[TenTec] OT: IP3 and Receiver Performance

[rick@dj0ip.de]

This is in response to Gary's comments on IP3.

 

"I do recall, however, his rather strident statement that IP3 is the most
important trait in receiver performance which is why the radios are listed
in the order in which they are listed, based only on close in IP3." 

 

The short answer is, take a look at the  "TIME STAMP"  on the origin of that
statement.  It is, as Gary himself acknowledges, 39 years ago.

In 1977 Rob published an article in "Ham Radio Magazine" explaining the
importance of close in IP3 and dynamic range.

This became the defacto standard for many years to follow.

It is obvious that all of us have moved on since then.

 

Some of you old-timers are aware of Rob Sherwood's work in the 2nd half of
the 1970's.

That was on your side of the pond.

 

On this side of the pond we had similar work going on.

 

Thomas, DL7AV published a similar article in our German CQ DL magazine in
the same timeframe that Rob published his.

Thomas was my partner in our CW field day contesting, with a 3rd member,
Joerg, DL7HE filling in occasionally when we took breaks.

Joerg was the head of Heinrich Herz Institute in Berlin, Thomas was an
engineering student and I was a technician working in the Rhode und Schwarz
lab in Berlin.

 

In order to measure IP3, you needed 2 very clean RF generators, each costing
about $100K.

I had one in my lab and Joerg had one in his lab.  Ours labs were only a few
hundred yards apart.

On weekends we would meet in one of the two labs and measure receivers using
our two generators and a home brew 50 Ohm splitter.

 

At the same time, Michael, DJ7VY published a method to measure IP3 using two
home-brew Xtal oscillators. It was low cost but only measured on one band
(20m).  We were measuring on multiple bands. 

 

At that time there were 3 lists floating around:

.        Rob's List

.        Michael's List

.        Joerg & Rick's list

 

For the record, Rick knew very little about all of this at that point in
time.  I was a part of the team mainly because I had a $100K signal
generator!  ;-)  

However I learned an awful lot just tagging along for the ride.

 

All of the above was going on by ham radio operators in their free time,
with no funding from anyone.

I am unaware of any similar work on the commercial front but surely there
was lots of it that we were not aware of.

After all, we had no internet to communicate with back then.

 

We published IP3 specs because that was pretty much the only thing we could
do, semi-easily.

 

The lists were on paper and  our lists are long lost.

Rob kept his up, and when he returned from his several years sabbaticals
away from ham radio, he published his list on his web site.

The list was based on Dynamic Range 3 Ranking.

Over the years Rob kept adding to the list.

 

However, as I have said many times, Rob's "List" is just one tiny piece of
information.  For each radio there is a long multi-page review showing ALL
measured results, followed by his subjective appraisal based on long
on-the-air tests.

See:  http://www.dj0ip.de/sherwood-forest/sherwood-xcvr-tests/ 

 

When the SDR radios came along, the better ones placed high on the list, yet
were crunching in the field, especially in multi-transmitter operations.  It
was immediately apparent that DR3 was not suitable for ranking SDR radios.

 

If not DR3, then what should we use?

 

IMO there is no longer any easy way to rank them based on one or two
specifications alone.

Too many factors affect SDR receiver performance. 

 

I am in favor of removing all SDR radios from the list and placing them in a
separate list, but I have no idea how we should do this.

My best guess would be to use NPR testing such as Adam Farson is conducting.

See: http://www.ab4oj.com/test/docs/npr_test.pdf 

 

Adam has two lists, one for traditional heterodyning radios and another for
SDR radios.

But this is not perfect either.

As you see, the 7300 tops the list and as we all know, it has overload
problems due primarily to improper gain distribution throughout its front
end stages.  Adam, Rob, and every test review I have read point out its
overload problem.  

 

Note that the two ANANs are ranked above the 6700, even though they do not
have dedicated BPF's for each ham band.

I'm not sure how Adam is ranking the SDR radios.  It is not only NPR
figures.  

 

At this point we are way outside (above) my pay grade.  I have no idea other
than to put these radios in the hands of contesters and let them report on
their experience.

 

In summary, we have come a long ways since first reporting on IP3 in the
1970s. 

We cannot compare SDR radios to heterodyning radios based on their IP3 or
DR3 results.

At this point in time, I am not aware of any agreed method of testing and
reporting performance levels of SDR radios.

There are ongoing discussions between Rob, Adam, Bob Allison (ARRL) and Ken
(ex Ten-Tec president) on how to do this.

 

ON THE BRIGHT SIDE, this new technology has brought us great improvements in
performance and as we will soon see, at an affordable price.

The 7300 was just the beginning.

 

73

Rick, DJ0IP