The LO1ref  Box
MT Jun 2009 rev Nov2013

In order to drive the Harmonic Mixers at each antenna with a clean and phase-accurate CW signal, an LO transport system is required. Historically, the one presented here is the third version. The first one (up to 1994) used a varactor-tuned VCO which offers fast  tuning  and good reliability but has poor phase noise. It has been superseded by the LOsystemII ,based on a DRO (dielectric resonator oscillator) in order to improve the LO phase noise @1 mm. Actually some units have exhibited very good phase noise characteristics but all suffered poor reliability from the mechanically-tuned DRO and intrinsically slow tuning. The presented version  is designed to provide low noise, fast tuning and good reliability by means of a YIG oscillator. The main change is in feeding  the harmonic mixer in the 15 GHz range. This has been  made possible by the availability of very fast digital frequency dividers. This range has several benefits:
    - Use much lower harmonic numbers,  yielding better S/N for the millimeter phase lock. (typically 7 vs 50)
    - Global phase noise is significantly improved , mainly due to the YIG performance.
    - Spurious lines at the HM excitation frequency are now created outside  the 4-8 GHz IF.  (also true for the future 4-12 GHz)

Block diagram of the new system, operational since Sept 2010.

The 0.5 and 100 MHz  references are now  transmitted thru the Hi-Q cable.   The Lo-Q cable  network has been disabled.
There are two LO1ref modules per antenna, operating at independent frequencies. One serves Band 1 only, the second is shared between Bands 2,3,and 4. No switches or amplifiers are inserted  between the  LO1ref and the harmonic mixers. Each  LO1ref module delivers four copies permanently. The unused outputs must be loaded.  When observing in one band, the LO1's of the 3 other bands must be tuned to a defined "parking frequency"  in order to avoid contamination of the active receiver.

Block diagram  showing possible evolution, as antennas are added.

The number of existing Hi-Q cables,  and LO1 rotators is currently limited to 12. The diagram above shows how additional antennas can be served. It requires :

- Designing new electronics and software for a 24-way Phaser  
- Building more Hi-Q cable terminals 
- Pulling a few extra Hi-Q cables to the building

More information on phasemeters