Envelope-to: dave@picks.force9.co.uk
Delivery-date: Tue, 22 Nov 2005 01:04:08 +0000
Message-ID: <001101c5ef00$7b99e990$0300000a@bob2l2u6k2n1g3>
From: "Vernall" <vernall@xtra.co.nz>
To: rsgb_lf_group@blacksheep.org
References: <NKEMLGOEKANPNJOBFBAFOEFFCIAA.g4wgt@tiscali.co.uk> <000701c5ee1e$02e663d0$0d00a8c0@oz7qcdell>
Date: Tue, 22 Nov 2005 14:02:56 +1300
MIME-Version: 1.0
Subject: LF: BYX10 as  LF PIN diode
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Transfer-Encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

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, and most of the PIN diodes are 
intended for HF and VHF and are more like rectifiers at lower frequencies. 
I searched for data but was unable to identify a PIN diode part number that 
I knew was rated for LF and what price and availability.  In searching for 
information I found that a 1N4007 is reported as making a good RF switching 
diode as the junction has a special PIN structure, even though the rectifier 
data sheet makes no mention of RF performance.  I understand that the 
Elecraft transceiver uses 1N4007s for RF power switching.  So I made up a 
test circuit to check 1N4007 and other diodes for suitability in an LF 
attenuator circuit, at 100 mV level (at a driver circuit level).  I found 
the 1N4007 and most other diodes were basically "rectifiers" in the LF band, 
and introduced obvious waveform distortion when forward DC was introduced 
through the diode.  If the 1N4007 is useful as a PIN diode equivalent, it is 
for frequencies above LF (which I did not test).  I was however pleasantly 
surprised to find that BYX10 diodes work well as current controlled 
resistors, with fairly linear performance for higher LF frequencies, and 
"rectifier distortion" not becoming obvious till frequencies were below 
about 50 kHz.  Several BYX10 diodes were tested and all were similarly good 
for the desired application.  In the band 130 - 190 kHz they gave about 30 
dB range in a voltage divider arrangement, for control current of 0 - 1 mA, 
giving smooth variation of current and attenuation, so it looks to be very 
practical for use in circuits running off a 12 volt rail, and involve only 
modest currents.  The BYX10 diodes I tried have a red plastic package, 
whereas a Philips data book mentions only a BYX10G diode, in a sealed glass 
envelope, so I don't know if all variants of the BYX10 are equivalent to LF 
PIN diodes.  Also there may be other high voltage rectifier diodes that are 
similar to the BYX10 diodes I tested, but they would need testing to verify 
what happens in an attenuator circuit.

If any reader of this report has information on PIN diodes intended for LF, 
or other types of diode that happen to work OK as PIN diodes, I would be 
interested in findings.

73, Bob ZL2CA