For frequencies above 30MHz, devices called I/Q modulators and
demodulators are used. These are not exactly the same as QSEs/QSDs, but
the end result is the same.
They are in essence image-reject mixers with an on-chip 90 degree phase
shift for the local oscillator. On transmit, I and Q signals are fed
into the modulator, along with the LO signal. The I/Q modulator does
the rest, giving a single, clean modulated output signal at the wanted
frequency. The amplitude and phase of the I and Q signals can be
adjusted in software just as with a QSE to give superb local oscillator
and image rejection.
The same is true on receive; the Rx signal is fed into an I/Q
demodulator along with the LO signal and out pops the I and Q baseband
signals to be filtered and fed into a soundcard, ADC or CODEC. No need
to convert to an intermediate frequency such as 70MHz, 10.7MHz or
455KHz - a single conversion takes the input signal straight down from
microwave to baseband. The Rx demodulators usually incorporate variable
gain amplifiers which can form part of a high-performance AGC loop; 60dB
of gain control range is not uncommon which can give the receivers a
huge dynamic range - necessary if there are strong signals nearby due to
radars for example.
IMHO the best IQ modulators and demodulators for use between about
100MHz and 3GHz are currently provided by Analog Devices, the AD8347 and
AD8349 for example being particularly good. They have a wide bandwidth,
usually 2:1 or 3:1, the AD8347 for example can operate from 800MHz to
2400MHz. Devices are available that work up to at least 30GHz as
surface-mounted devices. (That's thirty GigaHertz!)
The local oscillator can simply be generated by a microwave PLL
operating directly at the wanted frequency, no need for multiplication
from lower frequencies. The 90 degree phase shift is usually done
within the I/Q modulator or demodulator IC.
The IF bandwidth can be 10s of MHz, allowing for high-speed data rates
or television signals for example.
This isn't science fiction, it's happening right now. The technology
has been around for some time, and is getting better all the time. The
Microwave Software Defined Radio project (uWSDR) is doing just this, for
more details see
uwsdr.berlios.de
regards
Grant Hodgson G8UBN
for the UWSDR team
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