The focal plane array receivers of  the Pico Veleta deliver 18 IF bands, every one being one GHz wide. The pre-existent autocorrelator, VESPA, is very flexible, but does not offer the possibility of processing the totality of the bandwidth delivered by the receivers. For this purpose a special spectrometer has been designed, which has no flexibility at all, but which is capable of crunching all the bandwidth at once.
 
 

Architecture

The signal is delivered in the 100 to 1100 MHz range, to be compatible with the already existing spectrometers. This is not fortunate because an up-conversion is necessary, in order to reduce the relative frequency range.  An Image Rejection Mixer (IRM) changes the 1 GHz band into two 500 MHz subbands. Both USB and LSB are converted to digital (full 2-bit) after lowpass filtering and level adjustment. The digitized data (1024 Ms/s) is conditioned to a lower frequency, re-ordered and transmitted to the correlator boards pairs, which are clocked at 128 MHz.
 
 

The whole system has been squeezed in a single rack :

Its total power consumption is 2.4 KW

1Gb/s 1:8 Demultiplexer (view image)

The purpose of this board is to receive a dataflow at 1 Gigabit/s and to deliver 8 dataflows at 128Mbit/s to slower standard commercial electronics.

This board receives a digital dataflow at 1 Gigabit/s from the sampler, a 2-bit data and a 1024 MHz clock. After translation from NECL to PECL levels, the 2 bits are demultiplexed by two independent MAX3680. They deliver in LVTTL levels two 8-bit dataflows and their associated 128 MHz clocks, which are randomly phased at power-up. These 128 MHz clocks go in a synchronisation detector. Until clocks are synchronized, the detector requires from the 1024 MHz clock divider of the modulus demultiplexer, to drop one cycle. The modulus data timing is automatically disciplined to the sign timing in less than 8 tries at power up. A red LED on each module indicates if this mechanism has failed.
Then data flows into an FPGA (EPF10K10A) which generate  immediate and delayed dataflows that are buffered and sent to the correlator boards through the P2 backplane. A 128 MHz clock is also delivered to the two correlator boards that are involved with each demux board. Pairs of correlator boards may receive 128Mhz clocks of different phases, at random.

Boards have a 3U-VME format and are located at the rear of the rack.  This board also carries the I2C bus from the P2 backplane to the sampler.
 

Correlator boards (view image)

The correlator boards are the same ones that have been used for the Plateau de Bure interferometer correlator. They are operated with a time-multiplex factor of 8. Actually they work in cross-correlation, but they ignore that the demux/driver board delivers to them the same signal (with a half-length delay) on both inputs. They are populated with IRAM correlator chips, and are described in more detail here.

This configuration is summarized in the zero flexibility  table below :
 

Control board (view image)

1) The timing interface

The input signals are :
 

BLANKING

STATUS

PHASE

1SEC

 

 

An EPM7128 FPGA receives the inputs and delivers correlator timing signals under the supervision of a control register. Three acquisition rates can be selected : 128 Hz, 64 Hz or 32 Hz.
It also feeds the VME controller with interrupt requests.

The output signals are :
 

LDRST for the correlator

INHADD for the correlator

A 128 MHz clock for test

A 128/64/32 Hz clock for test

A 1 Hz signal for test if used

 

The VME controller is an EPM7192 FPGA that decodes the address of this board plus all the boards of the unit, generates 4 interrupts and send the DTACK to the CPU.

The 4 interrupts are :
 

1 : STATUS

2 : PHASE

3 : 128/64/32Hz

4 : I2C interface

Two PCF8584 I2C bus controllers are used to handle 9 sampler addresses.
Boards have a 6U-VME format and are located at the front of the rack.
 

Up-converter (view image)

The input frequency range  (0,1 to 1.1 GHz) needs to be converted to a range acceptable by the SSB mixer. A fixed LO at 1900 MHz is used. The output is from 800 to 1800 MHz. Since the input and output ranges do overlap, (from 800 to 1100 MHz) the design has been optimized to the best RF-to-IF isolation of the mixer. The resulting LO leakage at 1900 MHz is quite important and would lead the following stages to misfunction, so a special circuit has been implemented for its cancellation. It makes use of an I/Q modulator, controlled by two potentiometers, that are individually adjusted to reduce the leakage to less than one channel power level.
 

IRM modules (view image)

Image Rejection Mixers (also called SSB mixers) are used to convert the 0.8-1.8 GHz IF band into a pair of basebands, from 100 kHz to 470 MHz. They receive a 1300 MHz LO from a local synthesizer. They make use of a wideband 90 degree phase shift LC network. The 100 kHz cutoff frequency causes a discontinuity in the spectrum at the center of the band. To observe a spectral feature of interest, it is wise to place it not exactly at the center of the band, but a few tens of MHz apart.

Samplers  (view image)

The samplers operate in full 2-bit mode at 1024 MHz rate. Its analog section features an amplifier and a 470 MHz (@-3dB) low-pass filter for antialiasing.  A programmable (via I2C) attenuator allows to adjust the input level to the optimal RMS value, by 0.5 dB steps over a 15.5 dB range. On the back  panel, two LEDs indicate either too low or two high input voltage. This is useful for quick visual debugging.
The digitized signals are transmitted down to the demux boards. On the same connector, the sampling clock and the I2C controls are sent up to the sampler.
 

9-output synthesizer  (view image)

Wilma requires oscillators at  1900,1300,and 1024 MHz, each of them delivering 9 copies. A single design has been made for all purposes, only differing by the settings of the synth chip, and the VCO for the 1900 MHz unit. They are locked on the 10 MHz from the station.  There are two synthesizers in the Upconverter crate, and three in each IRM/Sampler crate. On each crate there is a 10 MHz  outer BNC connector with an impedance of 150 Ohms. The three crates should be connected in parallel to one output of the standard. Inside a crate, a buffered daisy-chain with SMA cables is implemented.
 
 

This machine has been built and installed on the Pico Veleta observatory by November,2003.  Another correlator slice of 1 GHz has been simultaneously built for the observatory of Yebes. It has been named Peebles.