Intro
We received the xband waveguide transition to help diagnose
problems with the feed/lna's. Prior to this debugging
i reran the xband monitoring of the receiver to see what
problems still existed (last runs were 01/21 jul21. The total
power results can be found here.
This page plots some spectra and standing
waves.
During data collection there was lots of
rain and lightning throughout the night.
- The 1 second spectra were first input.
- the dc spike in the middle was interpolated across using
the adjacent channels
- there were 7 x 172 mhz bands that were spaced by 164
MHz.
- The average spectrum for the 12 hours was computed and
then the 7x172 Mhz bands were interpolated to a single
spectrum.
- An average spectra was computed for each hour of
data and then the 7x172 bands were interpolated to a
spectrum for each hour.
- These spectra were then normalized to the 12 hour
average to remove the bandpass shape.
- Computing the acf:
- For each second of the 12 hours:
- The 7 spectral bands were interpolated to a single
spectra
- a gauss fit of the peak in the acf was then performed
to track the amplitude and lag of the peak (standing
wave location).
Plotting the spectra.
The first plots show the average
spectra (.ps) (.pdf) :
- top: Spectra by hour normalized to 12 hour average. PolA
- Each color is a different hour of data
- The bottom is 19:00, the the is 07:40
- an offset has been added for display.
- You can see a ripple that decreases into the night and
then increases. The decrease is probably where the rain
swamped out the other signals.
- middle: spectra by hour polB
- the ripple in the spectra is also visible.
- the normalized spectra remain flat across
frequency.
- Bottom:
- Blowup of 8400 to 8600 Mhz . I've removed the mean (and
added 1) to each hour to remove the tsys variation caused
by the rain.
- You can see the ripples in the bottom 3 (first 3 hours).
processing:x101/210919/xbspc.pro
Tracking the location of the ripple
with the acf
A sine wave in the spectra is a
spike in the acf (autocorrelation function). To track the
location of the peak in the acf:
- The average spectra was computed for the 12 hours, in each
of the 7 freq bands
- For each second of the 12 hours
- normalize the 7 spectra to the average value
- interpolate to get a single spectra of 975 MHz.
- compute the acf of the 975 MHz spectra (i didn't bother
to zero extend the spectra so i lost a little resolution).
- The time resolution of the acf was 1/975 MHz or 1.026
nanoseconds.
- Using 51 lags about the ripple peak (lag 71) fit a
Gaussian to the peak.
The plots show the results of the gauss fit to the
acf vs hour of day (.ps) (.pdf)
- Black is polA, red is polB.
- Top: amplitude of the fit.
- The amplitude of the standing wave falls until around 22
hours. After that the gaussfit to the acf channel did not
converge very often.
- Middle: the location of the peak in the ACF (in micros
seconds)
- the median value was about .0735 (A), .0733(B)
microseconds. If the peak is a reflection with vel=c, then
the dist stance is 150*[.0735,.0733] = 11.02, 11.00 meters
- polA saw a jump in the location of the ripple around
28.5 (4.5 am).
- Bottom: the width of the Gaussian fit.
- The large variations is where the fit did not converge
(mainly during the rain).
processing: x101/210919/xbspc.pro
Summary:
- There is a ripple in the spectra of both polA and polB.
- The amplitude of the ripple changes with time. It was
largest in the evening when we started.
- the location of PolA ripple varies (a few nano seconds).
polB ripple location does not vary.
- If the ripple is coming from a reflection in the system:
- the average time is .0734 usecs. using 150M/usecs to is
about 11 meters or 36.1feet
- if the velocity in the cable is .7 c, this gives 7.7
meters or 25.3 feet.
- This is the same result that was seen on 01jul21.
processing: x101/210919/xbspcrfi.pro