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RE: [linrad] For general interest
Hi Ron and all,
> Now a question.
> In most I-Q demod designs I note the phase shift is applied to the
> LO. In the simple 40 kHz 2x SBL-1 design that Leif published in QEX
> the shift is applied to the RF. What is the reason for this approach ?
The reason is that the input impedance of a schottky mixer changes when
the drive level changes. If one uses a simple quarterwave to produce
90 degree phase shift, it is extremely important that the 0.25 wl
cable is terminated in 50 ohms. Otherwise the amplitude will not
be equal in both ends.
If you put the 0.25 wl cable at the LO side and connect the two
RF ports in parallel, strong RF signals will change the impedance
of the LO ports causing degraded image suppression because the voltage
across the two LO inputs become different.
With the 0.25 wl cable at the RF side, the voltage on the LO ports will
always be the same. The drive level on the LO ports is constant so
the impedance of the RF port is constant and modest errors in balance
are taken care of by the Linrad calibration procedure.
It is an empirical finding:)
Leif / SM5BSZ