5.4.2 LO generation

The first local oscillator is a Gunn oscillator (a tripler is used for the 1.3mm receiver). The Gunn is phase-locked by mixing part of its output with a harmonic of a reference signal (used also as the second LO): the harmonic mixing produces a 100MHz signal, the phase of which is compared to a reference signal at frequency = 100MHz, coming from the central building. That reference signal is used to carry the phase commands to be applied to the first LO: a continuously varying phase to compensate for earth motion and phase switching used to separate the side-bands and suppress offsets.

The LO1 signal at may be locked either 100MHz above (``High Lock'') or below (``Low Lock'') the $\ensuremath{N_\mathrm{\scriptscriptstyle H}} ^{\rm th}$ harmonic of the LO2 frequency :

$$\ensuremath{\nu_\mathrm{\scriptscriptstyle LO1}} = (\ensurema... ...tscriptstyle 1}} ) \ensuremath{N_\mathrm{\scriptscriptstyle M}}$$ (5.17)

The multiplication factor is 1 for the 3mm receiver and 3 for the 1.3mm receiver.

The second local oscillator, at $\ensuremath{\nu_\mathrm{\scriptscriptstyle LO2}} =1875\pm25$ MHz, is phase locked =0.5 MHz below the frequency sent by the synthesizer in the central building (which is under computer control and common to all antennas):

$$\ensuremath{\nu_\mathrm{\scriptscriptstyle LO2}} = \ensuremat... ...yle SYN}} - \ensuremath{\epsilon_\mathrm{\scriptscriptstyle 2}}$$ (5.18)

The reference frequency is sent to all antennas from the central building in a low quality cable, together with the = 100MHz reference frequency for the first LO. The is sent to the antennas via the same high-Q cable that transports the IF2 signal. No phase rotation is applied on the second local oscillator. The relation between the RF signal frequencies (in the local rest frame) in the upper and lower side bands and the signal frequency in the second IF band is thus (for high lock):

$$\ensuremath{\nu_\mathrm{\scriptscriptstyle USB}} = \ensuremat... ...iptstyle 1}} - \ensuremath{\nu_\mathrm{\scriptscriptstyle IF2}}$$ (5.19)

and in the lower side band:

$$\ensuremath{\nu_\mathrm{\scriptscriptstyle LSB}} = \ensuremat... ...iptstyle 1}} + \ensuremath{\nu_\mathrm{\scriptscriptstyle IF2}}$$ (5.20)