U.S. patent application number 15/267451 was filed with the patent office on 2018-03-22 for group delay measurement apparatus and method.
This patent application is currently assigned to Guzik Technical Enterprises. The applicant listed for this patent is Guzik Technical Enterprises. Invention is credited to Anatoli B. Stein, Alexander Taratorin, Semen P. Volfbeyn.
Application Number | 20180080965 15/267451 |
Document ID | / |
Family ID | 61620933 |
Filed Date | 2018-03-22 |
United States Patent
Application |
20180080965 |
Kind Code |
A1 |
Stein; Anatoli B. ; et
al. |
March 22, 2018 |
GROUP DELAY MEASUREMENT APPARATUS AND METHOD
Abstract
Measurement of group delay for a device under test (DUT). A test
signal includes (i) a low frequency sine wave f.sub.LF, (ii) sine
wave harmonics at a high frequency f.sub.HF, (iii) L pairs of
sideband components at frequencies kf.sub.HF.+-.2f.sub.LF, where k
odd, and M pairs of sideband components at frequencies
kf.sub.HF.+-.f.sub.LF, where k is even. At DUT output, (i) phase
.phi..sub.LF at frequency f.sub.LF is measured, (ii) both sideband
phase .phi..sub.right(k) at frequencies kf.sub.HF+2f.sub.LF and
phase .phi..sub.left(k) at frequencies kf.sub.HF-2f.sub.LF for odd
k, are measured, and (iii) both sideband phases .phi..sub.right(k)
at frequencies kf.sub.HF+f.sub.LF and .phi..sub.left(k) at
frequencies kf.sub.HF-f.sub.LF for even k, are measured. Group
delay .tau..sub.k at frequencies kF.sub.HF, are determined from:
.tau..sub.k=(.phi..sub.right(k)-.phi..sub.left(k)-4.phi..sub.L)/(4f.sub.L-
F) for k odd, and
.tau..sub.k=(.phi..sub.right(k)-.phi..sub.left(k)-2.phi..sub.L)/(2f.sub.L-
F) for k even.
Inventors: |
Stein; Anatoli B.;
(Atherton, CA) ; Taratorin; Alexander; (Palo Alto,
CA) ; Volfbeyn; Semen P.; (Palo Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Guzik Technical Enterprises |
Mountain View |
CA |
US |
|
|
Assignee: |
Guzik Technical Enterprises
Mountain View
CA
|
Family ID: |
61620933 |
Appl. No.: |
15/267451 |
Filed: |
September 16, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R 23/175 20130101;
G01R 31/31725 20130101; H04B 3/462 20130101; G01R 23/02 20130101;
G01R 31/318328 20130101; G01R 27/28 20130101 |
International
Class: |
G01R 23/175 20060101
G01R023/175; G01R 23/02 20060101 G01R023/02 |
Claims
1. A method of group delay measurement for a device under test
(DUT) having an input for receiving a signal characterized by a
first domain, and an output for providing an output signal
characterized by a second domain in response to the input signal
received by the input, comprising the steps of: a. controlling a
signal generator to generate a test signal, wherein the test signal
includes as spectral components: i. a sine wave characterized by a
relatively low frequency f.sub.LF; ii. a set of harmonics of a sine
wave characterized by a relatively high frequency f.sub.HF; iii. a
set of L pairs of sideband components characterized by frequencies
kf.sub.HF.+-.2f.sub.LF, where k is an odd number; iv. a set of M
pairs of sideband components characterized by frequencies
kf.sub.HF.+-.f.sub.LF, where k is an even number; wherein phase
.phi..sub.right(k) of the sideband components with frequencies
kf.sub.HF+nf.sub.LF, and phase .phi..sub.left(k) of the sideband
components with frequencies kf.sub.HF-nf.sub.LF, are related to the
phase .phi..sub.LF of the component with frequency f.sub.LF and
phase .phi..sub.HF of the component with the frequency f.sub.HF, by
equations .phi..sub.right=k.phi..sub.HF+n.phi..sub.LF,
.phi..sub.left=k.phi..sub.HF-n.phi..sub.LF. b. applying the test
signal to the input of a device under test and c. obtaining from
the output of the device under test, an output signal responsive to
the test signal applied to the input with a phase measuring device
by: i. measuring phase .phi..sub.LF of a component thereof
characterized by frequency f.sub.LF; ii. measuring the phase
.phi..sub.right(k) of sideband components thereof characterized by
frequencies kf.sub.HF+2f.sub.LF and phase .phi..sub.left(k) of
sideband components with the frequencies kf.sub.HF-2f.sub.LF for
odd numbers k; and iii. measuring phases .phi..sub.right(k) of the
sideband components thereof characterized by frequencies
kf.sub.HF+f.sub.LF and .phi..sub.left(k) of the sideband components
with the frequencies kf.sub.HF-f.sub.LF for even numbers k; and d.
