5.4 Local oscillator system

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 $\approx$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 ($\times 2$, $\times 3$ or $\times 4$ in the case of IRAM systems) of the input frequency. The efficiency of the multiplication process is typically a few percent.