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[linrad] H mode mixers at 144 MHz
Currently the TUF-1h will perform several orders of magnitude better then
the Delta44 sound card. Even large signals that are inside the RF passband
but outside the IF passband at 2.5 Mhz will suffer from phase noise at levels
lower then that required to overload a simple TUF-1h. Myself, I think I would
have to see first hand evidence that my mixer is being overloaded and it is the
limiting factor before I would consider pioneering a complex mixer. Even then,
I would first look at adding a simple low noise 6db gain IF amplifier at 2.5 Mhz so
I could reduce the gain ahead of the mixer by 6db effectively buying 6db
more dynamic range in the front end. The only caviots to my comments would
be that it is important to provide good RF selectivity ahead of any mixer.
In my case the RF selectivity will be pretty extreme at about a 1 Mhz, -3db bandwidth
and a 10 Mhz, -100db bandwidth. This will provide optimal image rejection for
a 2.5 Mhz IF and performance several orders of magnitude better then my
Yaesu FT736r and MGF1302's.
I am not so sure that Leifs J310 mixers aren't already pretty similar to the H mode
mixer and certainly running at a high enough IP3 that it is not the limiting factor.
I think they are all using FET's to switch, instead of diodes? Other then that, there
is little difference between circuits conceptually .......? Or did I miss some
other magic of the H mode mixer?
If someone starting from scratch did not already have TUF-1h mixers bought and
someone had the H mode mixer perfected at 144 Mhz and they were cheap to
build, then it would certainly be a logical overkill choice.
I wonder if there aren't more simple J310 style mixers that couldn't beat a TUF-1h
at a lower cost...? Perhaps a single balanced configuration? At 27 cents each,
anything that uses J310's is pretty cost effective. I plan to buy 50 and do some
experimenting, first on my stage 2, 3, and 4 amplifiers. At 27 cents each, the only
thing better then one J310 is two of them or three or more in parrallel.
There are commercially available similar FET type high performance mixers, but they are
priced out of sight compared to a capable TUF-1h.
My opinions.... Your mileage may vary.
73, Jim Shaffer, WB9UWA.
On 31 May 2003 at 21:31, w3sz wrote:
> Hi, Leif!
> H-Mode mixers were made with J310's before people started
> using the Fairchild devices, I think. Given the IP3's of
> 40-45 dB quoted, to you think that it would be worth trying
> this at 144 MHz?
> > I made some experiments with the NC7SB3157, another member
> > of the Fairchild CMOS switch family. I tried to use it to
> > convert from 70MHz to 10.7MHz but the results were not too
> > encouraging. I do not remember the details any more, but
> > performance was similar to what I could get with the J310.
> > Since the microscopic devices are awkward to work with I
> > decided to use the J310......
> > Some day the modern switches will be superiour for use at
> > VHF. I do not know if the day has already arrived, my
> > tests did not give good enough IM3 performance but I did
> > not study very carefully since I do not have the tools to
> > work with these very small components.
> > Maybe someone will make a PCB with just the mixer on it?
> > 73
> > Leif / SM5BSZ