bbm compression levels from a sine wave input
The A/D rms values versus mixer input power were
measured using a sine wave input to the baseband mixer chassis. We
wanted to see how large a sine wave we could push through the mixer
chassis before compression set in. This tells us how much of the a/d
dynamic range we will be able to use when a radar signal passes through
the mixer chassis. The setup was:
The plots show the
A/D rms levels vs sine wave input to the mixer (.ps) (.pdf).
- A 830MHz Sine wave was injected via the if2noise source input
(there was only a sinewave, the noise was disconnected).
- This sinewave was mixed to with a 1Ghz lo to 170MHz.
- The power was measured using the if2 power meter sitting on beam
- This signal was then sent to the bbmixers via the normal path.
- The signal was only injected into the low band. Beam 1 had no
signal since that synthesizer was used to generate the birdie.
- Before the sine wave was injected, the gain levels in the bbm
were adjusted to give a 30 count rms on input noise. This is the
setting for using the spectrometer so the compression levels
should reflect what occurs during normal observing.
- Each plot has 4 traces:
black(polAI), red(polAQ), green(polBI), blue(polBQ).
- Page 1: A/D rms (in counts) vs bbm input power. We got to an rms
of 1400 with close to 1dbm input.
- Page 2: base band mixer compression vs input power.
- A linear fit was done to 20*alog10(atodrms) vs bbm Input power.
The last two points were not used in the fit.
- The data -fit was plotted to see where the compression set in.
- The residuals start to fall off above -1 dbm At 1dbm it is
close to 1db compression.
- -1 dbm is flagged for reference
- Page 3: A blowup of the Peak A/D counts vs input power.
- this shows how much of the digitizer range we use before
compression sets in.
- at -1dbm the a/d peak values are 1400 to 1700. This is
- at 1dbm the a/d peak values are 1800 to 2047. This is near 1db
- Using a sine wave the bbm starts to compress around -1 dbm with 1
db compression around 1dbm
- The A/D's have a peak value of 1400 to 1700 counts before
compression starts. The A/D values for 1db compression is 1800 to 2047.
- We could remove 6db of attenuation from the bbm output stage.
This would increase the compression free range of the A/D but it
doesn't look like it will gain us much.