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Agilent 4352S
VCO/PLL Signal Test System
Optimizing VCO/PLL Evaluations
and PLL Synthesizer Designs
Application Note
Introduction PLL Frequency Synthesizer
Frequency
Divider Loop Filter
Today's mobile communication systems demand
1/M Phase VCO RF
higher communication quality, higher data rates, Comparator
higher frequency operation, and more channels per X'tal
unit bandwidth. As much of this equipment is Programmable
portable, low power consumption and small size are Frequency divider
also required. All of these constraints combine to
1/N Prescaler
make the whole design including component selection
and evaluation quite challenging. One portion of this PLL IC
design that is very critical with regard to all of the
requirements mentioned above is the synthesized Set N data
oscillator. Typical synthesized oscillators combine
a Voltage Controlled Oscillator (VCO) with a Phase- Fig. 1: Basic Block Diagram of PLL Frequency Synthesizer
Locked Loop IC (PLL), frequency reference (e.g.
Crystal / TCXO) and a loop filter. The VCO is used to
generate the RF output frequency. The PLL (which is
of the "analog type"; i.e. different from a pure digital Design or Purchase VCO
PLL) is used to stabilize and control the frequency.
The loop filter design must integrate all of the compo-
nents to establish, among other things, a tradeoff
between noise and transient response (Figure 1). Evaluate VCO
This Application Note will describe the evaluation of
the PLL and VCO and relate those evaluations to infor-
mation that will allow the circuit designer to optimize
Design Loop Filter
the whole oscillator design including the loop filter.
Evaluate PLL Synthesizer
Fig. 2: PLL Frequency Synthesizer Design Flow
1 VCO Characteristic Parameters The absolute level of oscillation power depends prima-
rily on the DC control voltage. This level should be
and Problems with the Conventional maintained constant over the entire frequency range
Evaluation Method within which the VCO can oscillate. It is critical to
evaluate the oscillation power level vs. DC control
voltage characteristic. A program that can measure
1-1 VCO characteristic
the oscillation power level and display the measure-
parameters to be evaluated ment results in graphical form while changing the
Shown below are many of the common VCO evalua- DC voltage level is required.
tion parameters. To perform these evaluations, many
instruments and set-ups are required even including
special DC sources for both the power supply and
1-2 Impact of control voltage source
tuning voltage. noise characteristic on VCO
Using a general purpose DC power supply as the DC
1) Oscillation frequency [Hz] control voltage source for the VCO may degrade the
2) Oscillation power level [dBm] measured VCO phase noise characteristic due to large
3) Phase noise [dBc/Hz] noise component contained in the supplied power. As
4) Residual FM [Hz rms] a result, the measurement result obtained will be simi-
5) DC consumption current [mA] lar to that shown in Fig.3. This result does not represent
6) Tuning sensitivity [Hz/V] the real VCO characteristic. To suppress this noise
7) Harmonic/spurious [dBc] component, provide a noise elimination filter (low-
8) Frequency pushing [Hz/V] and frequency pass filter) with a low cutoff frequency at the DC
pulling [Hz p-p] control voltage input terminal of the VCO. The lower
the cutoff frequency the longer the time constant, and
Nine or more measuring instruments including a DC longer time it takes for the VCO frequency and its
voltage source (for VCO power supply), a DC control output level to stabilize. This results in longer measure-
(tuning) voltage source and a spectrum analyzer are ment time. The longer measurement time can cause
required to evaluate these characteristics. A controller the carrier frequency to vary due to change in temper-
and control and analysis programs are also required ature, humidity, or external noise, thus making it
to control these instruments and analyze measure- difficult to evaluate accuracy.
ment results.
For example, the VCO changes its oscillation
frequency if there is a change in DC voltage level
With Noisy Control Voltage Source
applied to its control terminal. To determine the VCO
basic performance, evaluate the output frequency vs.
DC control voltage characteristic (F-V characteristic).
A program that can measure the output frequency
with a frequency counter is required to accomplish
this. The program should display the measurement
results in graphical form as the DC control voltage
level from the DC voltage source is changed. Another
important VCO parameter is tuning sensitivity with
respect to the VCO control voltage. The tuning
sensitivity is the derivative of the VCO frequency by
the tuning voltage. It significantly affects the loop
With Low Noise Control Voltage Source
characteristics. PLL design should measure and
graphically display the tuning sensitivity of the VCO.
Fig. 3: Impact of Control Voltage Source Noise Characteristic on VCO
2
1-3 VCO oscillation frequency stability and Carrier Frequency
Drift
phase noise characteristic evaluation
Phase noise is a random noise. It is expressed as the
ratio of "power spectrum density at a specified offset
frequency" to "carrier signal level". Averaging is essen-
tial to ensure proper repeatability of measurement
results. (See Fig.4.)
