:
>>>>>
>>>>> Footnote Y --- "This is a testing anomaly of a direct-sampling
>>>>> receiver."
>>>>>
>>>>> Congratulations K3S
>>>>> Steve N4LQ
>>>>>
>>>>
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>>>
>>>
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>>>
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