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lbw receiver status jun05-sep05. 1400Mhz resonance and diode 1 dies

01sep05

The lband wide receiver has had a number of things happen to it between jun05 and sep05.

What has happened:

In 2004 tsys for polB had been drifting. On 01jun05 the dewar was warmed up and a bad cal cable in polB was found and repaired.
After the jun05 cool down a resonance was seen in polA at 1400 Mhz (see 01jun05 resonance in polA). This affected the daily Tsys measurements since they are made at 1400 Mhz with a 1 Mhz bandwidth (07mar06: the warmup actually created resonances in polA and polB).
On 22jul05 diode 1 failed (hcorcal). Users were switched to use diode 2 (h90cal). People using the circular option (hybrid after the dewar) got no cal in one polarization because of the 90 deg phase shift in cal going to polA and polB in combination with the hybrid used to create the circular polarization.
On 25aug05 the observatory lost power and lbw warmed up again. After the cool down, Tsys for lbw polA remained about 3K higher than it was before the warm-up.

Tsys april through sept.

The daily tsys measurements between may05 and sept05 (.ps) (.pdf) were plotted using diode 2 (the h90cal that was not replaced). Black is polA and red is polB. Epochs where things occurred are flagged in color:

Green. This is after the jun05 warm-up and cal cable repair. The the new cal values were measured and then installed on 21jun05. The jump in polA Tsys at 01jun05 is because there was now a resonance at 1400 Mhz where the tsys was being taken.
Blue: This is when diode 1 failed. The users were switched over to use diode 2.
purple: 24aug05 when the dewar warmed up. polB eventually returned to the pre warm-up value. polA stayed 3 to 4 K warmer. This may be do to the strength of the resonance changing after the warm-up.

01sep05 looking at the 1400 Mhz resonance

On 01sep05, cal onoff spectra were taken around 1400 Mhz with a 25 Mhz bandwidth centered on 1400 Mhz. This was sent to 4 separate boards of the interim correlator. The lbw spectra show the resonance at 1394 and 1400 Mhz (.ps) (.pdf):

Page 1: The cal on and cal off spectra. Black is polA while red is polB. The upper traces of each color has the cal on.
Page 2: CalOn - calOff spectra (with no bandpass correction). You do not see the bump in the cal deflection so it is not coming from the cal diode.
Page 3: (Calon-calOff)/normalized(caloff). This bandpass corrects the cal deflection. It also produces the dip at 1394 and 1400 because of the resonance in the calOff. The dip in polA is stronger than polB. The different frequencies may be do to the physical separation of the probes.

Summary:

There is a resonance in polA at 1400 Mhz that is increasing the daily Tsys measurement by up to 6 kelvins. Pol B shows a resonance at 1394 but it is not as strong. The resonance appeared after the jun05 dewar warm-up. It may have gotten worse after the aug05 warm up. I should check the "known" resonances in lbw to see if the polarization's are offset in frequency (the last test added the two pols together since a polarized source was used).
Diode 1 died on 22jul05. Diode 2 (the h90cal) is being used in its place. There is a 90 degree phase shift in the values that go to polA and polB for this cal. Users can create circular polarization's using a hybrid that sits after the dewar. When the h90cal value is passed through this second hybrid, no cal will end up in one of the circular channels (90 degree phase shift + 90 more from the 2nd hybrid cancels the signal). So users should not use the circular polarization option for lbw until the new hcorcal diode is reinstalled and measured.

processing:x101/050901/chkcal.pro

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