> I confirm the spur level on the spectrum analyzer. However the > 3048A always > says the spur is 6 dB lower than it actually is (6 dB plus/minus 0.5 dB). > > Can anyone tell me why this is so. > > I wondered if it had something to do with phase noise and amplitude noise > not being the same thing.
That sounds likely. It's interesting to think about where that 6-dB fudge factor might be coming from. The principle behind the most common measurement made with test sets like the 35601A, 11848A and 11729B/C is to use a double-balanced mixer to generate a simple baseband (0 Hz IF) output, given RF and LO inputs in quadrature from the reference and DUT respectively. The mixer acts as a phase-to-amplitude converter with a particular gain constant. Its baseband output is an AM signal that can be amplified as much as necessary and viewed on a low-frequency spectrum analyzer with the appropriate level correction. If you are calibrating such a measurement by hand -- e.g., say you're using an 11729B/C without any system software -- the HP-suggested way is to temporarily tweak the reference off-frequency by 10 kHz and drop its amplitude by 40 dB, and look at the amplitude of the resulting beat note on the spectrum analyzer. That amplitude is considered the "carrier" level for the measurement, with all measured data points being expressed relative to it. Then, at each data point, you subtract the system's PM-to-AM conversion constant, which is basically the post-mixer gain plus the aforementioned 6-dB fudge factor. If you have a 40-dB LNA between the mixer's IF port and the analyzer's input port, for example, that constant is -(40 dB+6 dB), according to HP's app notes. This procedure works as long as the mixer's conversion gain is constant; i.e., its RF port isn't driven into saturation during the real measurement. That '6 dB' part is probably what is bothering you. According to the notes written by the guy whose office door the 3048A authors would have knocked on for advice (see www.ke5fx.com/Scherer_Art_of_PN_measurement.pdf page 12), you need to subtract 6 dB from the noise trace for two reasons. 3 dB of it comes from a mysterious "Accounts for RMS value of beat signal (3 dB)" clause in Scherer's app note. Second, the AM noise at baseband contains energy from both conversion sidebands. You're trying to measure SSB noise, so there's the other 3 dB. I will confess I don't completely follow his reasoning. First, subtracting the first 3 dB from the noise trace implies that there's something different about the "RMS-ness" of SA-measured spur levels versus SA-measured noise levels. I don't see the mathematical justification for that. If nothing else, the difference between the crest factors of discrete spurs and Gaussian noise is much greater than a 3-dB correction would account for. Second, the two mixing products that are folded into the baseband spectrum by the mixer are identical, since the IF spectrum is symmetrical. Given 50R impedance at all three mixer ports, wouldn't the DSB output voltage be 2x (6 dB) the equivalent SSB value, not 1.414x (3 dB) as Scherer indicates? -3 dB is what you use to correct for addition of dissimilar sources, -6 dB for in-phase ones. So my thinking is that while Scherer has the right figure for AM-to-PM noise conversion (-6 dB), he may have reached it for the wrong reasons. I do not believe that any RMS correction needs to be applied for the calibration beat-note amplitude versus that of the sample-detected AM values that will eventually be interpreted as either phase noise values *or* discrete spurs. And I believe that the proper correction value for baseband downconversion is 6 dB rather than 3 dB, regardless of whether the spectrum is full of noise, spurs, or both. >From what you are saying, it sounds like the 3048A system software is going out of its way to avoid applying the -6 dB part of the gain correction factor to discrete spurs. In my opinion they should be treated identically. Disclaimer: Dieter knows what he is talking about. I don't. Caveat lector. -- john, KE5FX _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
