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
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