[evlatests] Results from P-band test run

[Paul Harden]

On 8/16/2013 3:48 PM, Rick Perley wrote:
>           a) the PSum and PDif values, being total power, are hugely
> sensitive to RFI contained within the subband.  The two 128 MHz-wide
> subbands each contain *a lot* of RFI

Attached is a typical spectrum analyzer display as seen at the antenna 
on the LBR receiver outputs, showing typical RFI and their fairly strong 
signal strengths.  This is with the antenna stowed with both 4- and 
P-band dipoles connected.  Power is heavily averaged.

>           b) the value of the Tcals will surely change dramatically as a
> function of frequency -- the 128 MHz-wide defaults are a large fraction
> of the observing frequency, so I would expect this variation to be
> significant.

In doing the hot/cold tests for Trx and Tcal, I use the following 
filters and noise bandwidths:

4-band: 73.8 MHz filter, 10 MHz bandwidth
P-band:  350 MHz filter, 25 MHz bandwidth

I have not measured Trx or Tcal at other frequencies within P-band.  If 
you think this would be a beneficial parameter, I'll see what other 
P-band filters I can find.

>      The stability of the correlations (amplitude and phase) is wonderful
> ('to die for'!).

Thanks.  I set the overall receiver gains to be within 3dB from 
unit-to-unit, and generally within about a 2dB difference between the 
two polarization channels to ensure consistent powers from all 
receivers.  In the lab, I have seen little gain variation over 
temperature changes, since these are not cooled receivers.

> The bandpass amplitudes are
> remarkably flat from 240 to about 450 MHz, above which many antennas
> begin to lose signal.

The P-band filters used in the receivers are quite sharp and flat from 
220-498 MHz.  Any slope or ripple in LCP/RCP is almost always due to the 
interconnecting coaxial cables.  Your 240-450 MHz realizable bandwidth 
is the bandwidth of the P-band dipoles, not the receivers.

>      Many antennas show a rapid bandpass oscillation (standing wave) of
> spectral period ~3.2 MHz.  This is seen in both amplitude and phase, and
> corresponds to a (free-space) length of 150/6.4 = 47 meters.

This is *close* to the 120-140 foot length of the coaxial cable runs 
from the LBR at the Apex to the Vertex Room, with about another 10-12 
feet of coax to the LO/IF system (T301 4/P upconverter).

>      Polarizations are swapped on ea03, 09, 14 and 26.  I'm also pretty
> sure they are swapped in ea01 (the near-dead 'RCP' channel makes this
> determination slightly dicier).

In the near future, we will be installing receivers into 19 and 21, and 
will attempt again to straighten out the P-band polarization swaps. 
I'll repair the receiver previously in EA01 as soon as possible.

>      Curiously, a number of antenna-IFs were set to too low a power (as
> seen in PSum) -- all the really low ones are in the 'RCP' side.  This is
> also seen in the visibilities from these antennas.

This is curious and sort of points to gain problems in the LO/IF system 
or attenuator settings.

>      In short -- this system really works well!  Far better than the old,
> narrowband one.

Really good to hear that.  Our biggest problem has been cabling. 
Re-using 25-year old coax cables and corroded connectors from the 
original systems has caused more problems with attenuated and reflected 
power problems than anticipated.  Thanks to Dan Mertely and interns, 
we're slowly getting the substandard cables replaced or repaired.

I hope the above fills in some of the questions you might have had.

As a reminder:
LCP = XLP = Horizontal
RCP = YLP = Vertical
:-)

Paul

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