FW: [AMPS] hot switching

[Richard W. Ehrhorn]

Hi Dave & all...

Relative to the T/R circuit you're asking about, I don't have Dick Frey's 
article or any of the older handbooks where I can put my hand on 'em, but I 
believe the technique he described was similar to what we used in the 
earliest ALPHA amps (70V, 70A, maybe some 77D/77S models).

Don't have manuals for any of those amps handy either, but to the best of 
my recollection we used a small capacitor (~1-3 pf?) to couple to the anode 
(hot) end of the pi-L output tank. That cap fed I think a 40673 dual-gate 
MOSFET (state-of-the-art at the time!) amp with a very low Q, broadly 
resonant circuit on its drain. Simply switched in appropriate off-the-shelf 
miniature inductors to approximately parallel resonate the MOSFET's output 
capacitance, as I recall.

An rf clipper consisting of back-to-back 1N4148 or similar diodes protected 
the MOSFET's input gate during xmit as I recall. As others have already 
mentioned, the obvious advantage is elimination of relays with a circuit  
 that's much cheaper and simpler than a PIN diode switch. The MOSFET made 
up the small amount of receive-path gain lost in the coupling scheme and 
didn't seem to significantly affect receiver overload susceptibility. (But 
some of the best transceivers in those days used the same MOSFET in their 
front ends, and today's front ends may be enough better that such a MOSFET 
T/R switch WOULD degrade IM/intercept point, etc.)

OTOH, the amplifier output tank provided some preselection that gave some 
receiver protection against out-of-band signals - including of course those 
on any band that the map wasn't tuned for. It might be an interesting and 
pretty simple project for some receiver bug to build such a T/R switch (or 
borrow an early ALPHA 70V?) and accurately evaluate these various factors 
in the context of today's best receivers.

73,    Dick


-----Original Message-----
From:	David J. Riehl [SMTP:driehl1@airmail.net]
Sent:	Wednesday, August 19, 1998 11:05 AM
To:	amps@contesting.com
Subject:	[AMPS] hot switching


Hello, everybody, I'm new to the reflector, but have been following some
of the threads for some time now.  I've reviewed much of the literature,
even back into the '60's and am wondering whatever happened to the
QSK/TR approach written up by Dick Frey, K4XU, in April '78 HR
magazine?  (This also appeared in the ARRL Handbook until the mid-80's,
as I recall.
Advantages IMHO:
1)No need to make/break contacts which will carry the high-power RF.
  The amp is always connected to the antenna.
2)Only items requiring switching are the grid-bias and (maybe) the
  receiver protection circuitry.
3)In receive mode, peaking on the noise, will normally get the amp
  within 5-10% of tune.

Disadvantages:
1)Requires a lo-capacitance connected to the hi-Z end of the amp's
  pinet, for the receive tap.  INSTALL WITH CARE, NOT FOR NEOPHYTES!!
  Also this connection is frequency sensitive, but should be good for
  a 20-and-up, or 2:1 frequency range.
2)Requires separate receive antenna port on the transceiver or
  separates.  Most modern rigs provide this.
3)There is some attenuation on receive, but an inexpensive pre-amp
  should overcome this as well as provide another "layer of protection"
  for receiver front-end.
4)There is possible damage to the receiver port if the protection fails,
  but this is not likely, if done properly.

QSK TR switching for the receiver port (well under 100w) could be
implemented by a medium-power RF reed relay, a simple SPST pin-diode
switch, or an electronic TR switch (tube or FET).

I'll try to keep future posts briefer, but this subject has been on
my mind for a while now and I wanted to do it justice.  I'm am really
hoping that a number of the experts, who have been involved in the
evolution of the art, can enlighten us as to why this approach is not
used (to speak of) in ham gear and whether it might be updated to be
fully useful at this time.

73,
Dave Riehl   AF5B

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm