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Instrumental Delay Measurements

DFB Delay Calibration

Instrumental delays in the DFB systems can be calibrated using a modulated bandpass signal. The modulation is provided by the Agilent pulse generator in Rack 5, Out 1 of which is fed to a pin modulator (HP 8731A for 20cm, 256 MHz bw; HP 8732B for 10cm, 1024 MHz bw). For 10cm and 20cm the pin modulator is placed in the downconverter input FC1 line (rear of Rack 6) and for 50cm it is placed in the FC3 line. The modulated A poln signal can be fed to both PDFB4 inputs by using a splitter (e.g., Minicircuits ZSC-2-4) on the A channel output of the switch-matrix system.

The function generator Out 1 can be monitored on the CRO, triggered by the 1ms pulse.

Settings for the function generator are given in the file DelayList.txt; a copy exists in /pulsar/psr/dev/cvshome/man082/soft_atnf/atas/caldelay/ on the Epping linux system. The entries are: instrument, config, min period (ms), nbin, bw, nchan, cal freq, pulse period (microsec), nr of cycles per 1ms group, duty cycle.

The pulse generator is set up as follows:

  • Connect trigger (Ext In) to 1ms output of Parkes Clock (pkclk00, rear of Rack 5). Select External Trigger.
  • Connect Out 1 to the PIN diode Bias input and to the monitor CRO.
  • Select "Pulse" function
  • Set Period (from Delaylist.txt according to config), Phase = 0.0, Leading Edge Delay = 2.5ns, Duty cycle (normally 40%), (Page 2) P-P Amplitude 2.0V, Offset -220 mV, Load Impedance 50 ohm, (Page 3) Output impedance 50 ohm, Polarity normal, Freq. Coupling Off, Freq Mult 1, Freq Div 1.
  • Select "Burst" function
  • Set # of cycles according to DelayList.txt

To make the observations, use TCS next, ensure that the relevant receiver is switched on, that the corresponding losetup has been run. The cal should be switched off on the MCP and "disable ccu" entered in the TCS Command box. Use an appropriate schedule file (e.g., ~/tcs/sched/P737/CalDly/P456_HOH_CalDly_DFB4.sch) setting the pulse generator appropriately for each configuration.

Processing

Timing templates are made using delay_template (/pulsar/psr/dev/cvshome/man082/soft_atnf/atas/caldelay/delay_template.c). This program reads in DelayList.txt (or a file with similar structure) and an impulse response, normally 6db_filter.txt and computes templates for a selected instrument.

Pat is used to analyse the data. Options are:

  • -r: list phase of the reference point, the delay in microsec and its uncertainty
  • -t -K 1/xs: plot the observation, template and difference profiles. The observation is rotated by the reference point phase and normalised to the same area as the template before differencing.
  • -u: a list of equivalent TOAs in Tempo2 format.