[evlatests] Some Small Nonlinearity in the T304?

[Michael Rupen]

RCP and LCP behave quite differently in terms of Pdif.  Which
T304(s) did you examine?  As I recall we have one such unit per IF,
i.e., a different one for {A B C D}.

             Michael

>    Bob Hayward and I performed some tests on the T304 module on May 12,
> which I've now had the chance to properly reduce.
>
>    The experiment reported on here inserted noise power from a noise
> diode into the C-band front end of ea24.  The inserted noise was varied
> over a range of ~22 dB through a selectable attenuator.  The maximum
> noise added was about a factor of 3
> .5 greater than the 'cold sky' noise -- a useful range over which to do
> the tests.
>    The normal calibration noise diode was modified to increase its
> value by a factor of a few -- to a level about 28% of the cold sky noise
> power.  This was done to make its contribution more easily visible in
> our analog power meter.   We manually switched the noise diode on to
> measure its contribution.
>    A 2-channel analog power meter was employed to measure the input
> power to the T304 within a 100 MHz-wide bandpass, and at the same time
> the output power from the T304, also within a 100 MHz-wide bandpass.
> (RF filters were employed to define these bandpasses, and the LOs were
> selected to ensure that both power meter channels were measuring the
> same RF frequency).
>
>    With the antenna pointed at the zenith (= 'cold sky'), and operating
> at a frequency of 4.9 GHz, we recorded how four quantities varied as the
> noise attenuator was varied in steps of 1 dB over a range of 22 dB:
>
>    o  The input power to the T304
>    o  The (input + cal) power to the T304
>    o  The output power from the T304
>    o  The (output + cal) power to the T304
>
>    All data were recorded in logarithmic units ('dBm'), which were
> converted to linear units for the analysis below.
>
>    Analysis:
>
>    A)  The noise diode power measured at the input to the T304 was
> determined by subtracting the first two quantities listed above.  The
> result showed a small but significant decline as a function of input
> power.  A linear fit gives:
>
>    Cal Power = 0.0557 - .0022*Pin    microwatts.
>
>    The input power ranged from 0.2 to 0.85 microwatts -- hence, the
> calibration signal apparently declines by about 2.5% over the input
> power range.  I can imagine two scenarios leading to this result:
>
>       a) The noise diode power actually does decrease by this value
> over that range (perhaps due to some interaction between the input noise
> source and the calibration noise diode), or
>
>       b)  The amplifier gain (either in the front ends, or in the T302)
> declines linearly with input power by ~2.5% over the power range.
>
>    B)  We can determine the T304's gain by subtracting the last two
> quantities in the list above.  This gives the output calibration power,
> after the T304 module.  This power also is seen to decline with
> increasing input power.  The relation is now:
>
>    Output Cal Power = .08981 - .0021*Pin  microwatts.
>
>    The slope is exactly the same (within the errors), and the power
> level is higher.  The T304 gain is the ratio of these:  Gain(T304) =
> 1.612.
>    This gain is independent of the actual input power -- evidence that
> the T304 is very linear over the 4:1 ratio of input powers employed.
>
>    C)  Another measure of the gain is obtained by plotting the output
> power as a function of the input power.  This gives:
>
>    Pout = -.0798 + 1.575*Pin    microwatts.
>
>    The negative offset is physically impossible, and indicates that (at
> least) one of the two power channels has an offset.   This *should* only
> be an offset, and should not affect measures of the gain.
>    The curious result here is that the slope of this relation -- which
> defines the T304 gain -- is significantly less than the gain  determined
> by the ratio of the calibration signals.  Yet this plot is linear, with
> no sign of curvature.  Perhaps the difference is some  manifestation of
> different sensitivities of the two channels, although it is not obvious
> to me how this could explain the different measures of the gain.
>
>    Despite the uncertainties given above, the changes in the
> calibration power as a function of the input power are nowhere near the
> values recorded by the switched power system, where a decrease in PDif
> is typically 10 to 20% over input power ranges varying by a factor of
> four -- such as in our experiment.  Our calibration power reduction is
> by only ~2%.  And for this particularly antenna (ea24) at C-band in RCP,
> the decrease in PDif claimed by the switched power system due to  a mere
> doubling of the input noise is by nearly 30% -- about 20 times higher
> than measured in this analog experiment.
>
>
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