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Re: [Amps] Looking for Alpha 87A expertise

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Subject: Re: [Amps] Looking for Alpha 87A expertise
From: "Paul Christensen" <>
Date: Tue, 16 Mar 2021 07:32:43 -0400
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>"Anything under 2ms  (2 milliseconds = 2000 microseconds, which is a whole 
>bunch of nano seconds) is WAY MORE then enough for most transceivers. (Which 
>was the point I was trying to make here)."

A diode's forward bias transition time is one important parameter but when 
considering PIN diodes for use in the Alpha 87A, the diode's minority carrier 
lifetime may be an even more important parameter.  The purpose of the intrinsic 
region between the P and N wafers is to store a charge long enough to keep the 
diode from rectifying RF during a complete cycle.  For us, the worst- case band 
of operation is 160m (excluding the two LW bands).  For 160m, the carrier 
lifetime computes to approx. 0.5 us.  But, typically much more time is required 
to minimize IMD products.  
When used as an HF T/R switch, Microsemi recommends 1A of DC current to fully 
bias the UM2100 series diodes -- and nearly 1KVDC reverse bias on the receive 
PIN when HF power levels reach 1KW.  

I believe ETO/Alpha originally spec'd the MA4P4006D for the transmit PIN which 
was superseded by the identical UM4006 when Microsemi acquired the product 
line.  I hear rumors that Alpha has been recommending 1N4007 diodes as transmit 
PIN replacements, but I've never seen an equivalent carrier lifetime spec that 
would give me comfort if a replacement is needed in my amps.  And, if these 
work reliably, I would like to know how close they are to the ragged-edge of 
acceptance.  And, are all 1N4007 diodes created the same and offer identical 
performance?  Even diodes from decades ago?

>BUT if you use an IC7000 or similar, you'll have trouble. But I think Alpha 
>compensates for crappy radios that still transmit RF AFTER the PTT line is 

To my knowledge, the Alpha 89 is the only ETO/Alpha amp that prevents a return 
from T=>R when RF is present.  Possibly the 9500 also protects this way but 
I've never tested it.  

A big deal at the moment is Icom's amp sequencing in the IC-7300, 9700, and 705 
models.  The amp key line releases approx. 3 ms  BEFORE RF decays to zero.  
This is a fault on amp break, not amp make.  As such, no amount of fiddling 
with a transceiver's menu helps to prevent hot-switching with fast amps.  The 
Elecraft amps DO have a menu selection to add delay on break.  

Icom has known about the problem since the 7300's release in 2016.  Myself and 
others have given up hope that they care enough to do something about it.  To 
that end, I created the new S-QSK Mini device that can be seen on my QRZ page.  
It's a tiny 1X1 sequencer that's smaller than a book of matches.  Precise 
timing is set by an Adafruit Trinket MO microcontroller and it completely 
compensates for Icom's problem through software.  The S-QSK Mini is powered 
exclusively from a Micro B USB cable.  For those with an interest, here's a 
quick link to my QRZ page:

Paul, W9AC 

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