analyzing the measured phases by: i. calculating with a digital
processor, a group delay .tau..sub.k at frequencies kF.sub.HF
according to:
.tau..sub.k=(.phi..sub.right(k)-.phi..sub.left(k)-4.phi..sub.LF)/(4f.-
sub.LF) where k is odd, and
.tau..sub.k=(.phi..sub.right(k)-.phi..sub.left(k)-2.phi..sub.LF)/(2f.sub.-
LF) where k is even; and ii. determining with a digital processor,
group delay for N frequencies in a range of interest by repeating
N/(L+M) times, the set of steps a, . . . , c, each time for a value
of the frequency f.sub.HF.
2. The method of group delay measurement according to claim 1,
wherein the generating of the test signal is performed by producing
two sine waves with respective frequencies f.sub.LF and f.sub.HF,
summing the two sine waves, and amplitude limiting the resultant
sum by applying the summed sine waves to than amplitude
limiter.
3. The method of group delay measurement according to claim 2,
wherein the measured phases of the spectral components of the
output signal are determined by a digital processor calculating a
Fast Fourier Transform (FFT) of the signal at the output of the
device under test.
4. The method of group delay measurement according to claim 1,
wherein the first domain includes frequencies in a first range, and
the second domain includes frequencies in a second range.
5. The method of group delay measurement according to claim 4,
wherein range of frequencies of the first domain is lower than the
range of frequencies of the second domain.
6. The method of group delay measurement according to claim 4,
wherein range of frequencies of the first domain is higher than the
range of frequencies of the second.
7. The method of group delay measurement according to claim 1,
wherein the first domain is an analog domain, and the second domain
is a digital domain.
8. The method of group delay measurement according to claim 1,
wherein the first domain is a digital domain, and the second domain
is an analog domain.
9. An apparatus for group delay measurement for a device under test
(DUT) having an input for receiving a signal characterized by a
first domain, and an output for providing an output signal
characterized by a second domain in response to the input signal
received by the input, comprising: a. a first oscillator providing
at an output, wherein the sine wave is characterized by a
relatively low frequency f.sub.LF; b. a second oscillator providing
at an output, wherein the sine wave is characterized by a
relatively high frequency f.sub.HF; c. an amplitude limiter
including a first input, a second input and an output, wherein the
first input is connected to the output of the first oscillator, the
second input is connected to the output of the second oscillator,
and the output is connected to the input of a device under test,
and wherein the amplitude limiter is configured to produce at the
output thereof, a test signal corresponding to an amplitude limited
sum of the inputs to the amplitude limiter, wherein the test signal
includes as spectral components: i. a sine wave characterized by a
relatively low frequency f.sub.LF; ii. a set of harmonics of a sine
wave characterized by a relatively high frequency f.sub.HF; iii. a
set of L pairs of sideband components characterized by frequencies
kf.sub.HF.+-.2f.sub.LF, where k is an odd number; iv. a set of M
pairs of sideband components characterized by frequencies
kf.sub.HF.+-.f.sub.LF, where k is an even number; wherein phase
.phi..sub.right(k) of the sideband components with frequencies
kf.sub.HF+nf.sub.LF, and phase .phi..sub.left(k) of the sideband
components with frequencies kf.sub.HF-nf.sub.LF, are related to the
phase .phi..sub.LF of the component with frequency f.sub.LF and
phase .phi..sub.HF of the component with the frequency f.sub.HF, by
equations .phi..sub.right=k.phi..sub.HF+n.phi..sub.LF,
.phi..sub.left=k.phi..sub.HF-n.phi..sub.LF.; d. a measurement unit
having an input configured to receive a signal from the output of
the device under test in a predetermined domain wherein the
measurement unit includes a phase measuring device operative on the
signal received from the output of the device under test for: i.