Uncertainty
Offset
Frequency
Unstable Offset
Frequency
Fig. 5: Impact of Carrier Drift on Phase Noise Characteristic
1-4 Phase noise measurement of DUT and
phase noise of measurement system
The measurement system should have better phase
noise performance than that of DUT (Device Under
Test). The measurement system cannot measure phase
Fig. 4: Phase Noise Measurement Results without Averaging noise of the DUT whose phase noise level is lower than
the system noise floor. However, even when the system
noise floor is lower than DUT noise, the measurement
A difficulty caused by VCO oscillation frequency drift result will have relatively large error if the DUT phase
or jump may occur when using a spectrum analyzer to noise level is close to the system noise floor. To get an
measure VCO phase noise. Such frequency drift or accurate and reliable result, the system noise floor of
frequency jump occurs when there is a temperature the measurement system must be carefully evaluated
change, vibration or shock during the measurement and compared with the DUT phase noise.
period. Measurement errors may occur if the previous
conditions were present during the measurement.
(See Fig.5.) Since these error factors are usually not
1-5 Phase noise of end product (equipment)
repeatable, they cannot be removed or calibrated by and phase jitter requirements
averaging or other methods. To make accurate VCO With today's mobile communication, digital modulation
phase noise measurement, it is important to keep VCO for improved frequency band use efficiency and a
oscillation frequency stable during the measurement. variety of schemes for reduced bit error rate are com-
However, VCO frequency is so sensitive to its environ- monplace. For example, ACPR (Adjacent Channel
mental disturbance that it is very difficult to keep VCO Power Ratio), that represents the purity of transmitted
oscillation frequency constant without sophisticated and received signals, includes not only distortion and
stabilizing mechanism in the measurement system. AM noise caused by digital signal but also phase noise.
For this reason, when it is desired to measure only
3
errors as well as bit errors due to irregular sampling.
Phase jitter is equivalent to residual phase modulation
and determined by phase noise. A dedicated phase
noise measurement system is required to calculate
phase jitter based on phase noise. As mentioned previ-
ously, it has been difficult to make efficient phase
jitter measurement and analysis.
2 VCO Characteristic Evaluation Using
the 4352S and Features of the 4352S
2-1 VCO testing capabilities of the 4352S
The Agilent 4352S is a self-contained solution for
performing virtually all measurements required for
Fig. 6: Example of ACPR (Adjacent Channel Power Ratio) Measurement
thorough VCO evaluation (Figure 7). Specialized
Using the E4406A
sources and measurement equipment have been com-
bined to achieve this dedicated task with ease and
accuracy. For example, the system contains a low noise
phase noise power in a specific frequency range, there power supply to power the DUT and an ultra low noise
is no alternative but to use a measurement system ded- DC tuning / control voltage source (Figures 5 & 6). The
icated to that purpose. This is in despite to growing system is integrated and includes the switching and
demands to reduce phase noise, eventually requiring a firmware to perform all of the following tests
large amount of time and cost. (Fig.6 shows an example accurately and with ease.
of ACPR measurement for PDC-modulated signals
using the E4406A vector signal analyzer.) 1) Oscillation Frequency [Hz]
2) Oscillation Power level [dBm]
Phase jitter measurement of wavelength or clock 3) Phase noise [dBc/Hz]
frequency is of significant importance for evaluating 4) Residual FM [Hz rms]
the performance of radar, laser length measuring 5) Drive consumption current [mA]
machine, A-D and D-A converters as well as digital 6) Tuning sensitivity [Hz/V]
communication systems. An increase in phase jitter 7) Harmonic/spurious [dBc]
causes transmit, receive distance, and quantization 8) Frequency pushing [Hz/V]
VCO Control/PLL Power
Ultra Low Noise DC Frequency
Power Source Frequency Counter
DC Consumption Current RF Power
Digital Multimeter RF Power Meter Local
Freq/
VCO Power
VCO Power PLL FM Dev./RF Transient
DC Power Source Set N Signal
F/V Converter
24 Bit I/O Generator
Phase Noise/Spectrum
Modulation Signal GPIB
FFT Analyzer
Audio Signal Analyzer Internal
Controller
Fig. 7: 4352S System Block Diagram
4
These functions will alleviate the tasks of configuring Note also that the 4352S can measure frequency push-
a measurement system, correcting measurement data ing (oscillation frequency change with change in DC
and creating measurement and analysis programs. The power supply voltage level) by controlling the voltage
result is improved design and evaluation efficiency. level of the VCO power supply through its IBASIC pro-
For example, the 4352S makes it possible to readily gramming function. Figure 10 shows an evaluation
measure and graphically display basic VCO performance example of the output frequency vs. DC power supply
parameters such as RF output power vs. DC control voltage characteristic. In this example, three different
voltage characteristic, RF output frequency vs. DC DC power supply voltage level settings are used.
control voltage characteristic (F-V characteristic) and
tuning sensitivity characteristic. (See Fig.8 and Fig.9.)
