The wideband IF processing system for PdB

MT March 2005
rev June 2010

he new receivers deliver an IF from 4 to 8 GHz. It is transported from the antennas via a monomode optic fibre network. The historical IF transport system used coaxial cable and was limited to 500 MHz BW. This paper describes the equipment that converts  the 12x4 GHz in narrower frequency slices so they can be accepted by the correlators.

The existing correlator input range is 0.1 to 1.1 GHz.  In the new scheme it is used for narrow line work and the IF processor gives it additional frequency agility to cover the whole range. To preserve its phased array capabilty, all the additional LOs that are introduced upstream must comply with VLBI frequency requirements.
The future wideband correlator is expected to have an input frequency range of 2 to 4 GHz, which is the ALMA standard.

Overall block diagram of the IF processing system

More detailed sections :

1. The optic receiver and noise source rack
1.5  The polarisation switches
2. The bandsplitter
3. The dual DDS-controlled LO2
4. The NB correlator IF extender
5. The 400 MHz generator/distributor

1. The optic receiver and noise source rack

The 12 optic fibers from the laser transmitters are selected from the patchpanel and converge here to be converted to electrical signals representing the 4-8 GHz firstIF. At this point there is no total IF power measurement, but digital power detectors will be fitted in the spectral
 correlator instead. In order to calibrate the phase shifts of the different analog paths to the correlator units, a noise source can be substituted to the whole IF, by means of one control bit sent to the CAN node. There are 3 physical switches, one for each "polarization" channel (for all six antennas simultaneously) and a third one that shuts off the noise source, to minimize the correlated leakage.
The photoreceivers have a built-in optical power meter which gives useful information about the optical attenuation of the antenna to building trip. They are  monitored by a microcontroller connected to CAN bus. Also monitored are power voltages, service temperatures...etc. Detailed information to be seen on the optic receiver page.

1.5. The polarisation switches

For polarimetry , the vertical and horizontal IF's of a given antenna can be swapped by a pin-diode transfer switch.

The commands are applied to the various antennas for 1 second phases organized in  a Walsh pattern.  The 4 polarization products are sequentially integrated and extracted  by software.

Commands are described in the Pol Switch user manual.

2. The bandsplitter

The 4-8 GHz band is split in two parts, that will feed the WB correlator in the 2-4 GHz range. The sampler of this correlator must be preceded by a very good bandpass filter, to prevent aliasing. The best location for this filter is  at the output of the bandsplitter, since its excellent characteristics can be re-used as a preselector by the further processing stage. The skirts of this filter are only 100 MHz wide each side.
The IF is split into Upper path (yellow) and Lower path (green).  For each path the useful downconverted band (2.1 to 3.9) is selected and two copies are made, one for the Narrow correlator and one for the Wide correlator. For each antenna there are two bandsplitters, named H-pol and  V-pol. (Although they may carry circularly polarized signals ;)
Hence the names of the 8 outputs.

the bandsplitter (1/6)

To achieve a good crossover, the LO2 frequencies are set at 8.1 and 9.9 Ghz. This makes the IF1=6.000 Ghz channel appear both at 2.100 L and 3.900 U.  Incidentally this sets a requirement on the channel spacing of the WB correlator. It has to be an integral fraction of 100 MHz. Both upper and lower subbands are frequency reversed.

the 2nd downconversion scheme

Since the two 2 GHz subbands have their own digital delay line system, there is an individual residual fringe rate that needs to be compensated for. This is performed by steering the phase of each LO2.

3. The dual DDS-controlled LO2

First note that "LO2" traditionally refers to the 2nd local oscillator of a receiving system. When this frequency plan enters operation, the historical (and  famous) "LO2 box" will stop being the LO2 and will have to find another name, like "LO1 reference" for example.

The LO2 frequencies must comply with VLBI frequency requirements, which are :
a) be a multiple of 250kHz, free of residual synthesis drifts.
b) have known and stable phase relationship with respect to the 5 MHz standard.

The two oscillators are derived from different harmonics of a master frequency of 400 MHz. The signs of the PLL's are such that:
LO2-U= 25*400 - DDS_U     (9900 MHz)
LO2-L= 20*400 + DDS_L     (8100MHz)

Each oscillator has 2 outputs, which are used to pump the upper (or lower) arms of both H and V bandsplitters.
The phase of the 100 MHz signal generated by the DDS gets transposed to the LO2 frequency,with + or - sign.

The LO2 synthesis scheme

The DDS generates a 100 MHz sinewave in 4 points, from its 400 MHz clock. The chip itself is clocked at 100 MHz but an internal x4 PLL is activated to produce the 400 MHz. This way was found 4 times less stringent regarding 1 pps timings than clocking them directly with 400.
The natural frequency resolution of the 32-bit DDS chip is 93 millihertz, so a resolution extender using a microcontroller has been implemented  to reach the required 1.5 millihertz, which represents half a degree of phase shift after 1 second period of time.
The microcontroller receives every second via CAN from the host computer the inital phase and frequency offset for next period. More detailed information is available in the Dual LO2 user manual.

4. The NB correlator IF extender

The natural input range of the NB correlator is 100 to 1100 MHz. Inside this range it has built-in frequency agility, thanks to its synthesized LO3 and LO4.
The purpose of the IF extender is to select a part of the new IF and change its frequency so that the NB correlator can ingest it. A new LO named "LO2 bis" does this function. This LO is actually the third LO in the new frequency plan of the interferometer. History has already attributed the label LO3 (and LO4) to the NB correlator. To avoid renaming these, this LO has been named LO2bis.
At the IF extender input the range is 2.1 to 3.9 GHz . The LO2bis  translates the 2.1 to 3.1GHz range or the 2.9 to 3.9 range, into 100-1100 MHz, depending  wether it is set to 2.0 or 4.0 GHz.  Once again the excellent characteristic of the 2-4GHz filter allows adequate and spurious-free conversion.

The LO2bis is designed to generate either 2 or 4 GHz depending on the IFx_F command bit. For 4 GHz  the NB correlator covers either range 1 or range 3. For 2 GHz, range 2  or range  4 is covered. Band inversion happens accordingly.

The 2nd-bis downconversion scheme

The NB correlator has two inputs, IF1 and IF2. Each is conditioned by a separate LO2bis and can be switched to either the H or V output of the bandsplitter by means of polswitch1 and  polswitch2 . In case  they are connected to H and V signals  the correlator units work at the same frequency in polarization diversity mode (or polarimetry as well) on a reduced band. In case they are switched to the same polarization, the NB correlator units can be tuned to any frequency over the 4 GHz band.
The command bits IF1_F, IF2_F, IF1_P and IF2_P are physically written in the Dual LO2 module. The relevant CAN frame is described in the Dual LO2 user manual.

 5. The 400 MHz generator/distributor

The 400MHz VCO (which is a 1600/4 actually) is divided by 4 by a synchronous 4-phase counter and later by 20 then locked to the 5 MHz standard. The 100MHz timing can be adjusted by 2.5 ns steps in order to correct once for different transit times on the 1pps and 100 MHz lines.

400/100 MHz generation

The 400 MHz is raised at a high power level, then divided and distributed to the harmonic mixers of each of the 6 dual LO2 assemblies. This solution improves phase stability.  The 1pps is locally resynchronized to the 5 Mhz in order to absorb wiring differences in the correlator room.