[TenTec] Isolation of diode T/R Switch on Corsair

Dr. Gerald N. Johnson geraldj at storm.weather.net
Tue Jun 9 09:13:02 PDT 2009

On Tue, 2009-06-09 at 06:26 -0700, Joe White wrote:
> Thanks for the replies, gentlemen.
> In answer to your questions,
> I made the measurements with a 10X probe on the transmitter running 100
> watts out (200V p-p).  Leakage amounts were on the order of 0.2 to 1.3V
> p-p, with the higher values observed at the higher frequencies.  This lower isolation at the higher frequencies
> could be attributed to the effects of leakage on the measurement, but
> perhaps more likely is that residual capacitance of the diodes simply
> makes the diode switch less effective at higher frequencies.

Its possible to measure that here, but I have other projects in line

Did you check the resistors for drift in value? A pin diode's resistance
(and that's critical in shunt mode) depends a great deal on the DC
current through it. Most carbon comp resistors tend to go high in
resistance with heating and that would reduce the diode DC current and
so the attenuation. Check all the way back through the switching
circuits to the supply.
> Because of the high signal levels, noise
> should not be a factor.  
> 40
> to 60 dB sounds like pretty good isolation for a two diodes cascaded
> followed by a shunt transistor, so it does sound like the output diode
> switch is doing it's job.  I was more concerned that if the leakage
> gets significantly above the 1.3V level that it could get close to the
> biasing level of the input PIN switch on the low-level driver board,
> but since that bias is about half of the 12V supply, maybe it's not an
> issue either.  
> Anyway, I
> was hopeful that someone might have actually measured that leakage at
> some point in making the same repair that I was making, and if the
> measured values that I was seeing were typical of a properly
> functioning Corsair.  They certainly seem to be, ..but there's always 
> that nagging doubt that somethings going to blow when I push up the 
> power since it did once before.

The other factor is the characteristics of the replacement diode
compared to the originals. Smaller junctions make for lower C, but
higher ON resistance.
> Thanks for taking the time to read and reply.
>     73
>      joe, 
>     WA4GIR

73, Jerry, K0CQ

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