Ampsmeasure NA
Ampsmeasure NA
Ampsmeasure NA
RF Component Measurements:
Amplifier Measurements Using the
Agilent 8753 Network Analyzer
Product Note
Introduction
Table of contents
2
3
4
4
4
4
6
7
8
8
10
10
11
12
12
12
13
15
15
Introduction
Amplifier Definitions
Measurements
Equipment Required
Linear Measurements
Transmission Measurements
Phase Measurements
Reflection Measurements
Non-Linear Measurements
Gain Compression
Non-Linear Measurements
with the 8753B
Swept Harmonic Levels
Two-Tone Third Order Intermodulation
Enhanced Manufacturing Techniques
Limit Lines
Test Sequence Function (8753B)
Accuracy Considerations
Appendix
S-Parameter Test Set Considerations
Amplifier definitions
This section contains brief descriptions of the amplifier parameters that
can be measured using the 8753 RF
vector network analyzer.
Gain
Amplifier gain is defined as the ratio
of the amplifier output power delivered
to a Zo load to the input power delivered from a Zo source, where Zo is the
characteristic impedance in which the
amplifier is used (50 ohms in this note).
In logarithmic terms, gain is the difference in dB between the output and
input power levels.
Since variations in frequency response
represent distortion, gain flatness is
often specified over the frequency
range of the amplifier.
Reverse isolation
Reverse isolation is the measure of
transmission from output to input. The
measurement of isolation is similar to
the measurement of small signal gain,
except that the stimulus signal is
applied to the amplifiers output.
Return loss/SWR
A commonly specified parameter is
the quality of the match at the input
and output of an amplifier relative to
the system characteristic impedance.
Impedance parameters are defined by
the following equations:
G = Vreflected/Vincident = r
Return Loss = 20 log10r
SWR =
1+r
1r
Complex impedance
The complex impedance of an amplifier consists of both a resistive and a
reactive component. It is derived
from the characteristic impedance of
a system and the reflection coefficient by the following equation:
Z = 1 + G *Zo
1G
Gain compression
An amplifier has a region of linear
gain, where the gain is independent of
input power level. This is commonly
referred to as small signal gain. As
the input power is increased to a level
that causes the amplifier to saturate,
gain decreases, resulting in the large
signal response. (See Figure 1-a.)
Two-tone intermodulation
distortion (Agilent 8753B)
When two or more sinusoidal frequencies are applied to an amplifier,
the output contains additional frequency components called intermodulation products. For an amplifier with
input signals at frequencies f1 and f2,
the output will contain signals at the
following frequencies: nf1 + mf2, where:
n, m = 0, 1, 2, 3. The order of the
intermodulation distortion product is
defined as i = |n| + |m|. Third order
products (i =3) are a major concern
because of their proximity to the fundamental frequencies. (See Figure 2.)
Group delay
Group delay is a measure of transit
time through an amplifier at a particular frequency. It is defined as the
derivative of the phase response with
respect to frequency. The 8753 has
the ability to derive the group delay
from the measured phase response.
Figure 1. Typical amplifiers characteristics: (a) output power versus input power
and (b) gain versus input power
3
Measurements
Equipment required
The following equipment is typically
required for the measurements
described in this note. Other configurations can also be used:
Agilent 8753B Network Analyzer/
Opt. 002
Test Set: Agilent 85044A,
85046A/B, 85047A
Calibration Kit
Test Port Cables
Coaxial Attenuators, as needed
GPIB Cables
Bias Supply
Power Meter and Sensor
Test Amplifier such as the Agilent
10855A Preamp
The 10855A has the following specifications:
Frequency range: 2 MHz to 1300 MHz
Gain: 24 dB typical
Output power for 1 dB compression:
0 dBm
Linear measurements
The following linear measurements
can be made with a single connection
to the S-parameter test set. (See
Figure 3.) A full two-port calibration
yields the most accurate results.
Before calibration, set the measurement parameters to their desired values. It may be appropriate to step
through some of the measurements
before performing the calibration to
ensure proper stimulus settings.
Power levels at the various ports are
of primary concern when measuring
the linear transmission and reflection
characteristics of amplifiers. If external attenuation is needed, calibrate
with the attenuator in the system to
remove its effects.
