blanking with Faa, 1290, and punta salinas radars.
az=180,za=17 to 17.5
ADC blanking uses the digitized signal to determine
whether to blank or not. If the digitizer exceeds the maximum value (ok
maybe equals the maximum value) then the current fft is marked as
being blanked. All ffts marked as blanked will not be added to the
accumulator. Some properties of the adc blanking is:
- When blanking is done, all spectra,cross spectra for that
integration are blanked as a unit.
- For each integrated spectra, the spectrometer returns the number
of ffts accumulated so you can tell how many ffts have been blanked.
- The spectral samples are still spaced the the requested fft
accumulation time they just have fewer integrations included. This is
important for pulsar observations that need a constant sampling
- As an example: a 172 MHz bw, 1024 sample fft has a
time duration of
5.9 seconds so you can include/exclude things at this time increment.
- See Ao
fits file for pdev spectrometer: status Heap for a description of
how the number of integrations are returned in the cima_fits fits files
(psrfits to come..)
adc blanking with Faa, 1290, and punta salinas radars.
az=180,za=17 to 17.5
On 17sep08 data was taken using the galfacts
configuration. The setup and telescope motion was:
- az=180, za=17 to 17.5 in 20 seconds (about 1/2 slew speed).
- pdev cfr=1375 MHz. 172 MHz bands centered at 1300 and 1450 MHz
- Data taken at 2 millisecond sampling with 16 bits and 2048
channels in full stokes mode.
- The low winking cal was cycled at 20 Millseconds on and 20
milliseconds off (synchronized to the ffts).
- Data was taken for 20 seconds with adc blanking off and adc
blanking on. Both scans covered the same za range (although the radar
phase was different for a given za).
Dynamic spectra were made for each 20 seconds worth of
data. The calOn+calOff cycle was summed to give a 40 millisecond
sampling. PolA and polB have been averaged. The top plot is the 172 MHz
centered at 1450 MHz while the bottom plot is the 172 MHz centered at
spectra 20 seconds with blanking Off (40 ms resolution) (.gif):
20 seconds with ADC blanking on (40 ms resolution) (.gif):
- at .65 and 12.65 seconds the FAA 1330 radar points at AO causing
- At 1.66 and 13.65 seconds the remy 1290 radar points at AO
spectra for 2,3 seconds where 1290,1330 radar are max.. 2 millisecond
- At 14.85 seconds the 1330 radar points at AO. There is no
horizontal line of power and there is a dropout in thte 1330 power.
- At 16.1 seconds the 1290 radar points at AO.
Total power and spectra during
the radar peaks (.ps)
- The top plot shows no blanking. The on/off pattern is the cal
firing every 20 milliseconds.
- the bottom plot show blanking on.
- At 14.9 seconds you can see that the 1330 power is gone. All of
the radar pulses during this peak were blanked.
- The total power for 1 MHz about the 1330 and 1290 signals
was computedand then plotted with 2 millisecond time resolution.
- About 60 spectra (2 millitsecond resolution) were plotted (with
offsets) to show the spectra
near the radar peaks.
- Page 1 blanking Off:
- The dashed green lines show where the spectra were taken
- There are lots of harmonics of the radar during saturation.
They match what happens when you clip a sine wave (see clipping
a sine wave).
- Page 2 blanking on:
- The dashed green lines show where the spectra were taken.
- The 1330 radar (red) drove the adc into saturation so blanking
- The blanking caused the 1330 MHz dropout near 14.85
- The 1330 spectra have the 1270 image of 1330 near the parts
that were not blanked. This is probably happening in the mixer.
- The 1290 radar (black) was not strong enough to cause adc
- There is no drop in the 1290 power. Since the adc did
not clip there are no harmonics from adc clipping.
- The 1310 image of the 1290 signal is present for the entire
time the 1290 signal is strong. The bbm has probably gone into
compression and not created i,q of the correct phase and amplitude.
- When a radar signal goes beyond the a/d limit you get a
clipped sine wave that creates many harmonics in the band.
- The adc blanking can stop the harmonics caused by the adc
clipping to be included in the integrated spectrum.
- If the radar does not reach the adc limit, it will not be
clipped. Since it hasn't reached the adc limit it will not create
harmonics caused from adc saturation.
- The baseband mixer will go into compression before the adc
clipping starts (see injecting
sine wave into bbm). This occurs around an ADC count of 1500 (out
of 2047). This can cause and image of the radar to appear.