measuring phase .phi..sub.LF of a component thereof characterized
by frequency f.sub.LF; ii. measuring the phase .phi..sub.right(k)
of sideband components thereof characterized by frequencies
kf.sub.HF+2f.sub.LF and phase .phi..sub.left(k) of sideband
components with the frequencies kf.sub.HF-2f.sub.LF for odd numbers
k; and iii. measuring phases .phi..sub.right(k) of the sideband
components thereof characterized by frequencies kf.sub.HF+f.sub.LF
and .phi..sub.left(k) of the sideband components with the
frequencies kf.sub.HF-f.sub.LF for even numbers k; and e. a
processing unit for analyzing the measured phases from the
measurement unit, to determine a group delay for frequencies
f.sub.LF and f.sub.HF for the device under test, by: calculating
with a digital processor, a group delay .tau..sub.k at frequencies
kF.sub.HF according to:
.tau..sub.k=(.phi..sub.right(k)-.phi..sub.left(k)-4.phi..sub.LF)/(4f.sub.-
LF) where k is odd, and
.tau..sub.k=(.phi..sub.right(k)-.phi..sub.left(k)-2.phi..sub.LF)/(2f.sub.-
LF) where k is even; and f. an interface unit having an input
connected to the output of the device under test and an output
connected to the input of the processing unit, said interface unit
being configured to provide the output of the device under test in
the predetermined domain to the input of the processing unit; and
g. a control unit configured to arrange measurement performance
step by step, to establish the frequency f.sub.HF for each step of
measurement and to determine group delay of the device under test
for harmonics kf.sub.HF of the frequency f.sub.HF.
10. The apparatus for group delay measurement according to claim 9,
wherein the processing unit determines phases of input signal
spectral components by performing a Fast Fourier Transform (FFT) on
the received signal.
11. The apparatus for group delay measurement according to claim 9,
wherein the amplitude limiter includes an adder configured to
receive the sine waves from the first oscillator and the second
oscillator and provide at an output of the adder, a sum of the
received sine waves, and provide at the output of the amplitude
limiter, an amplitude limited form of the sum.
12. The apparatus for group delay measurement according to claim 9,
wherein the amplitude limiter includes an amplifier with
differential inputs configured to receive the sine waves from the
first oscillator and the second oscillator, and to provide at an
output of the amplitude limiter, an amplitude limited form of a sum
of the signals at the differential inputs.
13. The apparatus for group delay measurement according to claim 9,
wherein the device under test is an analog to digital
converter.
14. The apparatus for group delay measurement according to claim 9,
wherein the apparatus is configured to receive a digital frequency
converter as a device under test.
15. The apparatus for group delay measurement according to claim 9,
wherein the apparatus is configured to receive an analog device as
a device under test.
16. The apparatus for group delay measurement according to claim 9,
wherein the apparatus is configured to measure group delay of an
analog up converter having an input and an output, as a device
under test, and further comprising: an adder including: i. a first
input configured to receive the sine wave characterized by a
relatively low frequency f.sub.LF from the first oscillator, ii. a
second input configured to receive the output of the device under
test, and iii. an output coupled to the input of the processing
unit, wherein the adder is configured to provide a sum of the sine
wave at the first input and output of the analog up converter to
the input of the processing unit.
17. The group delay measurement apparatus according to claim 9,
wherein the first domain includes frequencies in a first range, and
the second domain includes frequencies in a second range.
18. The group delay measurement apparatus according to claim 17,
wherein range of frequencies of the first domain is lower than the
range of frequencies of the second domain.