Transmission measurements
Transmission measurements can be
made using the basic setup shown in
Figure 3. An external attenuator at
the amplifier output may be necessary
to keep the power level at the B receiver
input below the maximum level (0 dBm)
for gain measurements. The test set
attenuator, accessed through the 8753,
affects only the source power path.
Both gain and isolation measurements
are possible with the same stimulus
settings by choosing a source power
level for the reverse isolation measurement, then using the test set attenuator
to reduce the power at the amplifier
input for linear gain measurements.
Figure 3. Basic setup for amplifier measurements using the 8753 with an S-parameter test set.
Measurement procedure
1. Connect the system as shown in
Figure 3. Preset the 8753 to return
the instrument to a known state of
operation.
[PRESET]
CAUTION: At preset, the source power
level is set to its default value of 0 dBm
and the internal attenuator for port 1
is set to 0 dB. The preset power level
available at port 1 is dependent on
the test set used. (See Appendix for
details.) If the amplifier under test
could be damaged by this power level
or will be operating in its non-linear
region, it should not be connected
until these parameters are set to a
desirable level.
Reverse isolation
Reverse isolation can be measured
with the small signal gain setup and
procedure, with some modifications.
6. Measure the gain variation or ripple in the frequency range. Set two
markers on the trace to define the
start and stop of the frequency range
of interest, then use the marker statistics function to view peak-to-peak
ripple. (See Figure 4.)
[MKR]
Position marker 1.
[MKR ZERO]
[MARKER 2]
Position marker 2.
[MKR FCTN]
[STATS ON off]
Measurement procedure
1. Connect the instrument as shown
in Figure 3.
2. Preset the instrument and measure
S21 in log format.
[PRESET]
[MEAS]
[S21 [B/R]]
[FORMAT]
[LOG MAG]
3. Enter the desired fixed frequency.
[MENU]
[CW FREQ] [500] [M/u]
4. Modify the parameters such that
the amplifier operates in its linear
region.
[MENU]
[POWER] [5] [x1]
[ATTENUATOR PORT 1] [30] [x1]
Phase measurements
Transmission phase measurements
are made with the same setup as
magnitude transmission measurements (Figure 3). For cases such as
complex impedance, both magnitude
and phase are displayed simultaneously. Power levels and number of
points are both considerations when
measuring phase.
Deviation from linear phase
The deviation from linear phase
measurement employs the electrical
delay capability of the Agilent 8753 to
add electrical delay to the amplifier
to remove the linear portion of the
phase shift. (See Figure 7.)
Measurement procedure
1. Follow the first two steps in the
Small Signal Gain Measurement
Procedure. If the instrument state
was saved, recall it from memory.
[RECALL]
[RECALL REG 1]
Reflection measurements
Reflection measurements can be
made with the same setup as transmission measurements (Figure 3). If
external attenuation is needed for
gain measurements, calibrate with
the attenuator in the system to
remove its effects. Since reflection
measurements involve loss instead of
gain, power levels are lower at the
receiver inputs. Therefore, it may be
necessary to increase power levels for
reflection measurements.
Alternatively, reduce the noise level
by decreasing the IF bandwidth or by
employing averaging.
Return loss, SWR, and reflection coefficient
Return loss, standing wave ratio (SWR),
and reflection coefficient () are commonly specified for the amplifiers
input and output ports. With the 8753,
scalar reflection measurements can
be displayed as return loss in dB,
SWR, or .
With the internal switching capabilities of the 85046/7A test set, you can
switch between displaying the input
reflection and output reflection with
the touch of a key; no rearrangement
of the test setup is required.
Non-linear
measurements
These measurements, with the exception of gain compression, are not usually associated with network analyzers. However, the Agilent 8753 offers
some added capabilities that allow
some non-linear measurements to be
made. Due to the nature of non-linear
measurements, measurement calibrations are not applicable. The exception
is gain compression measurement
where frequency response errors can
be removed from the system.
Gain compression
There are two ways to measure amplifier gain compression using the 8753.
The first method, swept frequency gain
compression, shows how to find the frequency at which 1 dB gain compression
first occurs. The second method, swept
power gain compression, shows the
reduction in gain as a power ramp is
applied to the amplifier under test.
Swept frequency gain compression
This measurement allows the user to
easily determine the frequency at which
1 dB gain compression first occurs
(Figure 12). This is accomplished
through normalization to the small
signal gain and by observing compression as the drop from the reference
line as input power is increased. This
frequency can in turn be used as the
fixed frequency for a swept power compression measurement, discussed in
the next section.
