QRP Labs QCX 5W CW, Assembly Instructions Manual

Page: 109 / 138

QCX assembly Rev 1.08
109
Despite the high IP3 and dynamic range, it is still prudent to provide some input band pass filtering
to protect the mixer from strong out of band signals. In this CW transceiver design, the T1
transformer provides a simple solution to all of these problems with a very low parts count.
The primary couples the incoming RF into the two secondary windings which feed the double-
balanced detector. One end of the primary is grounded which neatly keeps the DC potential of the
input at ground, so the transmit/receive switch is easily implemented by a single MOSFET (see
previous section). The two secondaries are connected as a centre-tapped single winding, which
means that the outputs have 180-degree phase difference as required.
The secondary “centre-tap” is connected to a DC bias formed by R1, R2 and C6 at mid-rail i.e.
2.5V. This simple bias does not source or sink any significant current due to the balanced nature
of the system, therefore no buffering is required. The DC bias feeds through the pre-amps, and
into later stages
– including the 90-degree phase shift network and the first three op-amps of the
CW filter. It is a great benefit not to have to AC couple each stage with coupling capacitors, and
then bias each stage individually. In this circuit, the same DC bias flows through all the way from
this centre-tapped input transformer. This reduces component count and - you guessed it
–
complexity and cost. Another benefit may be that since much of the receiver signal path is DC
coupled, it might be easier to reduce the inevitable “thumps” on switching between receive and
transmit.
Finally, the band pass filter is implemented by a fourth winding on the same transformer T1, with
some fixed capacitors and a trimmer capacitor forming the resonant circuit. It is only a single
resonant circuit band pass filter so has limited stop-band attenuation, but it does have the benefit
of low parts count, and simplicity of adjustment due to the single adjustment control.
5.6 90-degree audio phase shift
By this stage the I and Q outputs are each double-sideband, and we need to process them to
demodulate single sideband.
The circuit used here is an active two-path all-pass phase shift network based on four operational
amplifiers. The circuit is based on the same phase shift block as the Norcal NC2030
http://www.norcalqrp.org/nc2030.htm