19. The group delay measurement apparatus according to claim 17,
wherein range of frequencies of the first domain is higher than the
range of frequencies of the second domain.
20. The group delay measurement apparatus according to claim 9,
wherein the first domain is an analog domain, and the second domain
is a digital domain.
21. The group delay measurement apparatus according to claim 9,
wherein the first domain is a digital domain, and the second domain
is an analog domain.
22. The method of group delay measurement according to claim 2,
wherein the producing of each of the two sine waves is performed by
an oscillator.
23. The method of group delay measurement according to claim 2,
wherein the summing of the two sine waves is performed by an
adder.
24. The method of group delay measurement according to claim 2,
wherein the summing of the two sine waves and amplitude limiting is
performed by applying the two sine waves to differential inputs of
a limiting amplifier.
Description
FIELD OF THE TECHNOLOGY
[0001] The technology relates to a method and apparatus for
measurement of group delay caused by a device under test (DUT),
including but not limited to signal conversion devices, such as
high speed analog to digital converters, analog and digital
up-converters, down-converters and others.
BACKGROUND
[0002] Signal conversion devices are characterized by a frequency
response, consisting of amplitude and phase terms. Correction
(equalization) of the frequency response is essential for a high
quality converter and requires precise measurement of device
properties, and the group delay dependence on frequency in
particular.
[0003] Analog to digital converters (ADCs) operate in a wide
frequency region that extends from baseband frequencies up to the
limiting case of high frequencies. It means that group delay of an
ADC should be measured in the correspondent wide frequency range.
To ensure a precise correction of the frequency response of an ADC
or a frequency converter the group delay measurement should be
performed with a small enough frequency step: the distance between
the adjacent frequencies, at which the group delay is measured
should be reasonably small-sized. The apparatus for group delay
measurement should be straight forward: it should comprise as few
components as possible, and should not require employment of
laboratory devices and additional calibration.
[0004] Group delay measurements are conventionally performed using
Vector Network Analyzers (VNAs). For example, Agilent Application
Note 5965-7707E "Understanding the Fundamental Principles of Vector
Network Analysis" describes a group delay measurement method which
injects a known sinusoidal excitation to an input of the device
under test (DUT), and analyzes the phase of the signal at its
output. However, that VNA-based method has limitations. First, it
requires a DUT to have input and output ports of the same type, As
a consequence, that method is not applicable to such devices as
ADCs, where the input signal is analog and the output signal is
digital. Second, VNA-based method of measurement requires that the
input and the output signals lie in the same frequency range. For
these reasons, VNA-based group delay measurement cannot be used for
frequency converters.
[0005] Another prior art method for group delay measurement is
based on a time domain pulse shape analysis. The time domain method
uses an injection of a known periodic signal (e.g., impulses or
rectangular pulses) into a DUT and capturing a waveform at the DUT
output. This method is applicable to ADC group delay measurement.
Phase distortions introduced by the DUT can be determined using
Fourier Transform-based analysis, i.e., obtaining a spectrum (and
more generally, the phase response) of the DUT output signal and
comparing it with the spectrum (or the phase response) of the input
signal, thus obtaining the phase response of the DUT. However, high
frequency measurements require expensive and complicated tools for
test signal generation, such as a special picosecond pulse source.
Signal sources of that kind may have varying group delay, for which
reason they must be calibrated before the measurement, using, for
example, a high accuracy temporal resolution sampling scope. As a
consequence, the measuring apparatus becomes complicated and
cumbersome, ruling out the possibility of calibrating an ADC (or
like devices) under operating conditions.
[0006] European Patent Application No. EP1515147A1 (J. Kraus and C.
Kikkert) describes group delay measurement of an ADC based on
generating a test signal through modulation of an initial signal
consisting of a plurality of spectral components, by a low
frequency signal (FIG. 1). With that method, a digitized waveform
from the output of a DUT is Fourier transformed. The phase
differences between sidebands of initial signal spectral lines are
calculated. Group delay at the corresponding frequency is found by
dividing the corresponding phase difference by the difference of
the sideband frequencies.
