Sampler Statistics and Attenuator Levels

Attenuators control the level of the analog signals from each antenna just before they are digitised. The degree of attenuation is controlled automatically (by CAOBS), but you should check that this is operating correctly.

To ensure the samplers are running in automatic attenuation mode:

	CAOBS> enable auto
	CHECK:
	$\bullet\,\,$sampler statistics close to 17.3%		Use CAMON stats page.
			A small (1-2%) deviation is expected for a few integration cycles after source or attenuation change.

If any of the statistics are unacceptable for more than a few minutes, you can override CAOBS and change the degree of attenuation manually. To do this, perform the following steps:

	Ensure that the telescope is tracking.		Any where will do, but at this point it's probably best if you track on your calibrator.
	Display the sampler reference levels and attenuator settings for each Freq 1 and Freq 2:		Displays values for the XX (An) and YY (Bn) polarisations for Freq n.
	CAMON> samp		The line labelled attenuator shows the current attenuator settings. The four-digit attenuator setting jklm corresponds to the A1, B1, A2 and B2 setting for that antenna.
	CHECK the 'SAMPLER NEG' and 'SAMPLER POS' values. They should be:
	$\bullet\,\,$approximately equal and close to 128		Acceptable deviations are actually 10 - 246

	To correct sampler attenuation:
	CAOBS> disable auto		To disable automatic sampler attenuator mode.

	If 'SAMPLER NEG' too small:
	decrease the corresponding attenuator levels.		i.e., reduce the attenuation.
	If 'SAMPLER NEG' too large:
	increase the corresponding attenuator levels.		i.e., increase the attenuation. Do this even if the GTP levels are 0 -- the GTP levels sometimes saturate, and display 0, the actual level is much higher.

Do the opposite for `sampler pos' numbers.

Note: j, k, l & m can take integer values in the range 0 to 7. Each step in value for the fine attenuators corresponds to a 2dB change in power (GTP) through the system.

	CAOBS> set atten ca0n jklm		where n is the antenna number
			and j,k,l,m range from 0 to 7.

Alternatively, each frequency can be adjusted separately:

	CAOBS> set atten ca0n jk		For Freq 1: IF channels A1 and B1.
	CAOBS> set atten ca0n *lm		For Freq 2: IF channels A2 and B2.

Wait for 6 cycles while the new sampler levels are installed and take effect (i.e., 1 minute, if cycle time is 10 seconds.)

	CHECK the following is now true:
	$\bullet\,\,$`SAMPLER NEG' and `SAMPLER
	POS'in allowed range (128$\pm$64)		Will complain if $<$10 or $>$246.
	$\bullet\,\,$`GATED TOTAL POWER' is
	0.35 $<$ GTP $<$ 4.8		Operates to GTP $\le$5.0
	$\bullet\,\,$both `SAMP STAT NEG' and `SAMP STAT POS' are approximately 17.3%

If this is not the case, check you adjusted the erroneous sampler. If still not responding, consult your Duty Astronomer.

NB: if there is insufficient adjustment of the attenuators when using the set atten command, use coarse attenuator mode to change them. If you find it necessary to change coarse attenuator settings . Each step in value for the coarse attenuators corresponds to a 6dB change in power (GTP) through the system.

	CAOBS> show coarse		Show coarse attenuator settings.
	CAOBS> set coarse ca0n jklm		To change attenuator settings
			where j,k,l,m range from 0 to 3.

An example of the CAMON samp page is shown. At this time, observations were being carried out for one frequency only. The current attenuator levels can be seen for each antenna as can the sampler levels and the gated total power (GTP).

Figure 5.3: The CAMON /samp page displaying the sampler levels, the current attenuator levels and the gated total power for the array, when just the first frequency is being used.