provides 137dB of isolation in the “Off” state. Amazing! We don’t need anything so extreme (or
complex) here – but the principles are well described by W6JL and his work is inspiring.
It is possible to distill the whole topic into two important rules:
•
To switch the PIN diode Off, apply a large reverse bias VOLTAGE. The voltage should be
higher than the peak-to-peak RF being switched.
•
To switch the PIN diode On, apply a forward bias CURRENT; the more current, the lower
the insertion loss. At 10 mA forward bias, the insertion loss is only about 0.1dB. One can
increase it a bit further, to be on the safe side.
During the research phase of this project I studied in detail, the schematics of around 10
commercially available HF amateur radio transceivers that use PIN diode transmit/receive
switching. These are designs by some big, well-respected name manufacturers (who shall here,
remain nameless).
I was absolutely astonished to find that NONE of these 10, high performance transceivers, actually
follow these two guiding principles of PIN diode switching properly. My mouth hung open in shock
for quite a while… in ALL of the 10 schematics, I could find problems with the transmit/receive
switching. In some, you could see that the designers had found evidence of the problems, and had
implemented workarounds to reduce the symptoms, or additional components to minimize the
harm. I had also read for years, in my favourite RadCom columnist Pat Hawker G3VA (RIP: now
SK) “Technical Topics” column, of some of the problems of PIN diode switched transmit/receive
switches – for example, generating huge spurious spikes at the instant of transmit/receive
switchover. I myself noted these problems too, but I worked hard on the design to find out the
cause and fix it!
When the two “rules” are followed carefully, the performance of the PIN diode switch is excellent;
furthermore it is low cost and reliable to implement using inexpensive common components like
the 1N4007 and switches in a matter of microseconds so it allows the full break-in (QSK)
performance of the QCX to be maintained. It is necessary to use some additional transistors so
that a “Key-down” or “Push-To-Talk” (PTT) signal from the QCX transceiver (or other) can cause
the necessary biasing of the diodes.
To start with, take for example the switch named “SW1” in my
above block diagram. On the 50W PA schematic, this switch is
implemented by a 1N4007 diode as shown in this schematic
fragment (right).
The signal comes in from the RF input of the amplifier via C2. It
passes through the “PIN diode switch” D1, and via C3 to the 3dB
pad attenuator, then into the PA input.
The DC-blocking capacitors C2 and C3 are necessary to isolate
the DC conditions that we set up to bias D1 correctly, from the
RF signal path at the amplifier input and the PA input. At 7MHz a
1uF capacitor has a reactance of 0.023-ohms; for low loss we
want this reactance to be small in comparison to the 50-ohm
system impedance. 0.023 ohms is suitably small.
The 47uH inductors L1 and L2 are required in order to block the
RF from getting into the bias switching circuits, where it could leak around the amplifier and cause
positive feedback; and anyway cause unwanted insertion loss by dissipating power where it
should not go. At 7MHz, the reactance of a 47uH inductance is over 2 Kohms; this is sufficient in a
50-ohm system to prevent problems.
50W QCX PA kit assembly
1.00q
56