Vcc=12.5V
Vcc=12.0V
Vcc=11.5V
Fig. 10: Example of Frequency Pushing Measurement
(Examples of Frequency vs. DC Voltage Characteristic
Fig. 8: RF Output Power vs. DC Control Voltage Curve Obtained
during Change in DC Power Supply Voltage Level)
through the 4352S
2-2 Ultra low noise DC voltage source
The 4352S incorporates a VCO power supply (DC
POWER, 10nV/Hz@10kHz offset) and an ultra low
noise VCO control voltage source (DC CONTROL,
1nV/Hz@10kHz offset) for testing VCO. Because it is
Frequency not necessary to add a DC power supply for VCO test-
ing or low-pass filter to remove noise, it is possible to
quickly determine the real VCO phase noise character-
istic without waiting during DC power voltage setting
change.
2-3 High speed SSB phase noise
Tuning Sensitivity measurement function and automatic
frequency control function
The 4352S includes a "Carrier lock multi-mode PLL
Fig. 9: Output Frequency vs. DC Control Voltage Curve Obtained circuit", developed for high-speed phase noise
Using the 4352S
measurement. The stepped FFT technique and the
5
RF
4352B IF (24MHz)
Frequency DSP
Counter (FFT)
100MHz to 10MHz
PLL
CPU
2nd LO
1st LO
GPIB
Signal Generator
Fig. 11: Carrier Lock Multi-mode PLL Block Diagram
ultra low noise DC control voltage source, enable fast
and easy VCO phase noise measurements (several tens
of times faster than the conventional method). Figure
11 shows the block diagram of the carrier lock multi-
mode PLL circuit. The VCO frequency is measured,
and then the carrier frequency is translated into IF of
24 MHz through the mixer, with the local signal from
the external signal source. The phase noise of the sig-
nal is measured in the carrier lock multi-mode PLL
circuit with the orthogonal phase detection method.
This phase noise measurement circuit continually locks
to the drifting carrier frequency enabling quick and
accurate phase noise measurement with high repeata-
bility. Because the 4352S has automatic control over
all necessary settings, including the external signal
Fig. 12: Example of VCO Phase Noise Measurement Using the 4352S
source frequency, the phase noise measurement can
be made with ease. (See Fig.12.)
4352S
The 4352S Automatic Frequency Control, which oper-
Feedback
ates with the built-in frequency counter and DC tuning
voltage source, automatically controls the DC tuning
voltage. Therefore, VCO phase noise at a specific Control Voltage Frequency
Source Monitor
carrier frequency can easily be measured by directly
entering the desired frequency. (See Fig.13.)
VCO
Carrier Frequency
Target
Frequency
Fig. 13: Automatic Frequency Control Function
6
2-4 Phase noise characteristic of the 4352S The 4352S displays phase noise and integrated phase
noise power within the specified frequency range on
The 4352S is designed to have phase noise of -157dBc/Hz the same screen for improved development and design
(typical with 1 GHz carrier frequency @1 MHz offset efficiency.
frequency). This is sufficiently lower than the phase
noise level of VCO's used in ordinary cellular phones. Figure 15 shows a phase noise vs. offset frequency
(See Fig.14.) trace measured with the 4352S. The total phase noise
power [dBc] within the cursor-specified offset fre-
quency range (1 kHz) appears at the lower left corner
of the screen.
Fig. 14: Typical 4352B Phase Noise Characteristic
Fig. 15: Example of Phase Noise Power Measurement Using the 4352S
A consideration must be made to the phase noise gener-
ated by external local signal source when configuring
a measurement system. For more information on how
to select the optimal signal generator, see "1) Selecting Common logarithmic scale is normally used for offset
Local Signal Source (Standard Signal Generator) for frequency when a phase noise trace is displayed, how-
the 4352S" of "Considerations for Configuring Optimal ever, linear scale is used for offset frequency when
4352S System" in the last chapter. an adjacent channel power ratio trace is displayed.
Similarly, the 4352S selects linear sweep for offset
frequency when measuring phase noise power.
2-5 Integrated phase noise measurement
function and phase jitter calculation
Adjacent channel power ratio, a standard requirement
to be met by final products, is represented as the ratio
of power at the carrier frequency to integrated power
within the specified offset frequencies. This is unlike
the standard representation of phase noise.
7
The 4352S measures the integrated phase noise and Through IBASIC programming the 4352S calculates
can calculate phase jitter based on phase noise meas- phase jitter based on the above formula. The result is
urement results using IBASIC programming function. then displayed on the screen. The numbers shown on
The following example demonstrates how to calculate the upper left corner in Fig.15 is the phase jitter value
phase jitter: obtained through the above program.
The relationship between phase noise L(f) and phase The 4352S functions discussed above will assist with
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