Envelope-to: dave@picks.force9.co.uk
Delivery-date: Tue, 22 Nov 2005 16:14:18 +0000
Message-ID: <001001c5ef7f$913ff4e0$d6498351@Main>
From: "Alan Melia" <Alan.Melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
References: <000001c5ef77$d6fdc870$e6a4c593@RD40002>
Date: Tue, 22 Nov 2005 16:00:49 -0000
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Subject: LF: Re: RE: BYX10 as  LF PIN diode
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Hi all Jim has a fair point about this. The structure of an HV diode is
designed to make the depletion layer move far into the bulk material
reducing the voltage gradient mV/um to keep it below the ionising (pair
production) level. Guard ring structures are diffused in at the surface to
reduce the fields and constrain the breakdown into the bulk material. One
way to define the structure and maybe similarity of diodes of the same
generic type might be to measure the capacitance at a voltage of about
50volt. This would give an indication of how big an area the junction was
and how far the depletion layer was being pulled into the "intrinsic
material".

As an attenuator the "on" impedance needs to be low compared with the
circuit impedance but there is still the problem of non-linearity which
could produce IM products. I am not sure about this but it may be
advantageous to put the diode attenuator in a higher impedance circuit in
order to reduce the delta (signal current) compared with the bias current,
thus keeping the IMD low. So maybe a 1000 ohm circuit rather than  50ohm.
For ALC a potentiometer is required so this is not really a problem.

Cheers de Alan G3NYK


----- Original Message -----
From: "james moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: 22 November 2005 15:17
Subject: LF: RE: BYX10 as LF PIN diode


Dear Bob, LF Group,

If you have not already seen it, there is a PIN diode handbook available as
a pdf file from Microsemi corporation:

http://www.microsemi.com/literature/pinhandbook.pdf

This is quite informative. There is also a much shorter application note:

http://www.microsemi.com/micnotes/701.pdf

Microsemi make PIN diodes for high-power HF applications; the app notes
suggests they might work well at lower frequency too.

I believe high voltage rectifier diodes often work quite well as PIN diodes
because they also use the PIN structure to increase the breakdown voltage.
The thing you have to be wary of when using devices in an un-intended way
like this is that diodes marked "BYX10" and made by different manufacturers,
or the same manufacturer at different times, may use different processing to
achieve the same performance as a rectifier. This means they will all meet
their rectifier specifications, but since the RF PIN behaviour is not
specified or tested, they may vary a lot in performance as RF attenuators.
This is OK in amateur applications, where it is practical to individually
select the diodes for suitability.

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Vernall
Sent: 22 November 2005 01:03
To: rsgb_lf_group@blacksheep.org
Subject: LF: BYX10 as LF PIN diode

Hi all,

A brief report on some experiments to assess diodes as current controlled
resistors (as per PIN diodes) at LF.  I'm wanting to develop an ALC circuit
for a new LF transmitter and PIN diodes are quite good for gain control
using an attenuator circuit.  It seems that PIN diodes need thick junctions
to work properly at lower frequencies...