Agilent 8644A-1
Phase noise test with the Agilent 8644A and
8665A Signal Generators
Product Note
This product note describes the unique
characteristics of the FM scheme used
in the Agilent Technologies 8644A and
8665A and explains how it affects phase
noise measurements. Also included are
typical performance, limitations, and
specific operating instructions for using
these generators as the tunable reference in a phase noise measurement.
8644A and 8665A Theory of
Operation
The 8644A and 8665A have a unique
FM implementation that offers
benefits over the classical open loop
dc coupled scheme but have inherent
characteristics that are different from
most other signal generators. The
default FM is a digitized scheme that
is used for most applications and has
benefits such as center frequency
accuracy and good close-in phase
noise. A traditional open loop, nondigitized, linear FM is also available
as a special function for applications
that need true linear FM with low
group delay. Both schemes have
advantages and disadvantages that
will be explained more thoroughly.
Digitized FM
This is the default FM scheme automatically selected on power-up or
instrument preset. This scheme uses
a single phase-lock-loop with a fractional N divider (Figure 1). The modulation signal is applied to the loop at
two points, directly to the VCO and
indirectly after digitization to the digital divider. Only low rate signals
within the PLL bandwidth can cause
modulation at the divider, the high
rate signals must be applied directly
to the VCO input. The benefits of this
scheme are good dcFM stability since
the loop remains locked to the reference and good close in phase noise
because of the clean-up within the
phase-lock-loop bandwidth. The drawback of this scheme in a phase noise
measurement is the group delay and
quantization error from the digitization. Group delay varies with modulation rate (Figure 2) and can cause
phase shift and possible instability
when the generator is used as an element in a phase-lock-loop. The quantization error is small but can show up
when using small input signals <10 mV.
For these very small signals, the generator appears to have a changing FM
sensitivity which causes an incorrect
phase noise measurement within the
loop bandwidth. A true open loop,
linear FM, is available for phase noise
measurements that use small input
signals and are sensitive to group delay.
Linear FM
This FM scheme is true linear FM
where the modulating signal is applied
directly to the VCO and the feedback
path for the phase locked loop is
opened. This operation is selected
with special function 120. The advantage of this scheme is true linear dcFM
with group delay of < 1 msec. This
eliminates the instability caused by
large group delay and quantization
error from digitization. The disadvantage of this scheme is poor center frequency accuracy and higher close-in
phase noise because the loop is no
longer locked to the reference and
there is no clean up from the PLL.
Figure 1. Simplified Agilent 8644A/65A block diagram
Operation as a Phase Noise
Measurement Reference
The most common technique for
measuring the phase noise of a
source is to use reference source and
demodulate the phase instability
using a phase detector. Commonly
referred to as the Phase Detector
method, this method requires a reference source with equal or better
phase noise performance than the
source being tested. It is also
required that one of the sources have
tuning capability, like dcFM, in order
to maintain phase quadrature at the
input of the phase detector. The need
for good phase noise performance
and dcFM capability often results in a
signal generator being used as the reference source of a phase noise measurement system. The remainder of
this product note describes how to
use and optimize the Agilent 8644A
and 8665A as reference sources for
phase noise measurements. More
information on the measurement
technique itself can be found in literature related to products such as the
Agilent 11729C Carrier Noise Test Set
(Literature #5954-7362) or Agilent
3048A Phase Noise Measurement
System (#5953-8462).
Several features of these signal generators make them good choices for use
as a phase noise measurement source.
These include wide carrier frequency
range, high output power, good spectral
purity, and dcFM. The phase noise of
these signal generators is very low at
offsets greater than 10 kHz from the
carrier, as is commonly required for
testing channelized communication
devices or systems. Both signal generators have very low spurious on the
output which simplifies the detection
and interpretation of spurs from the
test source. The typical phase noise
and spurious performance is shown
in the graph included in the "Phase
Noise Performance" summary of this
product note.
Figure 2. Group delay for digitized FM synthesis
2
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