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 <k6...@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" <k6...@foothill.net> >> To: elecraft@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 >>>> >>> >>> ______________________________________________________________ >>> Elecraft mailing list >>> Home: http://mailman.qth.net/mailman/listinfo/elecraft >>> Help: http://mailman.qth.net/mmfaq.htm >>> Post: mailto:Elecraft@mailman.qth.net >>> >>> This list hosted by: http://www.qsl.net >>> Please help support this email list: http://www.qsl.net/donate.html >>> Message delivered to k3...@comcast.net >>> >> >> >> ----- >> No virus found in this message. >> Checked by AVG - www.avg.com >> Version: 2016.0.7227 / Virus Database: 4460/10976 - Release Date: 11/10/15 >> >> >> >> >> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to steven...@gmail.com > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
