[hpsdr] ALEX - Input protection

[Hollingsworth, Tommy]

Graham:
I am trying to understand what your criteria are.  I just completed an SDR
1000 clone and am trying to decide what kind of clamp to use on the input.
After spending 6 months building it, I really do not want the smoke to get
out of my working radio :)  Protek parts are not easy for me to obtain.
The parts below look interesting:

		Semtech	Semtech	DiodesInc	Protek
		RClamp0502B	SL05		DLP05LC		GBLC05C 
V standoff	 5		2.8		5		 5
volts
Vbr		 6		6		6		 6
volts
Vclamp	15		5.5		11		18.3	volts
I peak	 5		24		17		17 	amps
C		.6		2		1.6		 3 	pf

Package	SC75		SOIC8		SOT23		SOD323

   ***Note that Protek uses typical so I used typical for all C values
Note that similar parts are also available from On Semiconductor.  I really
doubt that these are more expensive than the Protek part, considering
Protek's $50 minimum per part and $500 minimum order. Note that all of these
parts are available from Digikey except for Protek. I assume next that you
have a target current/energy level higher than 5 amps so I looked for some
in that range.  The Diodes Inc DLP05LC looks really good to me at 17 amps.
You could have two in parallel for the capacitance of the GBL05C .  Note
that I see it as a good design idea to create a bidirectional clamp from two
unidirectional devices.  The spike really does ring, and in doing so dumps
energy into both device, not symmetrically but close enough. 

I am a little concerned that by trying to push the capacitance to a really
low value, a TVS is being chosen that will not really be robust enough.
Henry Vredegoor described using one or multiple 1N4148 diodes in series with
large TVS parts.  Note that the 1N4148 has a capacitance of 4.0 pF(max) and
the 1N4448 has a capacitance of 2 pF.  I am curious if we can stack two of
the TVS in series and cut the relative capacitance in half.  I am not
certain that it would work that way.  I do have a transient tester available
and may have to build a test board to find out. 

This sounds like we would be just as well off putting several of the DLP05LC
TVS parts in parallel on the line and forgetting about the smaller diodes.
I am really curious what Henry and some of the others might feel about this
part, considering it is easy to obtain.  Is this clamp voltage high enough?

Please let me know if this makes sense, or if I am overlooking something.  I
am anxious to get my radio on the air, but I plan on it lasting for a long
time so I won't rush it.  I also plan on building the Mercury and Penelope
in the near future.  
Cheers
73
Tommy Hollingsworth KV5T


-----Original Message-----
From: Graham Haddock [mailto:KE9H at austin.rr.com] 
Sent: Monday, August 06, 2007 8:50 PM
To: Hollingsworth, Tommy
Cc: hpsdr at hpsdr.org
Subject: Re: [hpsdr] ALEX - Input protection

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