# Lab Exercise N

```Microwave Metrology -ECE 684
Spring 2010
Lab Exercise N: Noise Measurements
In this set of measurements, you will measure the equivalent noise temperature of
each component shown in Figure 0. Then you will connect them together and measure
the equivalent noise temperature of the downconverter assembly. The results will be
compared to what you calculate using the Friis formula for cascaded stages.
Since the noise you are measuring is very small (microvolts), you need to be
careful that your measurement is not contaminated by external signal pickup such as
other signal sources in the lab or noisy power supplies.
Prelab
Before coming to the lab estimate how much power should be measured in the
two configurations in Figure 3.1 as described under Y Factor Measurement.
Procedure
Y Factor Measurement. To start off, you will measure the noise figure of the low noise
amplifier and IF amplifiers using the E4418B power meter and the HP346A noise source.
The noise source provides “hot” and “cold” temperatures to the input of the amplifiers.
The measurement setup is shown in Figure 3.1. The noise generated by the noise source
is small enough that additional amplifiers must be used to get sufficient noise power
output to the power sensor. The power meter has a sensitivity of -70 dBm. The filter is
necessary to set the frequency and bandwidth at which the amplifier noise figure is being
measured.
Before coming to the lab, estimate how much power should be measured in the
two configurations in Figure 3.1, the calibration configuration and the measurement
configuration. Assume the ENR of the noise source is 5 dB.
In the lab, apply 28 volts and 0 volts to the noise source to get the hot source and
the cold source respectively. Record the powers you read from the power meter and
calculate the Y factor. Use the Y factor and the calibrated ENR listed on the noise source
to determine the noise temperature of the amplifier chain. Do this measurement for both
the configurations in Figure 3.1. Use the results and the Friis formula to determine the
noise figure of the LNA and the IF amplifiers/filter block. Also use the power
measurements to determine the gain of the LNA and LNA+IMRF block. Compare these
gains to what you measured in Project I.
Lab Exercise N: Noise Figure Measurement
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Microwave Metrology -ECE 684
Spring 2010
noise
source
power
sensor
LNA
LNA
Power
Suppy
915 MHz BP filter
Power
Meter
(a)
noise
source
power
sensor
LNA
LNA
LNA
Power
Suppy
915 MHz BP filter
Power
Meter
(b)
Figure 3.1. Configurations for measuring the noise figure of the low noise amplifier (a)
calibration and (b) measurement.
Measurement with Noise Figure Meter. The Agilent N8973A Noise Figure Meter (NFM)
will be used for the remaining measurements. This is a more convenient and automated
way of doing what you did above. The Agilent N8973A automatically measures the noise
figure (NF) and gain of the device under test. In its basic configuration, the NFM can be
tuned to perform measurements over the 10-3000 MHz range. Noise measurements at
higher frequencies can also be completed using this system, however we will limit the
scope of this experiment to the useful bandwidth of the downconverter.
Before the system can be used, the ENR table for the noise source must be
retrieved. This is done by going to File-&gt;Load selecting the file ECE684 and pressing
ENTER.
Two measurement modes will be required to collect data. The sweep frequency
mode will be used to measure devices that do not perform frequency conversion, i.e. the
LNA, IMRF, IFF, and IFA. To use this mode follow the ensuing steps
a. Bring up the Frequency menu by pressing the FREQUENCY/POINTS key.
b. Set the appropriate START frequency and STOP frequencies (890 MHz and
940 MHz for example).
c. Press MORE and set the number of frequency points.
d. To calibrate the system, connect the noise source directly to the noise figure
meter input and press CALIBRATE twice.
e. Confirm that the calibration indicator changes from a white Uncorr display to
a white Corr display. Confirm that you see a value close to 0dB for both noise
figure and gain.
Lab Exercise N: Noise Figure Measurement
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Microwave Metrology -ECE 684
Spring 2010
f. Insert device under test, DUT. The gain and noise figure plots will update
automatically.
g. The noise figure and gain can be shown on the same graph by pressing
FORMAT, ensuring that Format(Graph) is on and toggling Combined to On.
For devices that perform frequency conversion, i.e. the mixer and entire downconverter a
second measurement mode must be used. Figure 3.2 illustrates the setup for measurement
of mixer noise figure and gain. To use this mode, set the LO to 975 MHz with 7 dBm
output power and proceed as follows:
a. Press MEAS MODE and select DOWNCONVERTER.
b. Press MODE SETUP and set the LO Frequency to 975 MHz, the sideband to
LSB, and enter the external power level (7.0 dBm).
c. Set the appropriate START frequency and STOP frequencies (30 MHz and 90
MHz for example).
d. Connect the noise source directly to the noise figure meter input and press
CALIBRATE twice.
e. Confirm that the calibration indicator changes from a white Uncorr display to
a white Corr display. Confirm that you see a value close to 0dB for both noise
figure and gain.
f. Insert device under test, DUT, and perform your measurements.
Noise
Figure
Meter
noise
source
HP8970B
mixer
LO
HP 8648C
Figure 3.2 Set-up for measurement of mixer noise figure using a Noise Figure Meter
1) Measure the mixer by itself as shown in Figure 3.2. Be sure to use a low noise source
for the local oscillator. Otherwise the oscillator noise sidebands may mix into the pass
band of the noise figure meter.
The noise figure and gain that you have measured are double sideband (DSB). The gain
calculated by the noise figure meter will be twice the gain you measured in project I.
2) Measure the noise figure and gain of the image reject filter by itself. What should the
relationship be between the noise figure and gain. Check at the filter center frequency
and at the frequency where the filter response has rolled off by 3 dB.
3) Put the image reject filter followed by a 3 dB attenuator in front of the mixer and
measure the combination. Use the Friis formula and your results from item 2 above to
calculate what the noise figure of the filter /attenuator/mixer combination should be.
Lab Exercise N: Noise Figure Measurement
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Microwave Metrology -ECE 684
Spring 2010
Remember, because of the filter the system becomes single sideband. Compare the
calculation to what your measurements show.
4) Remove the attenuator and put the image filter in front of the mixer and measure the
noise figure of the combination. How can you explain what you see?
5) Measure the noise figure and gain of the entire downconverter assembly with and then
without an attenuator between the image filter and mixer. To keep the power into the
Noise Figure Meter at an acceptable level an attenuator may have to be used after the
IFF.
6) Measure the LNA and the IFA. Compare these results to the results you got using the
brute force measurements in the first part of this project.
Make up a table with a column for each component and a row containing the gain
of each component (including filters) and a row containing the noise temperature of each
component. Make up a third row with the contributions each component makes to the
overall noise temperature (divide each temperature by the appropriate gain). Sum the
contributions and compare the result to the measured noise temperature of the entire
downconverter (item 5 above).
Equipment Required
E4418B power meter
E4413A Power Sensor
HP346A Noise Source
Power Supply (28 V)
HP8648C Synthesizer
3 dB SMA attenuator
Agilent N8973A Noise Figure Meter
Low Noise amplifier 1-1000 MHz
Band Pass filter 915 +/-34 MHz
Mixer
Band Pass Filter
IF amplifiers
Lab Exercise N: Noise Figure Measurement
(MiniCircuits ZFL-1000LN)
K&amp;L filter
(MiniCircuits ZFM-5X)
(MiniCircuits SIF-60)
(MiniCircuits ZFL-500)
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