LRO miniRF Linearity tests, recording with GASP, Aug 27, 2009

link back to main GBT LRO miniRF page . link to Aug 23 LRO obs

Project Code: AGBT09A_070_11

Tests were done similar to those on August 24, but this time recording was done with GASP.

Roger's report from Aug 27 tests:
    Yesterday, with help from Paul Demorest, we did more linearity tests on 
    the GBT S-band receiver and IF systems including GASP scans.
    Here Test Tone levels measured at the GASP-feeding Converter Modules' 
    (CM11 & 15) output port using a spectrum analyzer are reported.  It 
    appears the IF system through this chain is about 1dB compressed with 
    the Test Tone at +10 dBm.  Note that the ODM 5, 7 attenuators were set 
    15 and 16 dB lower than those in the Spectral Processor path, ODM 1 & 3. 
      Paul mentioned that at this level the GASP A/Ds were overflowed, but 
    he did not notice that strong a signal during the LRO observations.
    Measurement Details:
    S-X Pol -> ODM5 (6000/80 BPF, AT1=1dB) -> CM11 (AT1=25dB).
    S-Y Pol -> ODM7 (6000/80 BPF, AT1=0dB) -> CM15 (AT1=30dB).
    LO1 = 8380MHz, -10.6dBm (auto setting).
    System TestTone Generator HP 83620A.  Setting to 0dBm corresponds to -82
    dBm at S-band Rx LNA inputs (per Feb 2009 measurements).  Test Tone 
    frequency 2380.1 MHz (to avoid GASP center channel).
    Spectrum analyzer:  HP8562A, cf 404 MHz, span 10 MHz, rbw 100KHz, vbw
    10KHz, atten 0 dB.  Connected to CM11, 15 J5 via ~20 ft. RG 58. 
    Readings taken using marker on test tone peak.
      TT        CM11-J5 (X)     CM15-J5 (Y)
     -40dBm      In Noise       In Noise
     -30        -79               -81
     -20        -70               -72
     -10        -60.2             -62.1
      -5        -55.3             -57.1
       0        -50.4             -52.2
      +5        -45.7             -47.3
     +10        -41.0             -43.1

Paul's summary of GASP power levels:
    Hi Frank, Roger,
    I don't know if you guys have had time to look at any of the data from 
    yesterday.  I just took a quick look and generated the attached plot.  In 
    the bottom plot I've subtracted an offset to put the 0dBm value at 0. 
    The output power values come from looking at the power in a single 4 kHz 
    wide FFT bin near the tone's center freq.  I also subtracted off a 
    baseline power value computed from nearby channels using the -50dBm data 
    (we probably should have taken a "no tone" scan as well).  anyways, it 
    looks like the nonlinearity is at most ~1dB over the whole -50 to +5 dBm 
  • GASP Linearity tests:
    gasp linearity.png

-- FrankGhigo - 28 Aug 2009
Topic revision: r1 - 2009-08-28, FrankGhigo
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