[Elecraft] Analog vs. Digital Front Ends

Tue Sep 15 20:40:18 EDT 2015

> - According to the ARRL, the Flex 6700 has 14 dB worse preamp-off
> BDR than the K3.
>
> - If you turn the Flex's preamp ON, the BDR is probably 24 dB (or
> more) worse than the K3's. But if you turn the preamp on the 6700
> OFF, its 2-kHz IMDDR3 is 8 dB worse than the K3's. You can't win

Here are the comparisons I pulled from ARRL's reviews of the Flex-6700,
Flex-6300  (April 2015) and K3 (January 2009) and VA7OJ/AB4OJ's noise
power ratio testing (http://www.ab4oj.com/test/main.html#NPR):

ARRL Labs shows the *blocking dynamic range* of the Flex-6700 to be
much *worse* than the K3 at all spacings with the preamps off.   I've
added the MDS for comparison.  Note:  ARRL tested the original K3 with
KSYN3 *not the new KSYN3A*.

(preamp off)	20 KHz	 5 KHz	 2 KHz	 MDS
Flex-6700 	126 dB	126 dB	126 dB	-119 dBm
Flex-6300 	127 dB	127 dB	126 dB	-119 dBm
K3		142 dB	140 dB	139 dB	-130 dBm

(preamp on)	20 KHz			 MDS
Flex-6700 (+20)	130 dB			-135 dBm
Flex-6300	127 dB			-128 dBm
K3 (+10)	138 dB			-138 dBm

IMDDR3 is difficult to compare since ARRL did not report 2 KHz IMDDR3 with
the preamp turned on.  This means that the Flex MDS is more than 10 dB 
worse
than the K3.  Note that the Flex-6700 drops by nearly 9 dB at 20 KHz if
the preamp (+10 dB) is turned on.

IMDDR3		20 KHz/Off	20 KHz/On	5 KHz/Off	2 KHz/off
Flex-6700	103 dB		 94 dB		103 dB		103 dB
Flex-6300	 92 dB		 89 dB		 92 dB		 92 dB
K3		106 dB		103 dB		105 dB		103 dB

Using a different test protocol (Noise Power Ratio), AB4OJ/VA7OJ reports:

Noise Power Ratio	160M	 80M	 60M	 40M
Flex-6700 no preamp	71 dB	73 dB	75 dB	74 dB
Flex-6700 +20 dB	76 dB	71 dB	71 dB	70 dB
K3 no preamp		82 dB	82 dB	82 dB	not measured
K3 Preamp On		83 dB	82 dB	80 dB	not measured

Note: the Flex was tested with 10 to 15 less noise power signal than K3 to
prevent ADC clipping.

73,

   ... Joe, W4TV

On 9/15/2015 6:28 PM, Jim Brown wrote:
> On Tue,9/15/2015 1:52 PM, Tony Estep wrote:
>> On Tue, Sep 15, 2015 at 3:36 PM, Joe Subich, W4TV <lists at subich.com>
>> wrote:
>>
>>> ....noise power ratio testing hides the fact by not providing MDS
>>> values under each test condition *and* fails to indicate that even
>>> with *no preamplifier* the total noise signal is more than 10 dB
>>> *less*...
>> =========
>> I take this to mean that the test results are not comparable across
>> radios, and the SDRs are 10db worse than they show up on the
>> comparison chart. If this is true, it ain't good.
>
> You've got the concept right -- the only difference is that 10 dB is not
> a "hard" number -- it could be anything from 5 to 30 dB, depending on
> how much test signal strength has been turned down, or preamp gain
> removed, to get the input of the digital system out of clip!
>
> Here are some comments by an engineer i trust.
>
> 73, Jim K9YC
>
> =   =   =   =   =
>
> - According to the ARRL, the Flex 6700 has 14 dB worse preamp-off BDR
> than the K3.
>
> - If you turn the Flex's preamp ON, the BDR is probably 24 dB (or more)
> worse than the K3's. But if you turn the preamp on the 6700 OFF, its
> 2-kHz IMDDR3 is 8 dB worse than the K3's. You can't win
>
> This all stems from the poor NF of their ADC and its low input voltage
> range. The MDS of the Flex is only -118 dBm (according to Sherwood) with
> the preamp off. (A K3's MDS with preamp off is -134 dB.) So you have to
> turn the Flex's preamp ON to get good MDS. The 2-kHz IMDDR is then good,
> but the BDR is terrible. Sherwood was so baffled by the weird responses
> he saw that he simply put "A/D Limit" in the BDR column for the Flex.
> He's still thinking about how to test it.
>
> ARRL measured Flex's BDR, but only using the conventional method. To
> properly assess the damage caused by an ADC at the front end, you have
> to inject MANY signals into its input to give them a chance to combine
> in phase and hit the ADC's clipping threshold. And ADCs do NOT
> gracefully degrade.
>
>
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