[Elecraft] K4 DSP bit width

[W2xj]

The K4 is a direct sampling radio. It has no IF. It samples directly to IQ baseband and performs all filtering and processing in the digital domain. That requires a very high speed ADC. In this case it is just above the Nyquist limit for 6 meters. Getting a 16 bit converter that fast approaches the limit of available technology. These converters are pipeline architecture and have better than 90 db dynamic range.  Looking at S units, there is 88 db from S1 to 40 over S9 before any attenuation or AGC is applied prior to  the ADC. The problem comes when there are signals exceeding that dynamic range like working someone who might be S1 or S2 while someone else is transmitting nearby as a direct conversion radio has no front end selectivity. As Eric already explained, this is the reason for the K4HD option. This appears to essentially be a K3S superhet single conversion front end that feeds an approximate 8 MHz IF into the ADC. It would then provide a very narrow band reception option for the K4.

Looking at K3s architecture, the ADC is a PCM1804 which is a delta sigma converter which is basically a one bit ADC clocked at 768 times the sample rate or somewhere between approximately 24 and 36 MHz depending on what sample rate Elecraft selected. In the mode used in the K3S it can sample up to 48 KHz although there are other modes permitting up to 192 KHz. The one bit sample is decimated to 24 bits. This ADC is a fairly typical high end stereo audio converter.

The two approaches are very different and each fulfills a different need. Fortuneately, the K4 can provide both with the right options installed.


Sent from my iPad

> On May 29, 2019, at 1:08 AM, Bill Frantz <frantz at pwpconsult.com> wrote:
> 
> A friend and I were discussing the K4HD and he said that it sounds a lot like the K3S. Well, we both agreed that since the K3S has world class receiver specs, that would not be a bad thing.
> 
> I started comparing the information about the K4HD with the K3S manual and found:
> 
> K3S       K4HD   Feature
> 8.215MHz  ~8MHz  First IF
> 15KHz      ?     Second IF (if used in the K4)
> 24         16    ADC width
> ~30KHz?   122MHz ADC sample rate (WAG for the K3S)
> 5         2+1    Crystal filters (one empty slot is from a post on this list.)
> 
> 
> Note: In the K4 FAQ there are forward references to a K4HD section, but I couldn't find it on the web site.
> 
> 
> Analysis and wild speculation
> 
> Both DSPs and ADC technology have had at least 10 years to improve between the K3S and the K4. WHile I don't track ADC tech, DSPs are computers and we all know what has happened in computers. The basic processors are somewhat faster, and there are many more cores on a chip. The K4 certain to have taken advantage of these improvements.
> 
> The K3S uses a 32 bit floating point DSP, and I don't see any reason to change that specification for any of the K4 models. Not changing means that much of the K3 DSP code should easily port to the new DSP.
> 
> I don't understand how the basic K4 can get good dynamic range with a 16 bit DAC. The K3's 24 bit DAC seems a better choice, although getting high speed and wide bit width at the same time is hard and expensive.
> 
> In the wild speculation department, there seem to be several approaches for the K4HD. (1) Run a K3 like superhet with an ~8MHz IF and digitize that into the DSP. (2) Duplicate the K3's 2nd IF and use the 16 bit ADC at 15Khz. (3) Use a wider ADC at either 8MHz or at 15KHz. Note that one of the features of the K4 is ease of upgrade to new ADCs.
> 
> 73 Bill AE6JV
> 
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