[TenTec] FW: Corsair vs Corsair II

Wed Mar 13 04:42:20 EDT 2013

PLEASE DISREGARD MY PREVIOUS POST, WHERE I THOUGHT A BROADBAND TERMINATION
WAS NOT NECESSARY.

Guys, in my previous post on this topic, I WAS WRONG. :-(   
(sri)

Jerry was kind enough to point that out to me off-line, and showed me where
I could read more... where I discovered that my 1970's state of the art
mindset on this topic is well behind the power curve.

Looks like this old dog needs to go back to school and learn some new tricks
about RX technology.
I will switch into "READ-ONLY" mode for the rest of this thread.

JERRY'S RESPONSE TO ME IS BELOW...

THANK YOU JERRY!

73
Rick, DJ0IP

-----Original Message-----
From: Jerry Haigwood [mailto:jerry at w5jh.net] 
Sent: Wednesday, March 13, 2013 3:33 AM
To: 'Rick - DJ0IP / NJ0IP'
Subject: RE: [TenTec] Corsair vs Corsair II

Hi Rick,
    If you have ever looked at the output of diode ring mixer using a
spectrum analyzer, I doubt if you would have made the statement, "So as I
see it, it is not so important to have "broadband termination" as it is to
have "TWO specific terminations", one for the desired signal and one for the
unwanted signal."  There are many signals present at the output of a diode
ring mixer - not just two.  All of these signals need to be terminated
properly.  That is why diplexers are broadband.  Take a look at VE7BPO's
page, especially figure 9:
<http://www.qrp.pops.net/sbl1.asp>
Todd gives a pretty good description of what the diode ring mixer does and
how it does it.  He gives you a spectrum analyzer view of the output of a
diode ring mixer (figure 9).  Todd also discusses diplexers here:
<http://www.qrp.pops.net/dip2.asp>
I prefer the Bridge-Tee diplexer designed by Joe Reisert, W1JR.  It is easy
to build, broadband, and works very well at terminating a diode ring mixer.
Jerry W5JH
"building something without experimenting is just solder practice"

-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Rick -
DJ0IP / NJ0IP
Sent: Tuesday, March 12, 2013 8:11 PM
To: 'Discussion of Ten-Tec Equipment'
Subject: Re: [TenTec] Corsair vs Corsair II

The biggest problem with mixers feeding directly into a crystal filter is,
the mixer always has TWO outputs (the sum and the difference frequencies).
One of them is the desired frequency (i.e. 9 MHz), the same as the crystal
filter, and the second one is on another unwanted frequency.  

The crystal filter has a design impedance which should be designed to match
that of the output of the mixer, but it is only that impedance on its design
frequency (i.e. 9 MHz). It terminates the desired frequency just fine,
but...it's this second signal that causes the problem, because it is not
properly terminated by the crystal filter. This lack of proper termination
for this one specific signal causes standing waves at that point.  

So the culprit is specifically the standing waves caused by improper
termination of the unwanted signal, which may causes ~30 dB of degradation
the IP.

The best way to deal with this is with a "diplexer", which is a splitter
which separates the two signals, routes the wanted signal to the crystal
filter (where it is properly terminated) and routes the unwanted signal to a
tuned circuit or trap, resonant on its frequency.

The diplexer is inserted between the mixer output and the crystal filter and
traps the unwanted signal, preventing it from reaching the crystal filter,
thus preventing the standing waves and maintaining a high IP.

So as I see it, it is not so important to have "broadband termination" as it
is to have "TWO specific terminations", one for the desired signal and one
for the unwanted signal.  

Broadband termination is also a way of addressing this, and it also works,
but it is not nearly as effective as using a diplexer.

Les Hayward, W7ZOI popularized this concept in the mid 1970s.  
If I dig deep enough in my files, I can probably find one of his articles
explaining this, which as I recall, were published in Ham Radio Magazine.

BTW, as I recall, Ten-Tec introduced a diplexer with its first OMNI but that
was long ago and my old grey cells may be failing me on that point.  Easy
enough to check if someone has an Omni schematic handy.

73
Rick, DJ0IP

-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Barry N1EU
Sent: Tuesday, March 12, 2013 11:54 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Corsair vs Corsair II

On Tue, Mar 12, 2013 at 10:32 PM, GARY HUBER <glhuber at msn.com> wrote:

> Mike,
>
> You are welcome. I regret that I cannot locate the document where I 
> originally saw the reference to Corsair / Corsair-II having ANZAC 
> receiver design and later commentary on later TEN-TEC receiver design 
> being changed to avoid Copyright or similar legal issues.
>

Here you go, posted 2007 to this list by Jerry K0CQ.

73,
Barry N1EU

The bipolar RF (and first IF) stage uses transformer feedback with a three
winding transformer. Its a circuit developed for the output stages of cable
TV distribution amplifiers where acceptable intermod is 70 dB down and in
Ulrich Rohde's book on receiver design he sees nothing close to having its
performance. A double gate MOSFET doesn't come close. I've run Kenwood with
MOSFETs and those MOSFETS don't come close to the strong signal handling of
my Corsair II.

That circuit is covered by an Anzac patent or two and that may be why Tentec
went to the grounded gate JFET circuit in the Omni V and VI.
Those use four JFETS in parallel with individual source resistors to make
them balance better. I've not seem their performance compared directly to
the Anzac circuit, but the Omni V and VI seem to do quite well in the
intermod department.

Schottky ring mixers are hard to beat and the higher the LO power (so long
as the mixer is designed for it) the better the strong signal performance.
Its also important that at least two of the three ports of the ring mixer be
terminated over a wide range of frequencies, else NF, mixer loss, and
intermod can be a lot worse. The mixer output often sees a crystal filter
and that is the worst thing that can be done, one book on mixers says that
can cost 30 dB intermod range. The Corsair II, Omni V and VI use a broad
band IF stage with lots of dynamic range to provide that IF port broad band
load and it shows. It works.

Don't know about the Omni V and VI, but the "attenuator" switch on the
Corsair II actually removes the RF stage from the circuit. There is no
attenuator.
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