On Wed, 2007-10-03 at 10:36 -0700, Kevin Purcell wrote:
> I've wondered about this in the past too.
>
> Using "hand-built" mixers seems to be a part of the TT design style
> and has been for years.
>
> I would have though production costs would have swamped the component
> cost when you include adjusting the balance. How much do those little
> trifilar binocular core transformers cost to make? Or perhaps I'm off
> base and with large purchases they're saving several dollars per
> radio in mixers.
At least in the 2m to 6m transverter there are no balance adjustments.
Back in 1963 at Collins we figured a two leaded component cost 25 cents
to mount on conventional construction, not printed circuit boards. And
that was with skilled production line workers probably making under $3
an hour (I hope they made less than a Jr engineer, else I was working in
the wrong department teaching them how to read engineering drawings).
Machine mounting on PC boards should be less expensive today I think.
Could be they are saving several bucks a radio in mixers, plus they keep
more of the production inside with less dependence on vendors for
assemblies, but for the mixers it could be a performance damaging
saving. They don't save on vendor contacts because diodes, cores, and
wire would come from at least three different vendors. I doubt each
mixer gets tested at TenTec for IMD, and conversion loss, and thats a
lot more likely at a mixer maker like Minicircuits.
>
> Given the JFETs set the limit the high level mixer would be a bit of
> a waste to use a high-level mixer (plus an extra driver stage would
> be needed) but a TUF-1 or TUF-3 would just drop in or SBL-1 would
> drop in and retain the balance adjust.
Do the first stage JFETs set the limit? I think sometimes they don't
especially inside the pass band of the roofing filter. Because the JFETs
have gain and there might be a first IF gain stage to make up for filter
loss the signal at the next mixer is larger though band limited so
inside the roofing filter (which has to be wide enough to accept both FM
and fine tuning in some receivers) the second mixer is the limit, not
the first. This shows in almost ALL modern receiver tests where the
blocking range falls significantly inside the roofing filter passband.
Its a common, but curable malady.
Then again if the first mixer load is the filter its possible that first
mixer has higher gain in the filter passband, but many mixers perform
much better for intermod if they have wide band terminations on at least
2 of the three ports so that image outputs are not reflected back to the
diode for reprocessing.
>
> Does the Jupiter just have a single pair of JFETs in the front end
> like the 320? Or do they parallel the J310s?
>
> On Oct 3, 2007, at 9:30 AM, Dr. Gerald N. Johnson wrote:
>
> > I'd rather see them spend $3.95 on a packaged mixer made by an
> > expert in
> > mixer making than spend 2 cents on slightly slow diodes and 23 cents
> > each on almost good enough transformers. I think the receiver
> > performance would be detectably better with the packaged mixer, like a
> > MCL TUF-1H... though the H suffix may cost a little more than
> > $3.95, its
> > not much.
> >
> > 73, Jerry, K0CQ
>
> --
> Kevin Purcell
> kevinpurcell@pobox.com
73, Jerry, K0CQ
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