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Measuring Frequency Response
with the Keysight Technologies E5061B
LF-RF Network Analyzer

Application Note
Introduction
Evaluating frequency responses of components and circuits is essential for
ensuring the performance of electronic equipment. Especially in the case
of the high-reliability electronic equipment used in automotive, medical, and
aerospace and defense industries, it is necessary to evaluate a wide variety
of components and circuits in the low- to high-frequency ranges. Among these
applications, the low-frequency network analyzer plays an important role in
ensuring the stable and reliable operation of low-frequency analog circuits
such as sensor systems and power supplies. You need a better understanding
of low-frequency network analysis (gain-phase measurements) as well as RF
network analysis (S-parameter measurements).

This application note describes fundamentals of low-frequency network analy-
sis using the E5061B LF-RF network analyzer. Here we mainly discuss simple
low-frequency 2-port device measurements and associated topics such as
high-impedance probing techniques and high-attenuation measurements.
Table of Contents

E5061B-3L5 LF-RF Network Analyzer.................................................... 3
Basic Measurement Configurations ...................................................... 4
50 DUTs ........................................................................................... 5
Non-50 DUTs, example 1 .............................................................. 5
Non-50 DUTs, example 2 .............................................................. 7
In-circuit probing measurements .................................................... 8

IFBW Setting for Low-Frequency Measurement .............................. 10
High-Impedance Probing Methods ...................................................... 11
Signal Separation for Ratio Measurement ......................................... 13
High-Attenuation Measurement at Low Frequencies ...................... 15

OP Amp Measurement Example .......................................................... 20
Closed-loop gain............................................................................... 20
Open-loop gain, phase margin ....................................................... 22
CMRR ................................................................................................. 27
PSRR................................................................................................... 29
Output impedance............................................................................ 31

References ............................................................................................... 33

Table 1. Guideline for selecting test ports
Test ports Situation Application
S-parameter test port Transmission and reflection Passive filters, antennas,
measurements in the 50 cables, RF amplifiers, etc.
system impedance
Transmission measurements High-frequency op amps
with high-impedance probing
over 30 MHz using the
41800A active probe
Gain-phase test port Transmission measurements OP amp circuits
with high-impedance probing
in the low frequency range
Feedback loop measurements DC-DC converter loop gain
High-attenuation measurements CMRR and PSRR of OP amps
in the low-frequency range

3
E5061B-3L5 LF-RF Network Analyzer

The E5061B with the Option 3L5 vector network analyzer covers a broad test
frequency range from 5 Hz to 3 GHz in a single instrument. The E5061B-3L5
includes an S-parameter test port (5 Hz to 3 GHz, Zin = 50 ) and a gain-phase
test port (5 Hz to 30 MHz, Zin = 1 M/50 ). Both test ports can be used for
low-frequency applications, depending on your measurement needs. Table 1
shows an example of selecting the test ports.

E5061B-3L5
Gain-phase
test port block

ATT: 20 dB/0 dB
Zin: 1 M/50 S-parameter
test port block
T R

ATT ATT
R1 R2

Zin Zin T1 T2
DC bias
source

T R LF OUT Port-1 Port-2

Gain-phase test port S-param. test port
(5 Hz to 30 MHz) (5 Hz to 3 GHz)

Figure 1. E5061B-3L5 simplified block diagram

4
Basic Measurement Conigurations 50 DUTs

First let's summarize how to connect DUTs in typical low-frequency network measure-
ment applications. Here the focus is on configurations for 2-port transmission measure-
ments. The first case is a transmission measurement for 50 devices, such as filters and
cables. Figure 2 shows a configuration using the gain-phase test port. The R-ch receiver
VR monitors the source output voltage applied to the 50 system impedance (incident
voltage to the 50 transmission line), and the T-ch receiver VT monitors the transmitted
voltage. Then the analyzer measures the voltage ratio VT/VR which indicates the trans-
mission coefficient S21.

Figure 2 shows a configuration using the S-parameter test port. The S-parameter test set
has built-in directional bridges, and an external power splitter is not required. In most
cases, the S-parameter test port for 50 transmission measurements is used.

Most 50 transmission measurement can be covered with the S-parameter test port. For
high-attenuation devices such as, m impedance measurement for DC-DC converters and
large bypass capacitors using the shunt-thru method, the 50 transmission measure-
ment should be performed using the gain-phase test port rather than the S-parameter
test port. In this case, the semi-floating receiver architecture of the gain-phase receiver
ports eliminate the measurement error in the low-frequency range, which is caused by
the test cable ground loop between the source and receiver (discussed later).

