[Elecraft] RSGB RadCom K3 review posted on RSGB Members web page

[Jan Erik Holm]

Yes something isn´t right. This was his measurements:

CLOSE-IN INTERMODULATION ON 7MHz band,500Hz bandwidth, CW preamp off

  2.7 kHz roofing      1.8 kHz roofing   500 Hz roofing
2kHz  +19dBm 101dB    +12.5dBm 96dB    +2.5dBm   88dB
3kHz  +19dBm 101dB    +12.5dBm 96dB    +2.5dBm   88dB
5kHz  +22dBm 103dB    +12.5dBm 96dB    +8.5dBm   92dB
7kHz  +22dBm 103dB    +12.5dBm 96dB    +13dBm    95dB
10kHz +22dBm 103dB    +12.5dBm 96dB    +17.5dBm  98dB
15kHz +22dBm 103dB    +12.5dBm 96dB    +22dBm   101dB
20kHz +22dBm 103dB    +12.5dBm 96dB    +25dBm   103dB
30kHz +22dBm 103dB    +12.5dBm 96dB    +26.5dBm 104dB
40kHz +22dBm 103dB    +12.5dBm 96dB    +27dBm   104dB
50kHz +22dBm 103dB    +12.5dBm 96dB    +27dBm   104dB


He also wrote this:

Third order intercept and dynamic range
results were excellent, but results were a
little dependent on which roofing filter was
in circuit (see table). With the stock 2.7kHz
filter, the intermodulation limited dynamic
range held at over 100dB (in 500Hz DSP
bandwidth) down to 2kHz spacing,
probably the highest figure I have ever
measured at this spacing. However, with
the 500Hz 5 pole roofing filter, a
degradation was seen at close spacings and
non-linear effects such as hysteresis were
also observed. A step drop in performance
was triggered as the spacing was reduced,
which did not follow the reverse pattern as
the spacing was again increased.


Now if something similar did happen in the Orion case
a measurement error can be suspected. It isn´t logical
to think two bad filters.

de SM2EKM
.............................................................


Bill W4ZV wrote:
> 
> Jan Erik Holm wrote:
>> Interesting review.
>>
>> Considering IMDDR3 it seems like one should stay
>> way from the 500 Hz filter, looks like the 400 Hz
>> is the filter to get.
>>
> 
> I haven't read the review but he must have had a bad 500 Hz filter from what
> I've heard reported (i.e. the 2.7k better than the 500 Hz).  That makes no
> sense and is contradictory to previous measurements by both Elecraft and
> Sherwood:
> 
> Elecraft:
> 
> Filter            20kHz  10kHz  5kHz  2kHz
> 200 Hz, 5 pole    100+   100+   100+   95
> 250 Hz, 8 pole    100+   100+   100+   95
> 400 Hz, 8 pole    100+   100+   100+   95
> 500 Hz, 5 pole    100+   100+   100+   94
> 1 kHz, 8 pole     100+   100+   100    94
> 2.7 kHz, 5 pole   100+   98      92    n/a
> 2.8 kHz, 8 pole   100+   100     93    n/a
> 
> http://www.zerobeat.net/mediawiki/index.php/K3_Roofing_Filters
> 
> 
> Sherwood:
> 
> Filter - IMD @ 2 kHz
> 500 Hz - 95 dB
> 400 Hz - 96 dB
> 200 Hz - 101 dB*
> 
> *I listed the 200 Hz because it's a 5-pole filter (just like the 500 Hz) in
> case some believe there is an inherent problem with 5-pole filters.
> 
> http://www.sherweng.com/table.html
> 
> 
> It's odd but Peter Hart reported something similar when he did Orion's
> review.  The 2.4k filter was much better than the 1.0k, which again makes no
> sense unless the 1.0k was defective.  I'm sure Peter reported what he
> measured but common sense would dictate that one should suspect something
> was wrong with the filter and request another when anomalous results like
> this are measured.  It also reminds me when ARRL once reported better IMD
> performance with Preamp ON versus Preamp OFF.  Common sense would say
> "TILT"!
> 
> 73,  Bill  W4ZV