Re: K3 Filter IMD numbers

[Bill W4ZV]

W3WPR:

 >I believe you have stumbled onto something that should be carefully
defined when testing with wide roofing filters when the DSP IF is set to
the more typical testing bandwidth of 500 Hz.   The K3 has both IF shift
and narrow passbands available in DSP and they can be effectively used
together.  Then we hear about the 'narrow filtering with wings' in a
recent post and that would further complicate matters.

         ARRL has been doing testing to 1 kHz spacings since at
least 1998, although these measurements have appeared only in
their Expanded Test Reports until very recently (when they began
publishing 2 kHz measurements in QST reviews).  The test procedure
has always specified using any filters "closest" to the standard
500 Hz bandwidth.  This is to eliminate differences due to the
noise bandwidth of the filter selected.  With the advent of DSP,
I believe ARRL recognizes the problems that can arise when DSP
can "modify" the filter shape on demand.  I believe they just
proposed the following in response to this new capability (new
test methodology described in October QST):

**************************************************************
Equivalent Rectangular Bandwidth (ERBW)

 From time to time, members ask why
manufacturer's sensitivity specifications
are given in microvolts but our measurements
are reported in dBm (decibels relative
to a milliwatt). The chief reason is that
manufacturers typically do not include a
bandwidth in their specification, and measurements
in different bandwidths are not
directly comparable. All other things being
equal, there is more noise power in a 3
kHz bandwidth than a 2.4 kHz bandwidth.
To overcome that limitation, the sensitivity
testing is done with a 500 Hz bandwidth
filter, or as close to that as is available.
This permits reasonable comparisons of
different receivers.

With different filter types and with a
range of filter skirt slopes (see Figure D),
even two "500 Hz" filters are not exactly
the same. Additionally, some receivers
may only have a choice of 400 Hz or
700 Hz widths. Although the filters that
a receiver has cannot be changed, the
variation in actual bandwidth can be determined
by calculating the Equivalent Rectangular
Bandwidth (ERBW). This is the
width that the filter would have if it passed
the same noise power and possessed the
"ideal" shape of vertical sides and a flat
passband response.
***********************************************

 >It seems to me that a lot of the DSP setup characteristics must be
defined for any test to assure test repeatability.  I can envision test
setups to make the 2 kHz spacing 'numbers' for the wide filters look
good or bad depending on the rest of the parameters.  That may be why
the list posted by Eric had 'n/a' for the 2 kHz number for the wide
filters.  For instance when both signals are within the roofing filter
bandpass, the results will depend only on the DSP and based on the
number of bits used in the input DAC, the BDR should be in the vicinity
of 90 dB, but very different (and better) numbers can be obtained by
placing one signal inside and the other outside the roofing filter - so
I conclude that the receiver setup details must be better defined for
any test using 2 kHz spacing.

         2 kHz (and also 1 kHz) data is still very useful
even when interfering signals fall inside a roofing filter.
After all, this has been the case for most up-conversion
radios with 10-15 kHz roofing filters produced over the
past 25 years.  I interpreted Eric's "n/a" simply as meaning
they did not have the time or interest to test it since the
K3 has the possibility of very narrow roofing filters which
would be better suited to 2 kHz spacings.  However, I am
quite certain that ARRL, RSGB, Sherwood and others will provide
2 kHz test data, even with the wider filters, as they have
already been doing for many years.  This will be interesting
data anyway since it will give us a very good measure of the
quality of the K3's IF chain beyond the roofing filter.

                                 73,  Bill  W4ZV

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com