E5061B-3L5

VT 50 VR 50 50
T-ch R-ch
(Zin = 50 ) (Zin = 50 )

DUT 50 Power
50 splitter
Zout Zin

Figure 2. Configuration for measuring transmission coefficient of 50 DUTs with the
gain-phase test port

E5061B-3L5

R1 R2
A B

Port 1 DUT Port 2
(50 ) (50 )
Zout Zin

Figure 3. Configuration for measuring transmission coefficient of 50 DUTs with the
S-parameter test port
5
Basic Measurement Conigurations

Non-50 DUTs, example 1
Low-frequency 2-port devices often have non-50 impedances. The most
typical examples are low-frequency amplifier circuits. Figure 4 shows a con-
figuration example of measuring the frequency response of amplifiers with the
gain-phase test port. The DUT has high input impedance and the output port is
terminated with a non-50 load ZL. The load impedance ZL depends on require-
ments of the targeted application. The load ZL can be either a resistive load or a
reactive load.

The parameter to be measured is the voltage transfer function from the DUT's
input port to the output port, Vout/Vin. The difference from the 50 transmission
coefficient measurements which were shown in Figures 2 and 3 is that the R-ch
receiver VR directly monitors the AC voltage across the DUT's input impedance
Zin with high-impedance probing, instead of monitoring the voltage across the
50 system impedance. The output voltage Vout can be monitored using the
high-impedance probing, without affecting the DUT's load condition.

The analyzer's high-impedance receivers and the DUT should be connected with
coaxial test cables or 10:1 passive probes, depending on the requirements of the
maximum test frequency, the probing input impedance, the probing input capaci-
tance, and so on (discussed later). When you use the coaxial test cables, a
T-connector can be used at the R-ch probing point. To compensate the frequency
response and phase errors between two probes/test cables, the response thru
calibration should be performed. For example, by contacting the T-ch probe to
the point TP1.

E5061B-3L5

50
VT VR
T-ch R-ch
(Zin = 1 M) (Zin = 1 M)
Coaxial test cables,
or 10:1 passive probes Calibration:
Response thru cal.
TP2 DUT TP1 by contacting T-ch
Zout Zin probe to TP1
Low-Z High-Z
ZL
Vout Vin

Figure 4. Configuration for measuring amplifiers with the gain-phase port (up to 30 MHz)

6
Basic Measurement Conigurations

Non-50 DUTs, example 1 (continued)
If you need to measure the frequency response of an amplifier up to more than
30 MHz, or if you need to probe the amplifier with a very small probing capaci-
tance, the solution is to use the active probe at the S-parameter test port.
Figure 5 shows a configuration example. Unlike the configuration that was
shown in Figure 4, the ratio measurement is referenced to the 50 impedance
of the built-in R1 receiver, and the response thru calibration must be performed
by probing TP1 in order to correctly measure the voltage transfer function Vout/
Vin. If the response thru calibration is not performed (and if a feed thru is not
connected), the measured gain will be about 6 dB higher than the correct value
because the AC voltage measured at the internal 50 reference receiver will be
about half of Vin.

For measurements in high frequencies over tens of MHz, connecting the 50
feed thru to the DUT's input port can prevent the standing wave that may be
caused by the impedance mismatch between the analyzer's 50 source and
the DUT's high input impedance. However, it must be noted that connecting
the feed through will form a shunt signal path from the center conductor to the
ground of the test cable, and this may cause measurement errors associated
with the ground loop in the high-attenuation measurements such as CMRR and
PSRR. If this is of concern, it is better not to connect the feed thu.

E5061B-3L5

R1 R2
A B

Port 1 Port 2 with active probe
(50 ) (High-Z)
TP1 DUT
TP2 Calibration:
Zin Zout Response thru cal.
High-Z Low-Z by contacting active
Feed thru ZL probe to TP1
(optional) 50 Vin Vout

Figure 5. Configuration for measuring amplifiers with the S-parameter test port and
active probe (up to 30 MHz)

7
Basic Measurement Conigurations

Non-50 DUTs, example 2
Figure 6 and 7 shows configuration examples for measuring 2-port devices whose
input and output impedances are several hundreds of s to 1 or 2 k. Typical applica-
tions are low-frequency passive filters, such as ceramic filters and LC filters. In these
examples, impedance matching is implemented by simply connecting a series resistor.
The configuration of Figure 6 uses the gain-phase test port. The ratio VT/VR indicates
the transmission coefficient for the 1 k system impedance.

Some types of filters need to be tested by connecting a load capacitor CL in parallel
with the load resistor. The input capacitance of the high-impedance probe must be
small enough not to affect the filter's characteristics. So the high-impedance T-ch
receiver should be connected with the 10:1 passive probe which has the input capaci-
tance around 10 pF. Or, if the DUT is very sensitive to the capacitive loading, use the
S-parameter test port with the active probe, see amplifier measurement configuration
shown in Figure 5.

