QRP Labs QCX-mini, Assembly Instructions Manual

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outputs can be switched on and off under the command of the microcontroller. This
provides an opportunity for some simplification because the Clk0/1 outputs can be simply
switched off entirely during transmit. This relieves pressure on the transmit/receive switch.
There just cannot be any reception during transmit because there is no oscillator input to
the receive mixer. Conversely, the Clk2 output is switched off during receive.
A feature of the Quadrature Sampling Detector is that either the RF input, or the LO input,
must provide two paths in 90-degree quadrature. This is normally applied at the Local
Oscillator where it can be easily controlled for best performance. So, two oscillator signals
are required, with the same frequencies but a precise 90-degree phase offset. Generating
this quadrature Local Oscillator signal is always difficult. Analogue phase shift circuits have
limited accuracy. Often a divide-by-4 circuit is used, to produce quadrature oscillator
outputs from an oscillator input at 4x the reception frequency. This also creates challenges
particularly as you try to increase the reception frequency to cover higher bands. For
example, on 10m e.g. 30MHz, a local oscillator at 120MHz is required and the divide-by-4
circuit must be able to operate at such a high frequency. Devices such as the 74AC74 can
do so, but pushing it higher into the 6m band cannot be done with the 74AC74.
The Si5351A has a phase offset feature, which is not really very clearly described in the
SiLabs documentation. However, QRP Labs has perfected the technique to put two of the
Si5351A outputs into precise 90-degree quadrature, which is maintained without tuning
glitches as the frequency is altered. It’s a nice development because it eliminates one more
circuit block (the 74AC74 divide-by-4 circuit), again reducing complexity and cost. To the
best of my knowledge this is the first time the Si5351A has been implemented in a product
directly driving a QSD with two outputs in quadrature (no divide-by-4 circuit).
4.4 Transmit/Receive switch
Since the receiver is entirely disabled during
transmit, because of the absence of any local
oscillator signals to the Quadrature Sampling
Detector, the demands on the transmit/receive
switch are considerably reduced. Now the circuit
does not have to provide the massive amount of
attenuation necessary to prevent the transmitter
from overloading the receive circuits. All it has to
do is provide a reasonable amount of attenuation,
enough to stop the 5W signal (45V peak-peak)
from damaging the receiver input mixer.
The transmit/receive switch is implemented by a
single BSS123 MOSFET. The source is at DC ground (via the primary of input transformer
T1). The control signal from the microcontroller switches the MOSFET on or off.
Interestingly, capacitor C34 close to the MOSFET gate is found to be necessary to prevent
inductive pickup of the 5W RF from partially switching on the MOSFET.
The switch wouldn’t provide enough attenuation to mute an operating receiver; but during
transmit, our receiver isn’t operating; all the switch has to do is protect the Quadrature
Sampling Detector from seeing 45V peak-peak which would destroy it.
QCX-mini assembly Rev 1.05_Fr
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