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All the local oscillators in the IRAM telescope use Gunn oscillators.
A Gunn diode is a semiconductor device that exhibits negative dynamic
resistance over a suitable range of frequencies. Output powers of the
order of 10-50mW can be obtained between 60 and 120 GHz. To achieve
oscillation at a precise frequency, two means are combined. First, the
Gunn diode is coupled to a coaxial cavity that defines the oscillation
frequency, and whose high quality factor provides a good spectral purity.
Its resonant frequency can
be adjusted mechanically; this allows the desired frequency to be
approached within 10 MHz. Secondly, a fraction of the
millimetric radiation from the Gunn oscillator is used to produce a beat
with a reference microwave oscillator at a frequency of a few GHz;
actually, the Gunn oscillator signal beats with a harmonic (n=17-65,
depending on the systems) of the reference frequency. The beat signal is
used to ``servo'' by electronic tuning the Gunn oscillator to a multiple
of the reference. Actually, not only the frequency, but also the phase
of the local oscillator is locked to the reference oscillator, which is
essential for interferometry, whether connected-array or VLBI.
This description of the phase-lock system is over-simplified.
Local oscillator frequencies above 120GHz can generally not be generated
directly by Gunn oscillators. In that case, the Gunn power is fed to a
frequency multiplier, which is a non-linear device like the
mixer, but based on non-linear capacitance, and optimized to produce a
certain harmonic (, or in the case of
IRAM systems) of the input frequency. The efficiency of the
multiplication process is typically a few percent.
Next: 5.5 Local oscillator injection
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Anne Dutrey