The equivalent measurement can be achieved by using the 50 input instead of using
high-impedance probing at the T-channel and connecting another matching resistor as
shown in Figure 7. This configuration is simpler and has an advantage that no probe
capacitance is applied at the T-ch. However, it is not suitable for testing high-rejection
filters because the measurement dynamic range is degraded by the series matching
resistor. The degradation is 20*Log (50/1000) = 26 dB, in this case.

E5061B-3L5

VT VR 50 50
T-ch R-ch
(Zin = 1 M) (Zin = 50 )
10:1 passive DUT 50 Calibration:
probe Power
Zout Zin 950 50 splitter Response thru cal.
by connecting thru
CL 1 k 1 k 1 k device in place of DUT

Figure 6. Configuration for measuring passive IF filters with high-impedance probing
(for DUTs not extremely sensitive to capacitive loading)

E5061B-3L5

R1 R2
A B

Port 1 (50 ) Port 2 (50 )
950 Zin Zout 950
1 k 1 k
CL

Figure 7. Configuration for measuring passive IF filters with 50 input
8
Basic Measurement Conigurations

In-circuit probing measurements
The next application example is an in-circuit probing measurement, in which we
measure the frequency response between two test points in the circuit under
test. Figure 8 shows how to measure the frequency response of block-2 with
the gain-phase test port. The frequency response of the circuit block-2 can be
directly measured by probing TP1 and TP2 with dual high-impedance probing.

Similarly to the amplifier measurement configuration shown in Figure 4, connec-
tion between the analyzer's high-impedance receivers and the DUT should be
appropriately selected from either the coaxial test cables or 10:1 passive probes,
depending on the requirements on maximum test frequency, probing input
impedance, probing input capacitance, and so on.

E5061B-3L5

VT VR 50
T-ch R-ch
(Zin = 1 M) (Zin = 1 M)

Coaxial test
cables, or
10:1 passive
probe
Block-2 Block-1
TP2 TP1 T/R = TP2/TP1

Figure 8. In-circuit measurement using dual high-impedance probing with the gain-phase
test port (up to 30 MHz)

9
Basic Measurement Conigurations

In-circuit probing measurements (continued)
The maximum test frequency range of the E5061B's gain-phase test port is 30 MHz.
If you want to perform incircuit measurements up to more than 30 MHz, the solution
is to connect a single active probe to the S-parameter test port, and perform the two
step measurement sequence as illustrated in Figure 9.

First we measure the response of the block-1 by connecting the active probe to TP1
and save the measured data into the memory trace. And then we measure the entire
response of the block-1 plus block-2 by probing TP2. The measured data is stored
into the data trace. Then we can obtain the frequency response of the block-2 using
the data/memory trace math function of the analyzer.

The equivalent measurement is possible if we performing the response thru calibra-
tion by probing TP1 and then performing the measurement by probing TP2. This will
directly give the response of the block-2 referenced to TP1 without using the trace
math function.

If the DUT's output characteristic at TP2 is sensitive to the capacitance at TP1, the
DUT's condition in the step 2 will slightly differ from that of the step 1, and the mea-
surement result obtained by combining these two measurement results may contain
errors. To minimize errors, connect a dummy capacitor C2 whose capacitance is
about the same as the input capacitance of the active probe only when making
the measurement of step 2 as shown in Figure 9. For example, this capacitance
compensation method is required for measuring the phase margin of high-speed OP
amps using this dual-step measurement technique. (A measurement example will be
shown later.)

E5061B-3L5

R1 R2
A B

Port 1 Port 2 with active
(50 ) probe (High-Z)

Step 1 Step 2
(B/R1) (B'/R1)
(B'/R1)/(B/R1)
= TP2/TP1
Block-1 Block-2
TP1 TP2
C2

Figure 9. In-circuit measurement with a single high-impedance probe (up to 30 MHz)

10
IFBW Setting in Low-Frequency Measurements

The IFBW (IF bandwidth) setting is one of the most common questions that many LF net-
work analyzer users may first encounter. In high-frequency measurements, it is possible to
use a wide IFBW for faster sweep speed, but for low-frequency measurements we need to
set the IFBW to a narrow value to avoid measurement errors mainly caused by the LO feed
through. For example, let's assume the case of measuring a high-attenuation device where
start frequency = 1 kHz and IFBW = 3 kHz. The small signal attenuated by the DUT is up-
converted to an intermediate frequency (IF) and passes through the IF filter of the receiver.
Here the problem is that the leakage signal from the local oscillator (LO feed through)
also passes through the IF filter because its frequency is very close to the IF frequency as
shown in Figure 10, and this causes unwanted large measurement response.
Figure 11 shows an example of measuring a 60 dB attenuator with the E5061B's gain-phase
test port under the conditions of source level =

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