Oscillators – Beat frequency
Reexamination Certificate
2001-06-12
2003-05-27
Pascal, Robert (Department: 2817)
Oscillators
Beat frequency
C331S002000, C331S042000, C331S043000
Reexamination Certificate
active
06570458
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to automatic test equipment for electronics (ATE) and, more particularly, to the synthesis of low-noise, high frequency waveforms for testing microwave and RF circuitry.
BACKGROUND OF THE INVENTION
Significant improvements in the accuracy of high-frequency devices used in consumer products such as cellular telephones, pagers, and wireless personal data assistants (PDAs) have created a need for more accurate testing of these devices. ATE systems generally include one or more microwave synthesizer for testing microwave devices. In one typical testing scenario, a microwave synthesizer within the tester supplies a signal directly to the DUT. The DUT provides a response, which the tester measures and tests. In another testing scenario, a tester receives a microwave signal (e.g., 900 MHz) from a device under test (DUT). The tester mixes this signal with the output of one of its microwave synthesizers to generate an intermediate frequency signal (e.g., 10 MHz). The tester then samples the intermediate frequency signal to ascertain its characteristics. If the characteristics are within predetermined limits, the test passes. Otherwise, the test fails.
One common testing technique is to compute a power spectrum of the intermediate frequency signal derived from the device under test. A power spectrum reveals meaningful information about the DUT as well as phase noise. To accurately test the phase noise of the device under test, it is essential that the synthesizer's phase noise be small compared with that of the DUT. If the synthesizer's phase noise is large compared with that of the DUT, the DUT's phase noise becomes lost in the synthesizer's phase noise, and it becomes impossible to tell whether the DUT meets its phase noise specification. As devices are continually improved to deliver lower and lower phase noise, microwave synthesizers must correspondingly be improved if testing is to remain accurate.
FIG. 1
illustrates a conventional microwave synthesizer
100
, which operates as follows. A narrow-band synthesizer
112
generates an output signal that can be varied over a relatively narrow range, e.g., a 200 MHz range between 800 MHz and 1 GHz. Simultaneously, a wide-band synthesizer
122
generates an output signal that can be varied over a relatively wide frequency range, e.g., a 2 GHz range between 4.4 GHz and 6.2 GHz. Simultaneously, a comb generator
116
produces a series of harmonically spaced tones, or “combs,” e.g., at 200 MHz tone spacing. The output of the narrow-band synthesizer
112
, the wide-band synthesizer
114
, and the comb generator
116
are respectively fed to a narrow-band input
152
, a wide-band input
154
, and a comb input
156
of a drift-cancel loop
150
.
Within the drift-cancel loop
150
, a power splitter
130
divides the output of the wide-band synthesizer
122
into first and second circuit paths. Amplifiers
132
and
134
boost the levels of signals along the respective paths. A first mixer
138
combines the output of the amplifier
132
with the output of the comb generator
116
, to produce a different pair of sum and difference tones for each tone produced by the comb generator
116
. By appropriately tuning the frequency of the wide-band synthesizer
122
, one of the sum or difference tones from the mixer
138
can be made to equal a target frequency, F
K
. For normal operation, the inputs to the drift-cancel loop
150
are always adjusted to produce a tone at the output of the mixer
138
that equals F
K
.
A first band-pass filter
142
filters the output of the mixer
138
. The first band-pass filter
142
has a center frequency at F
K
, and has a narrow bandwidth for passing only the mixing product at F
K
and substantially rejecting all other frequency components. The output of the first band-pass filter
142
is passed to a second mixer
146
, which combines the output of the first band-pass filter
142
with the output of the narrow-band synthesizer
112
, thus producing another pair of sum and difference tones. These sum and difference tones are passed to a second band-pass filter
144
, which generally rejects the sum tone and transmits the difference tone to its output.
The transmitted tone is passed to a third mixer
140
. The third mixer
140
combines the transmitted tone with the output of the amplifier
134
to produce yet another pair of sum and difference tones. A low-pass filter
148
blocks the sum tone and transmits the difference tone to the output of the synthesizer
100
. The output may be coupled to additional stages (not shown), for selectively multiplying the frequency and adjusting the amplitude of the output signal.
The output frequency of the synthesizer
100
is adjustable in two ways. First, the wide-band synthesizer
122
can be adjusted to vary the overall output frequency in large increments. Second, the narrow-band synthesizer
112
can be adjusted to vary the overall output frequency in small increments. The narrow band synthesizer
122
generally operates via direct digital synthesis (DDS) to produce a nearly continuous range of output frequencies. The frequency range of the narrow-band synthesizer
112
preferably equals or exceeds the spacing of consecutive combs produced by the comb generator
116
, to allow the narrow-band synthesizer to fully tune between adjacent combs. With this arrangement, the wide-band synthesizer
122
effects gross frequency changes, whereas the narrow-band synthesizer
122
effects fine frequency changes. The combination allows the frequency of the synthesizer
100
to be adjusted over a wide range with high precision.
As is known, the wide-band synthesizer
122
tends to produce significant amounts of phase noise. This phase noise is greatly reduced, however, by the action of the drift-cancel loop
150
. Owing to the summing and differencing actions of the mixers
138
,
140
, and
146
, the frequency of the wide-band synthesizer
122
is made to cancel from the output of the synthesizer
100
. Along with the frequency of the wide-band synthesizer
122
, much of its phase noise is made to cancel as well.
In more elaborate implementations, a delay circuit
136
is placed between the second amplifier
134
and the third mixer
140
. The delay circuit
136
causes the inputs of the third mixer
140
to convey signals that represent the output of the wide-band synthesizer
122
at corresponding instants of time. By delaying the signal conveyed along the second circuit path to match the delay incurred by the signal along the first circuit path, a great deal of phase noise is canceled by making corresponding phase perturbations common to both inputs of the mixer
140
. Because the low-pass filter
148
passes only the difference of input frequencies produced by the mixer
140
, noise that is common to both inputs of the mixer
140
is cancelled out.
Even with the addition of the delay circuit
136
, the synthesizer
100
still fails to reject some of the phase noise of the wide-band synthesizer
122
. Low frequency, or “close-in,” phase noise (less than 1 MHz offset) of the wide-band synthesizer largely cancels out, whereas high frequency, “far-out,” phase noise (above 1 MHz offset) generally does not. In implementations that tightly control the phase noise of the narrow-band synthesizer
112
and the comb generator
116
, the overall far-out phase noise of the microwave synthesizer
100
tends to be dominated by the unreduced, far-out phase noise of the wide-band synthesizer
122
.
SUMMARY OF THE INVENTION
With the foregoing background in mind, it is an object of the invention to reduce the far-out phase noise of signals produced by microwave synthesizers in automatic test equipment.
To achieve the foregoing object, as well as other objectives and advantages, a microwave synthesizer according to the invention includes a drift-cancel loop having a narrow-band input, a low-frequency comb input, a wide-band input, and an output for providing an adjustable-frequency output signal. A narrow-band synthesizer is
Glenn Kimberly E
Pascal Robert
Rubenstein Bruce D.
Teradyne, Inc.
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