Measurement procedure
1. Connect the instrument as shown
in Figure 13. Preset the instrument.
Channel 1 will display the frequency
for which 1 dB gain compression first
occurs. Channel 2 will display amplifier
output power to allow easy determination of the 1 dB gain compression
output power.
[PRESET]
2. Set measurement stimulus parameters.
[MEAS]
[S21 [B/R]]
[START] [20] [M/u]
[STOP] [1.3] [G/n]
[MENU]
[POWER]
[ATTENUATOR PORT 1] [20] [x1]
3. Since the output power of an amplifier is important, the loss through the
test set between the amplifier output
and the receiver input needs to be
characterized. To do this, first perform
a response calibration on channel 2
as shown in Figure 13.
NOTE: When using the 85044 test set,
steps 3 and 4 can be omitted because
the amplifiers output is connected
directly to the 8753 receiver input.
[CH 2]
[MEAS]
[S21 [B/R]]
[CAL]
[CALIBRATE MENU]
[RESPONSE]
[THRU]
[DONE]
[DISPLAY]
[DATA MEM]
[MEAS]
[INPUT PORTS]
[B]
[DISPLAY]
[DATA/MEM]
5. Calibrate channel 1 for gain measurements.
[CH 1]
[CAL]
[CALIBRATE MENU]
[RESPONSE]
[THRU]
[DONE]
5B. (Optional for the 8753B only) For
increased accuracy, also perform a
power meter calibration at port 1
to remove source and test set nonlinearities with frequency. Attach the
438A power meter to port 1 of the test
set. Also, connect GPIB as shown in
Figure 14. Make sure the power meter
model and address is known by the
8753. Before proceeding, zero the
power meter.
[LOCAL]
[SYSTEM CONTROLLER]
[SET ADDRESSES]
[ADDRESS: P MTR/GPIB] [15] [x1]
[PWR MTR: [438A/437/436A]]
Toggle to select desired power meter.
[CAL]
[PWR METER]
[NUMBER OF READINGS] [1] [x1]
[PWRMTR CAL: ONE SWEEP]
Set the desired power at port 1 of the
8753B and take the cal sweep.
(Cal power) [20] [x1]
[TAKE CAL SWEEP]
A Power Meter Calibration Sweep
Done prompt will appear when
the calibration sweep is completed.
Reconnect Figure 3.
Measurement procedure
1. Connect the instrument as shown
in Figure 14 and preset the instrument.
Channel 1 will display the amplifiers
response to a power ramp. Channel 2
will display amplifier output power to
allow easy determination of the 1 dB
gain compression output power.
[PRESET]
2. Select a power sweep at the fixed
frequency of interest. This could be the
frequency at which 1 dB gain compression first occurs for a swept frequency
gain compression measurement.
[MENU]
[SWEEP TYPE MENU]
[POWER SWEEP]
[RETURN]
[CWFREQ] [20] [M/u]
3. Set the stimulus parameters. Power
levels must be set so that the amplifier
is forced into compression. The range
of the 8753s source is from 5 dBm
to +20 dBm. An external output attenuator may be required to prevent
overdriving input B.
[MEAS]
[S21 [B/R]]
[START] [-5] [x1]
[STOP] [10] [x1]
[MENU]
[POWER]
[ATTENUATOR PORT 1] [20] [x1]
4. Since the output power of the amplifier is important, the loss through the
test set between the amplifier output
and the receiver input needs to be
characterized. To do this, first perform
a response calibration on channel 2
as shown in Figure 13.
NOTE: When using the 85044 test set,
steps 4 and 5 can be omitted since the
amplifiers output is connected
directly to the 8753 receiver input.
[CH 2]
[MEAS]
[S21 [B/R]]
[CAL]
[CALIBRATE MENU]
[RESPONSE]
[THRU]
[DONE]
Non-linear
measurements with
the Agilent 8753B
Swept harmonic levels (Option 002)
Traditionally, harmonic measurements
are made with a spectrum analyzer at
several CW frequencies. Many frequencies must be tested for complete characterization, which can dramatically
increase test time. With the 8753B, however, you can make swept frequency
(and power) second and third harmonic
response measurements. This capability provides real-time update of the
measured harmonic response versus
frequency.
