EMRFD Message Archive 157

Message Date From Subject
157 2006-09-21 09:05:02 Stan H-mode mixers, etc.
This may be slightly OT for this list, but I believe most following
a list for EMRF text book would also really be into building.

If you have not noticed over the past couple of years the mixer
technology available and low cost components to implement H-mode has
just exploded.

There is one group on Yahoo who has been adding just about
everything in print on the subject to their files section (qrsstxrx
ยท qrss tx/rx)

While much of the actually building has been in just a couple
transceivers i.e. CDG2000 and a few DC QRP designs the application
of H-mode has numerous applications.

We the homebrewer now have the ability to build and modify receivers
that are not only the state of the art, but may be close to the
ultimate in what can be achieved.

If you read QEX a good reference is Jul/Aug 2004 issue.

Anyhow I thought if you are not aware of this emerging technique you
might as a RF designer find it very interesting and look forward to
the future designs and even modifications of our existing ham gear.

Cheers, Stan ak0b
158 2006-09-21 13:27:02 Wes Hayward Re: H-mode mixers, etc.
Hi Stan, et al,

No, I don't think this is not at all off topic. It is right in
there.

It is always difficult to trace "invention." The moment you think
you know of the source of the idea, someone else finds that the idea
was kicking around earlier and may even have been well understood
and appreciated. This is often true with mixers and may even be
true with the H-Mode mixer. Having said that, I first encountered
the H-Mode mixer as being the creation of Colin Horrabin, G3SBI.
That introduction occurred some 10 or 12 years ago. The work was
published in Pat Hawker's wonderful Technical Topics column in
RadCom.

The name (a presumption on my part) comes from the use of three
transformers. The schematic can be drawn so that the cores of the
three wideband transformers look like an upper case "H," as is seen
in Fig 5.37 on p5.15 of EMRFD.

We can think of this circuit as being a "mapping" from a FET ring,
much like a diode ring, into a configuration where the 4 FET sources
are all grounded. In a sense this is a FET version of the
old "STAR" mixer that used four diodes, all connected to ground. (I
don't have a reference on the star topology. I remember seeing it
in some early Tektronix spectrum analyzers.) Transformers driven by
a local oscillator then cause the 4 diodes in a STAR to conduct in
pairs to create a commutating action. Additional transformers
and/or taps on the existing coils serve to inject RF and extract an
IF output.

The H-Mode mixer is not related to new technology. The early
experiments, and still the best performers that Colin has measured
with IIP3 > +50 dBm, used the same FET quad that Ed Oxner used in
his well publicized FET ring mixer work, the Siliconix Si8901A.
The more recent work seems to have emphasized the Fairchild FST3125,
which is an inexpensive MOSFET quad with built in drivers. The
most recent work that I've heard about indicates that the mixer is
more tolerant of port termination quality than is a diode ring.
The H-Mode mixer is featured in the "Triad" receiver of Chapter 6 of
EMRFD. This is the collaborative effort of W7AAZ, W4ZCB, and
G3SBI. Much of that work has moved to the STAR transceiver, a SDR
effort predominantly in the UK. From what I have heard, the
performance of these radios is as good as, if not better than any of
the top of the line appliances. All of them (appliances and
homebrew) still seem to be limited by synthesizer performance.

About any quad of FETs of about any type are good candidates for
this mixer. It seems that some parts are disappearing while other
manufacturers are bringing replacements to the market place. This
leaves a lot of opportunity for the experimenter.

There are many interesting mixers that use FETs in switching
modes. They all seem to go back to one seminal microwave mixer
described by Steve Maas, "A GaAs MESFET mixer with Very Low
Intermodulation," IEEE Transactions on Microwave Theory &
Techniques, MTT-35, No. 4, April, 1987. This mixer just used a
single FET, but still offered stellar IMD performance with the same
gain and noise figure found with a diode ring. Some workers
expanded this grounded source mixer into balanced forms. The single
FET version has been recently rediscovered and named the "Y" mixer
by one semiconductor manufacturer. They seem to have missed the
prior work, including another manufacturer who has used this
topology for years to appear in what some estimate to be half a
billion (yes, with a "b") cell phones. Ah, the wonders of
marketing!

Thanks Stan for telling us about the other groups that are
discussing these problems and solutions. It's always good to have
more sources of information.

73, Wes, w7zoi
160 2006-09-21 22:09:39 LB Re: H-mode mixers, etc.
The Star transceiver has an active group on Yahoo
"PICASTAR" with loads of information including the 'H'
Mode.

