Bandpass Calibration

TIP Last Update: JeffMangum - 04 June 2009

Specification

  • Spectral Dynamic Range: 10000:1


Contents

Bandpass Calibration Memos (ALMA, EVLA, etc.)

A Note Regarding Bandpass Shape and Attenuator Response

On 2009-04-16 DarrelEmerson passed-on the following note from Peter Napier:

I like Darrel's suggestion of using the loss of quantization efficiency as a measure of the increase in variance across the spectrum. The resulting change in bandpass shape is probably more important than the sensitivity loss. As expected, the 4 level digitizer (DG) case (Correlator TDM mode) is a lot more sensitive to DG input power level than is the 8 level case (Correlator FDM mode). Here are the numbers for the 4 level case (from TMS(Ed2) equation 8.52 and Figure 8.5 (curve 2)).

E(dB)   QuantEffic
2            .963
1            .989
.5           .997
0           1.0
-.5          .996
-1           .985
-2           .936

where E is the DG input power compared to optimum and QuantEffic is the quantization efficiency compared to optimum. So with a tolerance  on DG power level of +- 1 dB the potential changes in bandpass shape are not negligible compared to the desired spectral fidelity.

So we are going to have to carefully develop a calibration strategy related to the allowed change in DG input power level. Certainly I think it will be necessary to do an amplitude and and phase bandpass calibration on either side of an IF or baseband attenuator change because I would not expect these attenuators to be spectrally flat to the desired level.

Peter

-- JeffMangum - 2009-06-04

Some Notes on ALMA Memo 505 (Bacmann and Guilloteau)

  1. It will be necessary to build-up a suitable database of bandpass calibration sources. The spectral indices of these sources will need to be known.
  2. Need to BP cal at observing frequency.
  3. To minimize bandpass errors due to the atmosphere, can use a variant of the "bandpass normalization" (uses normalization by the difference between the sky and load emission) technique by utilizing one of the calibrated loads in the Dual-Load relative amplitude calibration system.
  4. Tunable Filter Bank (TFB) measurements of total power will help with derivation of atmospheric opacity variations with frequency in BP measurements.

-- JeffMangum - 30 Sep 2005
Topic revision: r5 - 2009-06-04, JeffMangum
This site is powered by FoswikiCopyright © by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding NRAO Public Wiki? Send feedback