[0007] A phase difference between the sidebands of the initial
signal spectral line contains an unknown phase offset that equals a
doubled phase of the low frequency signal. This phase offset causes
a corresponding offset in the calculated group delay. As long as
the low frequency signal remains unchanged during the measurement,
the unknown offset delay is inconsequential--it does not cause
distortions of the measured group delay.
[0008] The number of spectral lines in the initial signal is
limited by the acceptable complexity of the measuring apparatus,
and usually is far less than the number of frequencies where group
delay is to be measured. A repetition of the measurement, with
alteration of the spectral lines frequencies in the initial signal,
presents difficulties because it is accompanied by random changes
in the phase of the low frequency signal with the a corresponding
appearance of the unknown offset in the measured group delay. As a
consequence of the limited number of frequencies where group delay
is measured, only an approximate correction of frequency response
based on these measurements is possible. This factor significantly
constrains applicability of this measurement method.
[0009] U.S. Pat. No. 8,983,796 (T. Bednorz and S. Neidhardt)
describes a different method of group delay measurement based on
generation of two sine wave signals with different frequencies
f.sub.1 and f.sub.2. This method is schematically illustrated in
FIG. 2. The input signals f.sub.1, f.sub.2 from two sine wave
signal generators 10 and 11 are added in a summer 12. The resultant
summed signal is split in a splitter 13 between a reference channel
40 and a DUT 20 followed by a measurement channel 30.
[0010] The phase difference of the input signals is measured in
reference channel 40 using a mixer 41 which uses a local oscillator
signal f.sub.LO to translate the summed signal from splitter 13 to
a low frequency signal including components derived from input
signals f.sub.1, f.sub.2. That two component low frequency signal
is processed to perform phase detection using a digital quadrature
circuit consisting of low pass filters (LPFs) 42 and 43 followed by
analog to digital converters (ADCs) 44 and 45 respectively,
followed by a phase detector 60. In this configuration, the outputs
of ADCs 44 and 45 are input to phase detector 60, which may be
implemented as quadrature phase detector.
[0011] The phase difference of the output of the DUT is measured in
measurement channel 30 followed by a phase detector 50, which are
similar to reference channel 40 and phase detector 50. In
particular, measurement channel 30 includes a mixer 31 which uses a
local oscillator signal f.sub.LO to translate the summed signal
from splitter 13 and the DUT 20 to a low frequency signal including
components derived from input signals f.sub.1, f.sub.2. That two
component low frequency signal is processed to perform phase
detection using a digital quadrature circuit consisting of low pass
filters (LPFs) 32 and 33 followed by analog to digital converters
(ADCs) 34 and 35 respectively, followed by phase detector 50. In
this configuration, the outputs of ADCs 34 and 35 are input to
phase detector 50, which may be implemented as quadrature phase
detector.
[0012] The frequencies f.sub.1, f.sub.2 of the input sine wave
signals are swept within a frequency band of interest and group
delay measurement is obtained by subtracting the reference phase
difference (at the output of phase detector 60) from DUT 20 phase
difference (at the output of phase detector 60) in a group delay
calculation unit 70. This method also has a number of disadvantages
and suffers from circuit complexity. It is not possible to measure
group delay of an ADC at low frequencies. Moreover, there is a need
for wideband mixers 31 and 41 which are used for providing signal
down-conversion. The down-conversion step includes use of low pass
filters 32, 33, 42 and 43, which may introduce additional phase
distortions and so must be precisely matched for four quadrature
branches (reference and measurement). Also, each quadrature channel
is digitized by separate ADCs 34, 35, 44 and 45, which may have
frequency mismatch and, as a consequence, introduce phase
errors.
[0013] There is a need in a method and a simple apparatus for group
delay measurement which may be applied to signal conversion devices
and which provide for precise measurement of group delay in a wide
frequency band at frequently repeated frequencies by separate tests
for different sets of frequencies.
