Topband: Sept/Oct NCJ article on DX Prowess of Receivers

[Bill Tippett <btippett@alum.mit.edu>]

        Here's a copy of a note I sent to SP7HT
regarding his recent article in the Sept/Oct NCJ.

			73,  Bill  W4ZV

Hi Tadeusz,

        I just got my new NCJ and read your
article.  I was surprised by the following statement
(next to last paragraph on page 4):

"These statements mean for me that somebody
unfamiliar with all measurement set-up and
procedures nuance cannot use data derived
"directly" from Swept BDR or IMD DR3 oscilloscope
graphs close to the listening frequency instead of
point results measured manually.  Therefore, I have
consequently used in my Table only manually
measured test point data at 5 kHz and 20 kHz
spacing."

I had believed that ALL discrete points plotted on the
ARRL graphs came from manual measurements, so I
asked Mike KC1SX who does ARRL's measurements.
I see that Mike copied you on his reply which did confirm
that ALL discrete measurement points in the ETR swept
graphs are done manually.  Thus the IMD and BDR
data down to 1 kHz spacings (always the innermost
point in all ARRL ETR plots since 1997) are perfectly
valid for comparison (although sometimes difficult to
interpolate, and I notice Mike has now included both
the 1 & 2 kHz IMD and BDR measurements in tabular
form for the IC-7800 ETR).

        In general, I agree with much of what you
said, but have some differences of opinion in the following
areas, primarily from the viewpoint of a contester (since
your article was also published in the NCJ in addition
to the recent issue of QEX):

1.  5 kHz spacing (BDR and IMD) is far too wide for most
contest interference in my opinion.  I prefer ARRL's 1 kHz
measurements as being much more realistic, especially
for contesters.  When do you last recall a contest with
signals spaced only 5 kHz?  In my opinion, signals
spaced at ~500 Hz is much more common, especially
in the major contests.

2.  By only looking at 5 kHz spacing, several of the
receivers you reviewed appear to be much better than
at much closer spacings.  A good example is the
Elecraft K2.  I have a K2 and like it very much for its
intended use, but the receiver basically falls apart
for close signal spacings.  ARRL shows BDR drops to
~116 dB and IMD drops to ~67 dB at 1 kHz spacings.
Compare this to Orion with BDR of ~119 dB and IMD
of ~84 dB at 1 kHz spacings (ARRL data using the
stock 1000 Hz roofing filter).

3.  Given "proper design" (similar to your "not good"
comment on page 11 about IMD DR2 and IP2), I would
rank IMD DR3 as the most important parameter for
contesters for the following reasons:

a.  Unless you have an extremely close neighbor, or
unless your BDR is terrible, BDR is not likely to be as
significant a problem as IMD DR3.  The reason is very
simply that BDR performance is normally MUCH higher
than IMD DR3.  Thus the latter becomes more critical
since you will reach the IMD DR3 limit sooner.  W8JI
cites an example on 160 below where he sees his
maximum dynamic range needed being around 95 dB,
which most of the receivers in your list exceed at 5 kHz.
As long as BDR is above this number, or unless you have
a very close neighbor or are in a multi-multi operation,
BDR of 95 dB at should be sufficient, although I would
add that this performance should be maintained at 1 kHz
or less instead of 5 kHz for the reasons cited in #1 above.

http://www.w8ji.com/receiving.htm

b.  Phase noise also becomes critical at very close
signal spacings.  An example of poor phase noise
performance is the IC-7800.  At close spacings of
1 and 2 kHz, you can see in ARRL and RSGB results
that Phase Noise overrides the receiver's IMD and
BDR performance (note asterisks by measurements
on pages 17 & 18 of ARRL's ETR).  As KC1SX
describes on page 34 of his August 2004 QST article,
Phase Noise becomes effective BDR (or IMD) on a
noise limited measurement.  I fully agree with this
definition since it makes no practical difference
to the operator whether the source of the noise is
due to BDR, IMD or Phase Noise.  If Phase Noise
performance is poor, then it becomes much more
important than either BDR or IMD DR3 since it will
over-ride those effects at close spacings.  Assuming
the receiver's Phase Noise is always below IMD and
BDR performance, then it is not so important.

c.  Assuming Phase Noise is below both BDR and
IMD DR3 effects, and assuming BDR is "adequate"
for your operating conditions, I feel that very close
spaced IMD DR3 (1 kHz or even less) becomes the
critical parameter for contesters.  This is mainly because
it is not uncommon to have many strong signals spaced
at 500 Hz (or even less) in a contest capable of creating
IMD DR3 products.  This was my primary motivation
in working with Inrad to develop the 600 Hz #762 filter
for Orion.  At this BW, even 650 Hz spaced signals
will fall just at the -30 dB BW of the #762, hence the
resulting IMD products should be attenuated by ~90 dB,
and we can even improve on that by using PBT (note
that these estimates for IMD at spacings <1 kHz have
not yet been confirmed by third-party measurements).

http://users.vnet.net/btippett/inrad_.htm

        As a contester, I rank very close-spaced IMD DR3
performance as my #1 criteria, as both W8JI and Sherwood
Engineering do.  Granted, this assumes that both BDR and
Phase Noise are "adequate" as described above.  W8JI has
some very practical advice below which relates to what levels
of MDS, BDR and IMD DR3 are "good enough".

http://www.w8ji.com/receiver_tests.htm

        I realize that contesters' needs are slightly different
than DX'ers, which was the primary focus of your article, but
even DX'ers sometimes experience DX stations that only
listen 1 kHz away (or less!), which places a premium on
receiver performance at spacings much closer than 5 kHz.

				73,  Bill  W4ZV

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