Harmonic response measurements on
the 8753B are made using the channel
trace math functions to normalize the
harmonic response to the fundamental
response. In this manner, the harmonic
response of the device under test
can be displayed directly in dBc. (See
Figure 16.)
10
Measurement procedure
1. Connect the instruments as shown
in Figure 19. Connect the external
frequency references together to synchronize the measurement frequencies. Preset the 8753B.
[PRESET]
7. Third order IMD should now be displayed on the 8753Bs display. To calculate the third order intercept point,
TOI, use the equation:
TOI = PO + (PO P3)/2
12
Accuracy
Considerations
Linear measurements
This section summarizes key accuracy
considerations for the measurements
described in this note. Vector accuracy
enhancement can be applied to the
linear measurements discussed to
greatly reduce measurement uncertainty. This is accomplished by removing systematic errors through measurement calibration with standards
such as a short, an open, and a thru.
To illustrate the differences between
the various accuracy enhancement
techniques, the following amplifier
characteristics were assumed.
Gain: 22 dB
Reverse isolation: 45 dB
Input SWR: 3.0
Output SWR: 2.2
An 85046A test set and 3.5 mm calibration accessories are used in these
examples. For a complete table of uncertainty improvements resulting from
various calibration procedures, see the
Agilent 8753 System Operating and
Programming Manual.
Gain
The major sources of error in a gain
measurement with the 8753 network
analyzer are the frequency response
of the test setup, the source and load
mismatch during measurement, and
the system dynamic accuracy. The frequency response of the test setup is
the dominant error in a transmission
measurement. A simple response calibration significantly reduces this error.
For greater accuracy, a full 2-port calibration can be used.
13
Non-linear measurements
The non-linear measurements discussed in this note make use of the
Agilent 8753s receiver to measure
absolute power. Vector accuracy
enhancement is not applicable to
power measurements. Uncertainties
for these measurements are affected
by the previously mentioned sources
of error. Thus, care should be taken
to reduce mismatch effects because
mismatch errors contribute to total
measurement uncertainty. For absolute
power measurements, additional
uncertainty is introduced by the
receiver power accuracy. The tuned
receiver of the 8753 is accurate to
1 dB in absolute amplitude measurements over the 300 kHz to 3 GHz frequency range (typically < 0.5 dB).
From 3 to 6 GHz with the 8753B, the
receiver is accurate to 3 dB (typically
1 dB). One method of increasing the
accuracy of absolute power measurements is to reference the receiver inputs
with a power meter. In this manner,
the re-ceiver can be characterized
and the measured data improved.
14
Appendix:
S-parameter test set
considerations
When configuring a test setup for amplifier measurements, knowledge of
power levels through the various signal
paths is vital. This appendix describes
the 85046A (300 kHz to 3 GHz) and
85047A (300 kHz to 6 GHz) 50-ohm
S-parameter test set, and explicitly
shows the signal paths and power levels
through them. The actual power levels
available at the test ports of the 8753/
S-parameter test set system are listed
in Table 1. Finally, an example is presented to clarify attenuator and power
level requirements for a particular amplifier. Figure 22 shows a schematic representation of the 85046A S-parameter
test set and lists the attenuation from
the RF input (0 dB reference point) to
the other ports. The same is done in
Figure 23 for the 85047A.
[ATTENUATOR PORT 1] or
[ATTENUATOR PORT 2] (depending on
the direction of the measurement) in
the 8753 power menu. Increasing the
port 1 attenuation to 20 dB, changes
the power level at port 1 (the amplifier
input) to 33 dBm, which is about the
required power limit.
Now consider the amplifier output.
Since the output power is approximately
7 dBm (33 dBm + 40 dB of gain) and
the loss in the test set from port 2 to
channel B is only about 6 dB, an external attenuator must be inserted before
port 2 in order to keep input B below
its maximum power level (0 dBm). A
20 dB pad at the amplifier output will
keep the maximum power at B 19 dBm,
which allows the receiver to operate
in its most accurate range.
85047A
Test Set
85046A
Test Set
3 GHz
6 GHz
83 dB
73 dB
88 dB
13 dB
3 dB
18 dB
+7 dBm
+ 17 dBm
+2 dBm
88 dBm
78 dBm
93 dBm
6 dB
16 dB
16 dB
15
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