The moral of the story is therefore: operate CW normally. Do not sit on the key for long periods,
which will probably blow up the transistors. A bit of tuning up or testing for a few seconds is fine.
Just don’t overdo it. Be kind to the transistors. If you don’t believe me, by all means try it. At least
IRF510s are low cost, which is a great benefit of this kit!
6.5 Low Pass Filter
A Low Pass Filter (LPF) is always required, following any amplifier (even a linear!) to attenuate
harmonics caused by non-linearities. Yes, even a linear amplifier has non-linearities! Though they
are smaller, so you often see less aggressive filtering. Here, our PA is operated in Class-C, which
is not linear. Therefore we use a 7-element Low Pass Filter design which is well tried and tested,
and is the same design as used in the QCX transceiver itself, and in the QRP Labs LPF kit
There are two important differences, both are related to power handling. Here, the amplifier
produces over 50W. That requires:
•
Powdered iron toroids which have enough iron in them that they will not saturate and
become lossy, at 50W
•
NP0/C0G capacitors which have a high enough voltage rating to withstand the peak voltage
excursions seen at 50W.
50W is a 141 V peak-peak sinewave. In the Low Pass Filter the sinewave is referenced to ground
so what matters, in theory, is the peak sinewave relative to ground – which is 70.7V. However, in
the case of a mismatched load (higher than perfect 1.0 SWR) the voltage is higher. In the original
QCX kit the LPF capacitors are 50V rated, it is fine for QRP but not here. So in this kit, 250V
ceramic NP0 capacitors are provided.
The reason for using NP0 (near zero temperature coefficient) capacitors is not that we care at all
about the very slight capacitance change with temperature which would occur with a non-NP0
capacitor; but because the same ceramic dielectric which gives the near-zero temperature
coefficient also gives very low loss at RF. Non-NP0 capacitors are rather lossy at RF. The
capacitors are also known as Class-I dielectrics (as opposed to “normal” non-NP0 Class-II
dielectric capacitors).
The toroid size question is solved with a little internet research, the conclusion of which is that
T50-size toroids are plenty sufficient for 50W power handling.
50W QCX PA kit assembly
1.00q
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