One end of the waveguide is the input of the mixer; the other end must be
terminated somehow. At the zero-order approximation, one would like the
junction to ``see'' an open circuit when ``looking into'' the rear end of
the waveguide. More generally, the junction should see a pure imaginary
impedance, so that no energy is wasted. A simple calculation shows that a
transmission line having a length l, and terminated into a
short-circuit, has an apparent impedance:
For various reasons (one of which is reducing the noise contribution from
the atmosphere) it is desirable that the mixer should operate in
single-sideband mode. We explain how this is achieved with a crude
zero-order model. Assume that the best impedance match of the junction is
obtained when the apparent impedance of the backshort seen from the
junction is an open circuit. Assume we observe in the lower sideband at a
frequency
, and want to reject the upper sideband
. That condition can be achieved if, at the
frequency
, the junction is short-circuited. So, we must meet the two
conditions:
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(3.7) |
Returning to practicalities, tuning a receiver requires several steps
(which used to make astronomers a bit nervous at the 30-m telescope when
all was done manually). First the local oscillator must be tuned and
locked at the desired frequency. Then the backshort is set at the
appropriate position, and the junction DC bias voltage is set. Finally the
LO power is adjusted to reach a prescribed junction DC current (of the
order of ). These adjustments are made by a combination of table
lookup and optimization algorithms under computer control. Altogether this
involves between 11 and 13 adjustments, mechanical or electrical, yet
this process takes only a few minutes with the current systems.