[hpsdr] mixers/QSD

[KD5NWA]

Beam deflector tubes (valves), ah the memories.

At 11:52 PM 6/10/2006, you wrote:
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>
>
> > 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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Cecil Bayona
KD5NWA
www.qrpradio.com

"Windows the worlds most successful software virus" 


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