[hpsdr] mixers/QSD

[John B. Stephensen]

Resistive FET mixers with the source leads grounded (like the H-mode mixer)
have the highest third-order intercept points today. Add an RF amplifier and
its similar to a 7360. Perhaps a LED/photodiode mixer would also work.

73,

John
KD6OZH

----- Original Message ----- 
From: "Lawrence Stoskopf" <stoskopf at tri.net>
To: <hpsdr at hpsdr.org>
Sent: Sunday, June 11, 2006 04:52 UTC
Subject: Re: [hpsdr] mixers/QSD


> ***** High Performance Software Defined Radio Discussion List *****
>
>
> > I've been doing some thinking about HORTON.
> >
> > It seems to me that the biggest problem with getting the QSD to run
> > fast is finding switches/muxes that are fast enough.
>
> I've commented on this before, but again my take:
>
> If you go back and look at the early papers on mixers: intermod and other
> problems occur two places: at the full switch on (not usually a problem as
> diode mixers are so overdriven that signal current is overwhelmed by the
> diode on current) and what happens at turn on and turn off.  In a diode,
you
> have the signal running up and down on the LO current during switching,
this
> is the most non-linear area and the signal is actually modulating the LO
> switching.  Thus the effort for multiple diodes, series resistors, etc. to
> allow for greater drive power with a steeper slope to switching before
diode
> limiting.  My gut feeling (as an anesthesiologist!) is that we kind
of/sort
> of skirt this with the FET/QSD type mixers in that the LO is not really
> directly in the signal path so they work great in the low freq
applications.
> But I don't see any way to overdrive the switch with LO to steepen the
> transitions and at high frequencies, the whole waveform starts to look
like
> a sine wave and worse......
>
> Faster switches in switching time are the key.  I doubt switch on pass
> frequencies are a problem in the applications these are designed for and
bus
> switching speeds are adequate also for the design applications.  As
always,
> we are using these in what is called in medicine, "off label
applications".
> For instance, we use Topamax, an antiseizure medicine in controlling
certain
> types of pain, and indication never intended by the manufactured and
> patients often lose 20-30 pounds and come back for more, even though the
> pain is still there.  Enough of that.
>
> Somebody with a whole lot more background help me out on this.  I still
have
> a couple of 7360 homebuilt mixer receivers that work in still another way
> with basically an electronic signal beam running Class A and being
deflected
> by electrodes back and forth between two plates with no switching
> linearities involved (mostly).  But you can't get them in silicon and
> Gilbert cells aren't the same.
>
> N0UU
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