Dealing first with the Receive → Transmit transition; PTT is initially at 0V. The transistors Q5 and
Q6 (above left) are therefore OFF. The Q5 gate is connected to PTT so when PTT goes to +5V
(on key-down), Q5 is switched on; this pulls the gate of Q3 to ground (0V) so Q3 is switched off.
With Q5 on and Q3 off, reverse bias voltage is applied to diodes D3 and D5, causing them to be
switched OFF; so the Receive bypass switch “SW2” of the circuit is OFF. The important point is
that this all happens extremely quickly, there is no delay.
At Q6 however (above right), there is a resistor-capacitor network R15 & C5. This delays the
voltage rise at the gate of Q6, delaying the turn-on of Q6; Transmit is enabled (switches “SW1”
and “SW2” closed) when Q6 is on. Therefore the combined effect of the immediate switch off of
the Receive switch (“SW2”) and the delayed turn-on of the transmit switches (“SW1” and “SW3”)
perfectly creates the required break-before-make timing. The positive feedback condition cannot
arise and there is no big spurious spike on key-down switching!
On the transition from Transmit → Receive, PTT changes from +5V to 0V. Q5 is switched off
immediately. The voltage at the Q3 gate rises but this is not instant; it is delayed as C13 has to be
charged up via 470-ohm resistor R12. So there is a delay before “SW2” is closed.
The turn-off of Q6 (transmit PA switches “SW1” and “SW3”) is also delayed, as C5 discharges
through R15 (1.5K). This discharge is faster than the charging of the “SW2” capacitor C13, which
is charged through 470-ohms but is 1uF. The positive feedback situation is completely avoided.
The choice of component values is quite non-critical, I chose the capacitors after some rough
calculations of approximate values from the time-constant (product of resistance and capacitance)
They were found to work, the positive feedback condition is reliably voided so there is no more
huge spike at turn-on and turn-off.
7
Amplifier set-up and operation
7.1 PA Precautions
To avoid dramatic failure of components, observe the following precautions:
1) Check all the components again, and again, BEFORE applying power to the amplifier!
Check the resistors are in the correct places, check that all the enameled wire inductors
and transformers are properly soldered with continuity through the windings. The same
applies to diodes, transistors and the voltage regulator IC. Pay particular attention to ground
connections of all components: A very common fault is failure to apply adequate heat to
ensure a good solid joint to ground – the ground plane will dissipate heat and draw it away
from the joint.
2)
Before applying power, make sure the R5 trimmer resistor (PA bias) is turned all the
way anticlockwise.
3)
Check for short-circuits between the IRF510 metal tab, and ground! Check that there are no
other shorts, the DC resistance between the +12V tab and Ground should NOT be near
zero.
4) Do not apply power to the amplifier unless the heatsink installation has been performed and
all screws are tightened.
50W QCX PA kit assembly
1.00q
64
