The waveguide mixer is an upgraded version of the design of
Walker et. al. (1992) and consists of a corrugated feedhorn followed by
a circular to full height rectangular transition and a waveguide ``T''
section. The vertical part of the ``T'' contains a sliding, rectangular
backshort, often referred to as the E-plane backshort. The SIS junction
is located one-half wavelength behind the E-plane backshort. The junction
is formed on a quartz substrate with dimensions 3.5 
millinches (mils). The full height rectangular waveguide has dimensions
of 18.0 
mils. Behind the junction is a second sliding backshort
referred as the junction backshort. This backshort has a circular crossection
and is can provide better performance than a rectangular backshort.
One side of the junction substrate is connected to ground. The ``hot''
side of the substrate is connected to an impedance matching network
via a 1.0 mil gold wire. The wire is soldered to the matching network and
silver painted (by hand!) to the tip of the junction substrate. The purpose
of the matching network is to transform the I.F. output impedance (160 
,
if we are lucky) to 50 , which is the input impedance of the
first I.F. amplifier. For the mixer to provide stable operation, a
magnetic field of about one quantum flux unit must be maintained
across the junction. The magnetic field works to suppress the Cooper
pair tunneling across the junction which is a source of both noise and
instability. High mu metal field concentrators are embedded
in the mixer block for this purpose. The required magnetic field is
generated by a superconducting electromagnet attached to the mixer
via the field concentrators. The intensity of the magnetic field can
be varied by adjusting the bias voltage and current across it.
Typically, a magnet current of 
mA is sufficient for stable
operation of the mixer.
The SIS junction (provided by Caltech) has a tuning structure fabricated
directly on the junction substrate. The tuning structure provides a
better impedance match to the waveguide than can be provided by the
backshorts alone. In fact, the match is almost perfect! This is both
a blessing and a curse. The blessing is that high Y-factors ( 
)
can be obtained over much of the receiver's passband. The curse is that
the excellent impedance match produces flat photon steps. For normal
operation the junction is biased in the middle of the first photon
step below the knee in the I.V. curve. A special 4-wire bias box is
used to maintain a constant bias voltage or current even on a flat photon
step. Biasing the junction on a flat
photon step can make the I.F. impedance of the junction look
extremely high to the cooled I.F. amplifier which wants to see
50 . This impedance mismatch can produce standing waves
between the mixer and amplifier. A cooled rf isolator has been inserted
between the mixer and amplifier to absorb these standing waves.
Even with the isolator, it may still be possible to tune the receiver
in such a way that standing waves or other instabilities are seen in
the IF. If these instabilities are observed, they can usually be eliminated
by detuning the receiver slightly. The easiest way to do this is by
adjusting the mixer's backshort.