Walsh system

From setiquest wiki

Jump to: navigation, search

The Walsh system is used in the ATA back-end. The last analog local oscillator is modulated by a square wave prior to mixing with the RF signal. This introduces a phase flip to the RF data - e.g., when the modulating signal is +1, data are in phase, when the modulating signal is -1, output data are 180 degrees out of phase from their original data. The phase switching is undone in our digital electronics before the beamformer and correlator blocks.

Contents

Explanation

So - why do this? There are 8 unique Walsh functions (at the rates 100 Hz ... 12.8 kHz), and they ensure that, where there is a potential for cross-talk in the analog system, neighboring signals have different Walsh functions. The short of it is that cross-talk signals don't get properly "decoded", and tend to integrate away. This is great in a correlator (for imaging), but can leave artifacts in the beamformed data - first and foremost in that any cross-talk signals can be seen - if we look hard enough - as very weak tone sidebands.

You might already have noticed that the analog "Walshing" is probably not exactly 0/180 degrees, and this can introduce artifacts (e.g., if the analog is 0/179, and the digital is 0/180, we'll end up with a 0/1 degree phase modulation in the resulting beamformed data). This leads to many weak sidebands at the harmonic rates of each Walsh function. Further, if the analog "Walshing" and digital "un-Walshing" are not very-well aligned in time, we'll get artifacts at the transition edges of the Walsh square wave. This will tend to create artifacts at 2x the Walsh rate and up.

There are 8 unique Walsh functions at the ATA, and they are phase-locked with the master clock. The system is wired such that each ADC unit (which accepts 4 analog inputs) has inputs that are on unique Walsh functions. The X-polarization antpols use 100 * 2^ functions, for exponents 1,3,5,7, and the Y-polarization antpols use 100*2^, for exponents 0,2,4,6. Most of the data sets that have been posted are for X-pol beams.[1]

The analog transition time (phase switch) was measured to be much longer than 1 sample[2] which makes perfect reconstruction impossible.

Flow diagram

This diagram describes how the GPS synchronized clock and Walshing unit fit into the ATA system.

ATA system clock walsh.png

Upgrade

In October 2011, Jack Welch mentioned that when funds are available the Walsh generators should be replaced with a more accurate phase switching system.[3] This would reduce the Walshing artifacts but it would not eliminate them completely.

A December 2011 test comparing the current RG316 cables used in the IF-ADC path to thicker LMR100A cables demonstrated that the LMR100A cables had crosstalk that was -30 dB lower.[4] The picture of the back of the RFCB rack[5] shows a large number of RG316 cables. A combination of shorter LMR100A cables with an improved wiring layout may make the Walsh circuitry unnecessary.

References

  1. http://setiquest.org/forum/topic/baudline-analysis-psr-b032954#comment-560
  2. http://setiquest.org/forum/topic/community-meeting-2011-10-25#comment-2744
  3. http://setiquest.org/forum/topic/community-meeting-2011-10-25#comment-2744
  4. http://log.hcro.org/content/if-coaxial-cables
  5. http://log.hcro.org/content/521-radio-frequency-converter-boards

External links

Personal tools
Namespaces
Variants
Actions
Navigation
Toolbox