This page is a sub-page to
PRECISION MEASUREMENTS OF NOISE FIGURES.
The image below shows the Linrad screen in the morning. The two maxima on the signal level are caused by the sun shining on the directional coupler and the LNA, heating them up. The LNA is the ATF33143 with negative input impedance and 6 dB return gain.
The change in the coupling factor is in the order of 0.005 dB. In this experiment the dummy load is kept in an ice/water slurry.
|Figure 1. The sun shines through a window onto the directional coupler in the early morning. The window is divided in two parts. That is why there is a double peak. The dummy load is in ice/water.|
The day after this experiment was repeated with the
dummy load in free air,
exposed to the sunshine a little later than the
The increased temperature on the dummy load reduces S/N by about 0.23 dB. That corresponds to a temperature change of about 15 degrees. By how much the coupler and the LNA are heated is not clear from this experiment.
|Figure 2. The sun shines through a window onto the directional coupler in the early morning. The window is divided in two parts. That is why there is a double peak. The dummy load is in the air and hit by the sunshine a little later than the LNA and coupler.|
Figure 3 shows the effect of heating the coupler.
The temperature is rised from 25.3 to 36.5 degrees and the S/N ratio
is then improved by 0.008 dB.
This is in the opposite direction but a similar magnitude
compared to figure 1.
The conclusion is that about 10 degrees will change the coupler
to increase the coupling by 0.008 dB while a similar temperature
increase will degrade the NF of the LNA by about 0.015 dB.
The conclusion is that the coupler should be kept at a constant temperature well within one degree while the dummy load is moved between the hot and the cold areas.
The NF of a GAAS FET amplifier varies with the temperature. When comparing the LNA to measurements made at EME2012 it is necessary to study it at the same temperature.
|Figure 3. The coupler is heated by 11.2 degrees.|