SUMMARY
[0014] Method and apparatus for group delay measurement according
to the present technology comprises means for generating a test
signal using two sinusoidal signal sources at low and high
frequencies, followed by amplitude limiting of a sum of those
signals. This test signal is injected into a DUT. A digitized
waveform of the amplitude limited signal is obtained, and group
delay, is determined by simultaneous measurement of signal sideband
components and low frequency fundamental phases. The method is
applicable to signal conversion devices, such as ADCs,
up-converters and down-converters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 depicts a method for generating components of a
signal spectrum using a mixer for use in a prior art group delay
measurement system;
[0016] FIG. 2 depicts a block diagram of prior art group delay
measurement system using separate reference and measurement signal
paths;
[0017] FIG. 3 shows a block diagram of an exemplary group delay
measurement system according to the current technology;
[0018] FIG. 4a shows an exemplary amplitude limiter of the
configuration of FIG. 3, configured as a cascade connection of an
adder and an amplitude limiter;
[0019] FIG. 4b shows an exemplary amplitude limiter of the
configuration of FIG. 3, configured as a limiting amplifier;
[0020] FIG. 5a illustrates an exemplary waveform at the output of
an amplitude limiter in a form of the technology;
[0021] FIG. 5b illustrates an exemplary spectrum at the output of
an amplitude limiter in a form of the technology;
[0022] FIG. 6 shows a block diagram of group delay measurement
configuration for an analog up converter according to the
technology;
[0023] FIG. 7a illustrate results of a numerical simulation of the
method of technology for an experimental group delay measurement of
a 40 Gs/s ADC; and
[0024] FIG. 7b shows a zoomed region of FIG. 7a in the range 10700
to 13300 MHz.
DETAILED DESCRIPTION
[0025] A block diagram of an exemplary apparatus for group delay
measurement, according to the current technology, is shown in FIG.
3. The apparatus comprises two sine wave oscillators: a low
frequency sine wave oscillator 1, generating a low frequency
f.sub.LF, and a high frequency sine wave oscillator 2, generating a
high frequency f.sub.HF. Output signals of oscillators 1 and 2 are
connected to the inputs of an amplitude limiter 3. A signal formed
in amplitude limiter 3 is applied to an input of a DUT 4. A signal
from an output of DUT 4 is processed in a processing unit 6. An
interface unit 5 is coupled between the output of DUT 4 and an
input of processing unit 6. The interface unit 5 converts the
signal from the output of DUT 4 into a form that matches the input
of processing unit 6.
[0026] A control unit 7 manages the process of measurement step by
step, establishing a frequency f.sub.HF for each step of
measurement, and determining group delay of DUT 4 for harmonics
kf.sub.HF of the frequency f.sub.HF that had been set.
[0027] The amplitude limiter 3 may be constructed as a cascade
connection of an adder with a limiting amplifier (for example, as
shown in FIG. 4a), or as a limiting amplifier with a differential
input (for example, as shown in FIG. 4b). In both cases, the formed
test signal presents a limited sum of two sine waves.
[0028] A signal at the output of amplitude limiter 3 is shown in
FIG. 5a, where, as an example, a mix of two sine waves is amplitude
limited at a threshold level of 10% from the original amplitude. A
sum of the low and high frequency signals at the output of the
amplitude limiter 3 is transformed into a sequence of width
modulated pulses. During a positive half period of the low
frequency, those pulses have narrower negative-going, and wider
positive-going, widths. During negative half period, the
negative-going polarity pulses are wider, and the positive-going
polarity pulses are narrower. This pulse width modulation is
periodic, with the period of low frequency signal. As a result, a
low frequency component at f.sub.LF is present at the output of
amplitude limiter 3, with the phase of this low frequency signal
being equal the phase of original low frequency sine wave.
[0029] The relationship between the voltage at the output of the
amplitude limiter, and the voltage at its input, may be
approximated by a Taylor series decomposition having odd
components, i.e., by a polynomial of the form ax+bx.sup.3+cx.sup.5.
