[Elecraft] Sherwood on ARRL Testing Methodology

[Bill Tippett]

W5WVO:
 >I don't know what the test configuration of the SDR-5000 was -- I 
don't think
it uses any discrete hardware roofing filters, does it? -- but Elecraft is
already claiming that the IMDDR3 at 5 kHz spacing (400 Hz CW roofing filter)
will be greater than 100 dB. (See K3 Specs page) I don't believe Eric would
have let that spec be on a public web page unless he was pretty certain it
would still be true when the final numbers come out.  :-)

         The QSD design of the SDR-1000/5000 is completely different than
that used by the Orion/K3, so it does not need roofing filters.  Specs
like IMD/BDR are determined by the resolution and linearity of the ADC
in the sound card.  Although Sherwood did not publish 2 kHz
measurements, he did say the IMD/BDR performance is basically
independent of signal spacing, so 2 kHz IMD performance is
likely 96 dB also.  BDR at 100 kHz was measured at 123 dB,
which will also likely apply at 2 kHz.

         Phase noise was reported as:

Phase noise (normalized) at 10 kHz spacing:    123 (flat) dBc *
* Phase noise does not fall off at 6 dB per octave as expected.  Flex 
believes the present phase noise limitations are caused by A/D clock jitter.

By contrast the K3's phase noise is:

Rig     1kHz    2   10   20   50   100   1M
K3      -110  -119 -136 -140 -143 -144  -150
Bottom line is that QSD-based SDRs are very different beasts, so
our assumptions based on classical designs are probably wrong.

         It's also dangerous to assume Elecraft's measurements
will be identical to ARRL/Sherwood.  There are often differences
due to different test methodologies, people and equipment.  For
example Ten-Tec maintains Orion's 5 kHz IMD spec is "101 dB typical",
but nobody else has ever measured more than about 95 dB, so I
would wait for independent measurements on the K3.

                                 73,  Bill  W4ZV