Wednesday, 13 November 2013

Subband to frequency checking

On Monday, we reported on the checking of the cells within the HBA tiles. As part of the same test, we also checked on the frequencies. Although there had been some discussion regarding the registration between the subband and the frequency due to some confusion related to the documentation, we had this pretty well resolved a while ago. However, there is nothing like actually checking it for real. Therefore, as part of the general testing day, for the cells, polarisations and other things, we checked of the frequencies as well.

To do this, we built a test aerial. This was a crude dipole made with some wire, a broom and a snow-post. The coax lead from the dipole went back into the site office, where we had a calibrated signal generator.

Our hi-tech aerial! (Photo: D. McKay-Bukowski)

Using this, we could inject signals and look at the corresponding response. Here is one example output, using RCU mode 5:

The colour indicates signal strength. Red is strong, yellow is medium and blue is weak. Of course, most of the spectrum is weak. The injected signals, in time order, were 149.902, 150.000 and 150.097 MHz. Each sample is 10 seconds in duration. We ran each for a about 30 or so seconds, with a similar gap in between.

As you can see, there was the best response at exactly 150.000 MHz, with no spill into adjacent channels. Additionally, the signals injected on the subband boundaries do tend to appear roughly equally in both channels.

Like our other tests, this came as no surprise. However, it is always useful to double check all aspects of the measurements and verify the data integrity.

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