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Why don't I get the same answer as my spreadsheets

Question asked by rulonv Employee on Jul 31, 2003
SPECTRASYS accounts for VSWR between stages, sneak paths, reverse isolation, frequency response, channel bandwidth, gain compression, broadband noise and image noise.  Spreadsheets do not!  Cascaded noise figure equations assume no image noise and perfect matching between stages.  Cascaded intermod equations assume no frequency rolloff for the interfering tones.  This can be a bad assumption especially for a receiver blocking test.

In order to correlate SPECTRASYS data with a spreadsheet (or other math packages or programs) the schematic must be reduced to the spreadsheet case.  That is:

    1. Remove VSWR and frequency effects
      a. Behavioral filter have return loss which is a function of the ripple.  Set the ripple to something really small like 0.001 dB
      b. Set all ports and stages to the same impedance 
      c. Replace S Parameters elements (or other frequency dependent elements) with attenuators or amplifiers of the equivalent gain.
    2. Remove sneak path effects
      a. Set isolations very high (100 dB)
      b. Set reverse isolation very high (100 dB)
    3. Remove gain compression effects
      a. Gain compression is based on total node power not channel power.  All unwanted signals including noise will contribute to the total node power.
      b. Increase the P1dB, PSAT, IP3, and IP2 points of all non-linear stages
    4. Remove image noise effects
      a. Set the image rejection high (100 dB) in all mixers (be sure to reject the image frequency band not the desired channel band)
After making these changes you will get excellent correlation.

[Spectrasys, Gain, Cascaded Gain, Cascaded Noise Figure,  Intermods, GAIN, CGAIN, CNF, NF, IM3]