A/D rms values vs mixer gain & compression


    The A/D rms values versus mixer gain were measured on the sky with alfa  to see how much range we had around the optimal operating level (an A/D rms of 30 counts). The A/D rms  counts were also measured using a noise source to check the dynamic range of the mixer/AtoD combination.

    Gain vs attenuation in the mixer chassis:

Measuring A/D rms counts vs mixer gain on the sky:

    The setup and processing was:
The plots show the A/D rms counts vs bbm gain. Each color is a pair of A/D converters (I,Q). Red in lowband polA, green is lo band polB, Blue is hi band polA, purple is hi band polB.

Measuring A/D rms counts vs mixer gain using a noise source

    The downstairs if/lo noise source was  used to measure the linearity of the bbmixer , A/D system. The setup was:

Mixer levels at 1db compression:

    The table below shows the computed power levels at stages in the mixer chassis. I've used the lowband with the 250 Low pass filter and the 500 Mhz noise input bandwidth. I've used attn=3 (gain=28) w(close to the 1db compression).
Computation uses lowband(250 lowpass filter) and (gain=28 or attn=3)
Input  Output  Component
-17.0 -21.0 H1 splitter
-21.0 -25.0 LPF1
-25.0 -29.5 H2 ProgAttn set to: 3. db (gain=28)
-29.5 -7.5 H3-1 Amp
-7.5 -12.5 U5 Attn
-12.5 7.5 H3-5 Amp
7.5 3.5 H1-2 Splitter
3.5 -0.5 U4-Attn
-0.5 -11.5 H4 Mixer
-11.5 -19.5 U3 Attn
-19.5 -22.7 LPF2
-22.7 -28.7 R2 attn
-28.7 -8.2 H5-1 Amp
-8.2 -17.2 U1 Attn
-17.2 -2.2 H6-1 Amp
-2.2 -8.2 U2 Attn

The measured output level from the mixer chassis (beam5a low band) was -4dbm so there is 4db more gain in the mixer chassis than expected.


    processing: x101/080424/chkadjp.pro,chkadjpNoise.pro

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