Text preview for : 5967-5661E English _ 2014-07-31 _ PDF 870 KB c20140827 [12].pdf part of Agilent 5967-5661E English 2014-07-31 PDF 870 KB c20140827 [12] Agilent 5967-5661E English _ 2014-07-31 _ PDF 870 KB c20140827 [12].pdf
Back to : 5967-5661E English _ 2014 | Home
Keysight Technologies
8 Hints for Making Better
Measurements Using Analog
RF Signal Generators
Application Note
Signal sources provide precise, HINT 1. HINT 5.
highly stable test signals for a
variety of component and system Improve Source's Effective Combine Source Outputs for TOI
test applications. Signal gener- Harmonic Distortion Measurements
ators add precision modulation
Use a low pass ilter at the output of Use a proper setup to isolate sources,
capabilities, and are used to sim-
your source to decrease its harmonic improve match.
ulate system signals for receiver
distortion.
performance testing.
HINT 6.
This Keysight Technologies, Inc. HINT 2. Improve FM Deviation Accuracy
guide helps you improve the Increase Power Level Accuracy
accuracy of your measurements Use the Bessel Null method to set
that involve using RF analog signal Use a power meter to increase the your signal generator's frequency
sources. You may increase the accuracy of the signal level at your deviation.
accuracy of your data by using device under test (DUT).
more than one of the hints in your HINT 7.
test setup. HINT 3. Extend the Amplitude Range
Improve Frequency Accuracy
Use an ampliier or an attenuator to
Select the appropriate frequency increase or decrease respectively, the
reference to improve absolute or amplitude range of your signal source.
relative frequency accuracy.
HINT 8.
HINT 4. Select the Optimum Phase Noise
Improve Source Match Proile
Use a ixed attenuator to reduce the Choose the appropriate phase noise
mismatch error. proile to optimize in-channel or
out-of-channel measurements.
Control
Frac-N ALC Burst Output
Modulator Modulator Attenuator
Phase
Detector Source
X 2
Output
VCO Multiplier
Function ALC
Generator Detector
d/dt
ALC Burst Mod
Driver Driver
Divide Function
by x Generator
Ext. AM ALC
Hold
From Ext Pulse
Reference
Section External FM/PM input
Typical Signal Generator Block Diagram
2
HINT 1.
Improve source's effective harmonic distortion
Accurate harmonic distortion measure- Note: You can calculate the percent
ments require a spectrally pure signal distortion for a particular harmonic, mth
source and a spectrum analyzer. The harmonic as,
harmonic distortion of the signal source
and the dynamic range of the spectrum (dB)
20
%dm = 100 x 10
analyzer limit the quality of the measure-
ment. However, the signal source is often
the limiting factor, with harmonic distortion Or you can calculate total harmonic
performance on the order of 30 dB below distortion: calculate the distortion for each
the fundamental. Figure 1 shows a typical harmonic as above and find the root sum of
harmonic distortion measurement. The the squares,
harmonic distortion of a signal is often
%THD= (%dm)
2
specified by stating the amplitude of the
largest harmonic in dB relative to the
fundamental.
Use a low pass filter to improve the
source's effective harmonic distortion,
as shown in Figure 2. Choose the cutoff Fundamental
frequency of the low pass filter such that
the fundamental frequency is passed largely
intact, while the harmonics are attenuated
dB Relative harmonic distortion
significantly. You can verify the performance
of the source/filter combination directly Largest
with the spectrum analyzer. harmonic
If the loss through the filter at the
fundamental frequency is significant, the
loss should be accounted for when setting
the source output level. Use the spectrum
analyzer to check the fundamental level at freq
the output of the filter, or for better level
accuracy see Hint 2.
Figure 1.
Spectrum
Analyzer
8563A SPECTRUM ANALYZER 9 kHz - 26.5 GHz
Device
Low pass Under
FREQUENCY
WCDMA
T Waveform: Recon Filter: 2.5 MHz
RF MOD
ON ON
Filter Test
ON Sample Clk: Arb Ref: Int
DUT
Figure 2. The harmonic distortion of a signal source improved by installing a low pass filter at the
source's output.
3
HINT 2.
Increase power level accuracy
In your test setup, you are likely to use Note: the accuracy of the power meter
passive devices such as cables, filters or measurement depends on the calibration
switches between your source and the DUT. factors of the sensor; be sure to enter the
The accuracy of the signal level at the DUT calibration factors into the power meter
is effected by the use of these components. prior to calibration.
In some applications, for example receiver
sensitivity measurements, the accuracy of Once you have completed the calibration
the input signal level is critical. To have the of your power meter, set the power meter's
desired power applied to the DUT, perform measuring frequency to the signal frequen-
the following test prior to making your cy. Connect the sensor in place of your
measurements. The setup consists of your DUT as indicated in Figure 3 and measure
signal generator, power meter with a power the power level. If there is a difference
sensor and the cables or switches that are between the power meter's reading and
necessary in the measurement as shown in the indicated level on the source, use your
Figure 3. source's amplitude offset feature to make
the necessary adjustments. Match the
Calibrate the power meter to the power displayed power level of your source to the
sensor for an accurate power measurement. power meter's reading. Once you adjust the
It is assumed that you are familiar with the amplitude at a particular frequency, then the
calibration and zeroing of the power meter source will automatically display the correct
in use. value for different amplitudes at that same
frequency. Since the accuracy of the power
meter is very high (uncertainty in the tenths
of a dB range), you can have confidence
that the power level is accurate.
Power Meter
FREQUENCY
RF MOD
WCDMA
T Waveform: Recon Filter: 2.5 MHz ON ON
ON Sample Clk: Arb Ref: Int
Test Port
Power Sensor
Cables, Switches,
Etc.
DUT
Figure 3. Setup for improving level accuracy of the signal.
4
HINT 3.
Improve frequency accuracy
For certain measurements, the absolute When the absolute frequency of the
frequency of the stimulus signal is most signal is important, increase the frequency
important, but other measurements require accuracy of your source by finding the
accurate relative frequency spacing most accurate external-frequency reference
between multiple signals. For instance, available. Choose the instrument in your
to create multi-tone inputs with known setup with the most accurate time base
frequencies more than one signal generator and connect all the other equipment to this
is often used. The frequency accuracy of reference.
each source relies on its internal frequency
standard. It is very possible for these Some instrument manufacturers offer
standards to be slightly off in frequency, high-stability ovenized reference oscillators
thereby causing relative frequency errors in as an option. These frequency and time
the measurement. standards are extremely accurate, but can
be expensive.
For example, assume you are trying to set
a 1 kHz separation between two signals You can always improve frequency accuracy
centered at 200MHz, and your sources have by using a house standard (a high-accuracy