Microwave Metrology -ECE 684
Spring 2010
Lab Exercise G: Gain Measurement
You will use the Agilent E5062A network analyzer to measure the downconverter’s
(see Figure 1) amplifiers and filters. Then use the Agilent spectrum analyzer to measure
the conversion loss of the mixer.
Mixer
Low Noise
Amplifier
Image
Reject
Filter
Channel
Select Filter
IF
Amplifier
Local
Oscillat
or
Figure 1.
SuperHeterodyne Downconverter
Prelab
Prepare a table of downconverter components like the one at the end of this project
description, but include a column for “component identification number.”
Network Analyzer Measurements
Use the 3.5 mm calibration kit to do an SOLT calibration of the network analyzer for
measurements 10 MHz to 1210 MHz. You can expect to measure gains of as much as 25
dB so set the power level such that port 2 will not be over driven. You will use this
calibration to make all your measurements. Your most important measurements will
occur in a narrow band around 915 MHz (for RF components) and below 250 MHz (for
IF components). You should be familiar with this type of calibration from your earlier
courses. The difference here is that you will be using a 3.5 mm calibration kit. These
kits are very expensive. They need to be handled carefully and kept clean. Download all
your measured S parameters to a USB thumb drive for use in writing your report.
Image Reject Filter. This is a multisection coaxial filter with SMA connectors on
either end. Measure S21, S11, S22 magnitude. Note particularly the filter gain in the
passband, at the -3 dB bandwidth, -20 dB bandwidth, and at the image frequency of the
receiver.
IF filter. Often this is a Surface Acoustic Wave (SAW) filter with a very narrow
bandwidth (less than one MHz) around the IF frequency (60 MHz for this receiver). This
would select the channel you are receiving within the ISM band. In this case, however,
we will just be using a band pass filter with a center frequency of 60 MHz and a 1 dB
bandwidth of 20 MHz. Measure the S parameters of this filter especially noting the gain
near 60 MHz and near 915 MHz (small gain hopefully).
Low Noise Amplifier. Connect the ZFL-1000LN to the network analyzer and
connect power to it (15 V). Measure the S parameter magnitudes and plot the results.
Especially note the gain from 900 to 1100 MHz (covering both the RF and Image
frequencies).
Lab Exercise G: Gain Measurement
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Microwave Metrology -ECE 684
Spring 2010
IF amplifier ZFL-500. Measure and plot the amplifier’s S parameters especially
noting their gain around 60 MHz and at 915 and 975 MHz. In theory, there should be no
signal at the RF frequency, thus the latter is not that important unless there is local
oscillator leakage.
Spectrum Analyzer Measurements
Now you will measure the conversion gain of the mixer. First you will need to
check the power levels and frequencies associated with the local oscillator (HP8648C
Signal Generator) and the test RF source.
Set the Signal Generator (Source1) frequency to 975 MHz and the power to 7.5
dBm. Verify the LO power level using the Agilent E4419B power meter. Then connect
Source1 to the spectrum analyzer and check the frequency and (again) the power. The
SA is not the best instrument for measuring absolute power; that’s what the power meter
is good for. The principle use of the SA is showing what frequency components are
present and what their relative powers (dB) are. Use the difference between the power
meter and SA measures of absolute power to correct future SA measurements of absolute
power.
Your RF source will be a second signal generator (Source2). Set the frequency of
Source2 to 915 MHz and power level at -5 dBm and verify these settings with the SA.
Set Source2 to 1035 MHz (image frequency) and record the power. Reduce the Source2
power by 5 dB and measure the output powers at the two frequencies. Now you have a
record of the powers you will be applying to the mixer.
Connect the mixer as shown in Figure 1.1. Using the 915 MHz, -5 dBm RF input,
locate the IF frequency spike on the SA. Record the power and calculate the conversion
loss. Check that the mixer is not saturating by reducing the RF power by 5 dB and
measuring the conversion loss again. There should be little change of conversion loss.
Measure the RF to IF isolation and the LO to IF isolation by observing the SA response
at the appropriate frequencies. Change the sweeper frequency to 1035 MHz (the image
frequency) and measure the conversion loss.
Connect the image reject filter to the RF port and measure the conversion loss of
the filter/mixer combination at 915 MHz. Does adding the filter make the conversion
loss change as much as you think it should change based on the loss in the filter?
mixer
RF IF
LO
Sourc
e2
Source2
spectrum
analyzer
Sour
ce1
Source1
Figure 1.1 Set-up for mixer measurement.
Lab Exercise G: Gain Measurement
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Microwave Metrology -ECE 684
Spring 2010
Gain of the Downconverter
Calculate the gain in dB of the downconverter chain shown in figure 0. Do this
calculation at 915 MHz and again at the image frequency. Note the image rejection ratio.
Connect all the components together with the LO and the Spectrum Analyzer.
Use the signal generator as an RF source. You need to make sure that the RF power is
small enough not to saturate the last IF amplifier. You may need an attenuator for this.
Look at the amplifier data sheet to get an estimate of the 1 dB compression power. Then
work back to determine the corresponding maximum RF input power. Set the input
power 10 dB below the maximum. You may need to use an attenuator. Measure the IF
output power at 60 MHz for a number of input frequencies from 875 to 1050. You
should take enough points to get a smooth plot showing the selectivity of your
downconverter. Note the image rejection.
Compare your measured conversion gain to your calculations. Compare your
measured image rejection to calculation. If there are discrepancies of more than 2 dB,
find out why.
Report
Your report should include plots and tables of measured quantities. Your
narrative should be succinct, but include enough to clearly explain your measurements.
You should include diagrams, tables, etc, where needed to enhance clarity. Be sure (1) to
make comparisons to calculations and (2) to explain the significance of the tests and your
results. Clarity is extremely important. Your report will not be graded by weight. In
fact, including a lot of useless repetitive material inhibits clarity and will be graded off.
Type your report. Figures can be hand drawn; however, many of the system components
you use in this report may be used again in following reports so an electronic drawing
may be useful in the long run.
Equipment Required
Spectrum Analyzer
2xHP8648C Synthesizer
Variable Attenuator
Agilent E5062A Network Analyzer
Low Noise amplifier 1-1000 MHz
Mixer
Band Pass Filter
IF amplifiers
Band Pass filter 915 +/-34 MHz
Lab Exercise G: Gain Measurement
(MiniCircuits ZFL-1000LN)
(MiniCircuits ZFM-5X)
(MiniCircuits SIF-60)
(MiniCircuits ZFL-500)
K&L 4B121-915/T68-0/DP
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