Lou wb4wrr/ei2je


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161 2006-09-22 11:15:56 w4zcb77 Re: H-mode mixers, etc.
(I
> don't have a reference on the star topology. I remember seeing it
> in some early Tektronix spectrum analyzers.) Transformers driven by
> a local oscillator then cause the 4 diodes in a STAR to conduct in
> pairs to create a commutating action. Additional transformers
> and/or taps on the existing coils serve to inject RF and extract an
> IF output.
>
> The H-Mode mixer is not related to new technology. The early
> experiments, and still the best performers that Colin has measured
> with IIP3 > +50 dBm, used the same FET quad that Ed Oxner used in
> his well publicized FET ring mixer work, the Siliconix Si8901A.
>
> 73, Wes, w7zoi
>
The major contribution that Colin made to the science was the
topology. The commutating mixers go all the way back to a ZS in the
late 60's who used a CD4044 as a mixer. (And prolly before that
somewhere in the dust of history) Colins "H"-mode is the first I'm
aware of that takes the signal path out of the switching path and
thereby prevents a high level incoming RF signal from modifying the
switching function.

While Darsbury Labs has the intellectual rights to the design, it has
been released to the public domain.

Regards
W4ZCB
162 2006-09-22 16:20:52 jr_dakota Re: H-mode mixers, etc.
What I really find interesting about the Pic a Star's H mode mixer is
it uses a 1x LO while all the other H mode mixers I've seen (Including
the ones in EMRFD) use a 2x LO ... there is also some great stuff on
making the DDS IC's as spur free as possible including the use of a
"CU Sandwich" mounting that outperforms any double sided PCB design
... lots of good stuff on that forum

In fact I'm in the middle of building an original Pic n Mix DDS LO for
a self designed radio (Just got the rest of the parts I didn't already
have on hand about 15 minutes ago) and am looking to make a Pic a Star
for use as a baseline to compare against my own superhet designs ...
plus it gives me an excuse to get the KK7P DSP module

Since I got a dual 9952 circuit for use in my SDR testbed (If I can
program the dang thing right) I'm also going to build an R2-Pro
receiver using AA0ZZ's IQ-VFO .... I'm just kinda waiting for Craig to
show us his new front end design he's working on (hint hint) so I
don't have to do all the preamps and band filters twice ... this will
be a baseline for other direct conversi
163 2006-09-23 02:33:56 Craig Johnson, AA... IQ-VFO front end (was H-mode mixers, etc.,)
Hi JR,
164 2006-09-23 04:42:12 jr_dakota Re: IQ-VFO front end (was H-mode mixers, etc.,)
165 2006-09-23 08:03:21 Stan Re: H-mode mixers, etc.
JR tells us more about the CU Sandwich, any references, etc.
Thanks, Stan ak0b

166 2006-09-23 08:42:50 James Duffey Re: H-mode mixers, etc.
Wes - This caught my eye in your recent post:

"They all seem to go back to one seminal microwave mixer
described by Steve Maas, "A GaAs MESFET mixer with Very Low
Intermodulation," IEEE Transactions on Microwave Theory &
Techniques, MTT-35, No. 4, April, 1987. This mixer just used a
single FET, but still offered stellar IMD performance with the same
gain and noise figure found with a diode ring."

I will look up the paper the next time I am at the library, but is this
essentially the same single FET mixer that G3RJV describes in his
"Minimalist Radio" monograph? I was intrigued by the mixer when I saw
it in George's little book as I do not recall seeing it previously in
the amateur literature. It is as George puts it "delightfully simple".
- Duffey
--
James Duffey KK6MC/5
Cedar Crest NM 87008
DM65
167 2006-09-24 12:35:12 jr_dakota Re: H-mode mixers, etc.
Basically the DDS IC isn't mounted on a PCB but more or less
breadboarded on a 28 pin wide spaced IC socket ... a small piece of
pcb material is placed in the gap between the pin rows, the DDS IC is
placed crossways and all ground pins are soldered to it .... an piece
of copper foil goes over the top of the IC and the pins that connect
to Vcc are bent up and soldered to it ... that just leaves a a few
lines to bring out to pins on the IC socket with enameled wire ... The
foil over the top is bypassed by a stack of SMT caps .001, .01 and .1
in 2 or all four corners

The author used this method 1st about 10 years ago when he developed
the Pic n Mix which still forms the heart of the Pic a Star .... When
making the Pic a Star the author and several other people in the past
couple of years have tried mounting the DDS IC on a double sided PCB
and every time spur performance suffered, the reas