[evlatests] Some Small Nonlinearity in the T304?

[Keith Morris]

I'll mine the monitor data archive for TPD values during this experiment. 
Should be interesting to see which way they fall...



> 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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