To get things started, in the absence of any signal, I also pass in the supply voltage via D6/8. That
ensures that the switch is “off” even when there is no RF signal.
What about the fact that the diodes are all implemented by two 1N4148 diodes in series? Why?
Because, according to the datasheet (Vishay, but all the manufacturers are similar), the absolute
maximum rating for a 1N4148 “Repetitive peak reverse voltage” is 100V; the “Reverse voltage” is
75V. Therefore we will easily breach these conditions with 50W power.
I looked for other commonly available small signal diodes but could not find any which had the
right combination of:
•
Easy availability, common everywhere
•
Low cost
•
High reverse voltage specification
•
Fast reverse recovery time (so that I could be confident that the diodes would accurately
rectify and double the RF)
The easy solution was then just to make each diode of the doubler from two in series. 1N4148 are
easily available, inexpensive, and fast. Two in series doubles the reverse voltage specification and
makes it adequate for the task. There will be a tiny leakage through the diodes which will suffice to
balance the workload between each diode of the pair.
The fact that the doubler voltage is somewhat larger than the peak-to-peak RF voltage, and the
fact that the entire amplitude of the RF voltage never arrives instantaneously – because there is
envelope shaping provided by the QCX to prevent key-clicks (which, by the way, would be all the
more important when transmitting 50W than at QRP power levels) – means that the voltage
available at the doubler output is always higher than the peak-to-peak of the RF. As the RF
amplitude rises, relatively slowly over the course of a few milliseconds, the doubled voltage does
not instantly keep up because the rectifier capacitor has to charge up – but it is nevertheless
always “ahead” of the RF! I love it when a plan comes together (TM – Hannibal Smith, A-Team).
Therefore this doubler circuit works very well to provide a large (but low current) DC voltage to the
Receive diode switch to keep it OFF during transmit.
Now, you might wonder why I used BS170 transistors for the N-type MOSFETs Q5 and Q6; and
Q4 is a P-type IRFU9024 of course; but why the TO220-package FQPF2N60C for N-type Q3?
What is that about? That is another subtle design problem arising during the development!
The problem arises when you
observe the waveform at the Q3
drain, when the Q3 transistor is a
BS170. You see these horrible
spikes! In the ‘scope trace (right)
the top trace is the RF output
waveform (151.4V peak-peak is
57W); the lower trace is the signal
at Q3’s drain. We have a lot of
horrible spikes. Why?
The answer is in the BS170
datasheet (ON Semiconductor
version, in this case – but again
they are all the same). The
“Maximum Ratings” section
specifies the maximum Drain-
50W QCX PA kit assembly
1.00q
60
