Temporary IF processing  for Antenna # 7

MT October 2012 , revised Dec 2013

The signals from the 7th antenna will need to be connected to WideX, which can correlate up to 8 antennas, but whose IFprocessor is limited to 6, for historical reasons .  This memo describes how to extend the IF processing with minimal interruption of the routine 6-antenna observation. The existing hardware does not need to be modifed. A new piece of hardware named "Strapontin" will be dropped on the table hosting WideX and the IF processor.  It will be dismantled  when PolyFiX gets installed.

1. Signal Path
The  new receivers will deliver 4-12 GHz IF, and  correlation with the old ones is limited to 4-8GHz. The upper part is truncated by a filter to avoid image response at the 2nd conversion. The process  for ant #7 is no different from the others.  Four 2-4 GHz bands are delivered to samplers #7 of the four Widex units. Of course there is no processing for the narrowband correlator.

2. Oscillators and phase issues
They make use of the Dual LO2 ( fixed 8.1 and 9.9 GHz ) modules for which there are two spares already available in the lab. These modules are DDS-controlled to compensate for the slightly different fringe rate of the two bands. However, since a CAN interface is not available  for module #7 , it is preferred to  have its phase and rate permanently set to zero and to correct the values of the six others by software. This technique has already been applied when antenna #5 was added to a 4-antenna  IF processor .
The Dual LO2 modules are fed by 400 MHz, for which there is a generator in the IFP.  Better than extracting it from there, the spare 400MHz generator, also locked on the 5 MHz,  will be used to drive module #7. A commercial synthesizer could be used as well but probably at the expense of a lesser phase stability.

3. Noise source
The analog response of the Strapontin channel needs to be calibrated. A fraction of the central noise source is coupled and can be directed to the  Ant7 inputs, in a very similar way as the 6 other inputs.

Physical location