To switch ON the diode D1, we arrange for a forward bias CURRENT to flow through D1. For this,
we set A to the supply voltage, and B to ground. The current flows through the diode, limited only
by R1. Using Ohm’s law (I = V / R) with 12V supply voltage leads to a current of 12 / 1000 = 12mA
(neglecting voltage drop across the diode and other losses). At 20V supply voltage the current
would be 20mA. Recall that with 10mA forward bias, the insertion loss is under 0.1dB. So this
“ON” condition is adequate for the switch.
To switch OFF the diode D1, we arrange for a negative bias VOLTAGE to be applied. To get this,
we set A to 0V and B to the supply voltage. The 12-20V (supply voltage) reverse bias voltage is
enough to cause D1 isolation to be at least 30dB (in the worst case, at 30MHz). Since D1 is Off
during Receive, and no large signals are expected, not anywhere near 12V peak-to-peak (which
would be an ENORMOUS, receiver front-end-frying signal) – this is plenty adequate for the Off
state.
These A and B signals are generated by transistor switching which is shown around the PIN diode
switch at the other end of the amplifier, which is SW3 in the block diagram at the start of this
section.
This PIN diode switch “SW3” is responsible
for switching OFF during Receive, to isolate
the PA and LPF output from the Receiver
signal path.
Note that it would be possible to just leave
the PA connected all the time (eliminating
SW1 and SW3) and this is indeed a short-cut
that some designs take. However, I wanted to
ensure that the antenna was cleanly
connected to the Receiver during Receive,
properly isolated from the PA sections so that
proper 50-ohm antenna matching to the
Receiver is maintained. In this way, the
excellent Receiver performance of the QCX is
not degraded in any way by the presence of
the 50W amplifier.
In the diagram (right) you can see the D7
“PIN diode” switch again, just as in the
diagram discussed just now. Again we have
DC blocking capacitors C15 and C17 which
isolate the DC bias conditions from the RF
signal path. And again we have 47uH
inductors L9 and L10 which keep the RF from
leaking into the bias control circuits.
Notice that here the capacitors C15 and C17
need to be high voltage types, because they will handle 50W (141V peak-peak). But high voltage
capacitors are expensive, particularly a 1uF capacitor. Initially in my developments I was using an
0.022uF junkbox capacitor for C17. One day, I smelt burned plastic and heard crackling in my
headphones. Upon inspection I found C17 was very hot, it was melting internally and liquids were
bubbling out. Why? Because that particular capacitor was lossy at RF, effectively dissipating
power. Power dissipation in a small capacitor volume can cause significant heating! No wonder it
got so hot it started to break down and bubble!
50W QCX PA kit assembly
1.00q
57
