What we did:
Spectra taken 24mar21 showed large 64MHz ripples
when the all deflection was used to remove the IF
7apr21 a 2 minute on,off position switch was
done on casA. Plotting (spcPosOn-spcPosOff)/spcPosOff showed
Plotting spcCalOff/(spcCalOn-spcCalOff) for data taken after
the position switch still showed the ripple.
I also computed the sefd using a flux value o 620 Jy for
casA (from chris salter).
When processing the spectra:
- there were 7 172.032 MHZ bands each with 8192
channels. The spectra were spaced by 164 MHz.
- I averaged each of the 120 second scans.
- the bands were interpolated to a continuous band with
- When plotting, a median filter of length 5 channels was
used to get rid of DC.
The plots show the position
switch and calon,off spectra (.ps)
- Page 1: on,off position switching
- Top: (on -off )/ off
- black polA, red polB
- The vertical scale has units of Tsys
- The junk on the left comes from the mock
bandpass having no power (the IF filter cut it off).
- There is no 64MHz ripple seen
- the dip at 8750 MHz may be a resonance in the
- Bottom: SEFD using casA
- I used 620 Jy for the flux of casA at xband
- The sefd is (Tsys/Tsrc)* srcFlux
- Notes no the SEFD:
- casA is extended. (chris salter in his srclist says
it's about 5')
- the xband beam is around 10'
- I didn't do any corrections for the source being
extended. A point source would give a better value
- I found a 21' offset in the elevation encoder
(relative to the model made back in 2016.. maybe the
encoder jumped during one of our earthquakes?). ..
anyway, i came pretty close to optimizing the correction
on this source, but a new pointing model might give a
little better signal.
- Page 2: calon, caloff spectra
- After the position switch, i did a 10 second
calOn,calOff in the off source position.
- the off source was 35 amin from the on.. so we were well
away from the source.
- Top: calOn - calOff in spectrometer units
- You can see the 7mock bands of
- Bottom: CalOff/(calOn - calOff)
- this shows the ripple that was seen on 24mar21. It is
worse in polb.
What we were measuring.
- Assume we can break the signal path into 3 sections where
each section has a constant temperature value and a
- Tsrc .. assume this is flat..
- Tsky, Gstw(f) tsky is sky and ground. let Gstw
come from reflections on the structure or in the horn
- Tcal, Gcal(f) cal including the
- Trcv Grcv(f) this is every thing after
the cal injection
- Position switch:
- Ton = ((Tsrc + Tsky)*Gstw + Trcv)*Grcv
- Toff = ( Tsky*Gstw + Trcv)*Grcv
- (on-off)/off = (Tsrc*Gstw)/ (Tsky*Gstw + Trcv)
- Since Trcv/Trcv is about 9/100. expand the
- (on-off)/off = Gstw*(Tsrc/Trcv) * ( 1 -
- So any frequency dependence would likely come before
the cal coupler in Gstw
- CalOn= (Tsky*Gstw + Tcal*Gcal + Trcv)*Grcv
- calOff = (Tsky*Gstw + Trcv)*Grcv
- (calOn-calOff)/calOff = (Tcal*Gcal)/(Tsky*Gstw + Trcv).
expanding the denominator gives
- (calOn-calOff)/calOff = Gcal*(Tcal/Trcv) * ( 1 -
- In this case the frequency dependence would come from
the Gcal and Gstw
- Since Gstw came out small when we did the pos on/off , the
cal lines or coupler must be generating the frequency
dependence in the (calOn-calOff)/calOff..