QCX assembly Rev 1.08
107
The Si5351A chip requires a 3.0 to 3.6V supply (nominally 3.3V) but the rest of this transceiver’s
digital circuits operate with a 5V supply. For the reduction of complexity and costs, two 1N4148
diodes in series are used here to drop the 5V to a suitable voltage for the Si5351A. It works well.
There are three outputs of the Si5351A synthesiser and these are all used to good advantage. The
Clk2 output is used to feed the transmit power amplifier, and the Clk0/1 outputs are used to drive
the Quadrature Sampling Detector (QSD) during receive. These 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
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).
5.4
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 BS170 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.
Summary of Contents for QCX 5W CW
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