. . . As a result, the spectrum of the signal at the output of
amplitude limiter 3 comprises multiple combination frequencies
kf.sub.HF.+-.nf.sub.LF, where k, n are integers and k+n is an odd
number. An illustrative example of the spectrum at the output of
the amplitude limiter 3 for f.sub.LF=25 MHz, f.sub.HF=150 MHz, is
shown in FIG. 5b. The main spectral peak corresponds to the first
harmonic of the high frequency sine wave; the magnitude spectrum is
normalized to 0 dB at f.sub.HF=150 MHz. As seen, the spectrum has
distinct low frequency spectral component at 25 MHz, and a number
of combination frequencies (sidebands). The highest sideband
magnitude is around second harmonic of the high frequency signal,
i.e., at 2f.sub.HF.+-.f.sub.LF, with -10 dB magnitude. The
dominance of sidebands at the second harmonic is explained by the
fact that the signal consists of a sequence of width-modulated
rectangular pulses. It is well known that asymmetry of pulse period
generates even signal harmonics while a perfectly symmetrical pulse
train has only odd harmonics. The asymmetry of modulated pulses is
opposite for positive and negative half periods of the low
frequency signal. Therefore, the average asymmetry over a full
period of the low frequency signal is zero and the second harmonic
of the high frequency signal is not present. However, modulation of
pulse width asymmetry with frequency f.sub.LF causes an appearance
of sidebands around a second harmonic at 2f.sub.HF.+-.f.sub.LF.
Weaker sidebands are also present at the fourth harmonic
4f.sub.HF.+-.f.sub.LF; second-order sidebands are present at
f.sub.HF.+-.2F.sub.LF and 3f.sub.HF.+-.2f.sub.LF. All sideband
pairs can be used for phase and group delay measurements. The
sidebands at the second harmonic of the high frequency signal are
preferable due to stronger spectral magnitude and better
signal-to-noise ratio.
[0030] DUT 4 may be an ADC, for which group delay is to be
measured. Alternatively, DUT 4 may be a digital frequency converter
with an ADC as the converter component. The current technology
makes it possible to measure group delay of analog devices as well.
In such cases, an ADC is incorporated in the interface unit 5. In
any event, the signal at the input of the processing unit 6 always
has a digital form.
[0031] In a form, when the technology is used for group delay
measurement of an analog up-converter, measures should be used to
ensure presence of the component with the frequency f.sub.LF in the
spectrum of the signal at output of the DUT 4. An exemplary block
diagram for this form is shown in FIG. 6. In this block diagram,
DUT 4 has an additional input 10 connected to the output of the low
frequency oscillator 1. Inside the DUT 4, an adder 9 is coupled
between an analog up-converter 8 and the output of the DUT 4. The
additional input 10 of DUT 4 is connected to a first input of the
adder 9. A second input of adder 9 is connected to an output of
analog up-converter 8. The sine wave with frequency f.sub.LF
produced by the oscillator 1, is applied by way of additional input
10 of DUT 4 to the first input of adder 9. In adder 9, the sine
wave of the frequency f.sub.LF is mixed with the output signal of
analog up-converter 8, and as a part of the mixed signal passes to
the output of DUT 4. The availability of the sine wave of the
frequency f.sub.LF at the input of the processing unit 5 makes
possible the measurement of the group delay of the analog
up-converter 8 in accordance with the technology.
[0032] The signal coming applied to the input of the processing
unit 6 may be Fourier transformed, resulting in a complex Fourier
spectrum. This operation can be performed using an FPGA, a computer
or a dedicated digital processor. Thus, phases of all spectrum
components can be obtained from a single Fourier transform. By
sweeping the high frequency signal f.sub.HF in a band of interest,
phase measurements can be obtained for a range of frequencies.
[0033] In explanation, DUT 4 has a phase frequency response
.psi..sub.DUT(f), so that a sine wave with the frequency f passing
through DUT 4, experiences a phase shift .psi..sub.DUT(f). At the
input of DUT 4, the right sideband for the harmonic number k of the
high frequency f.sub.HF, has a frequency=kf.sub.HF+nf.sub.LF and a
phase .phi..sub.right=k.phi..sub.HF+n.phi..sub.LF, where
.phi..sub.HF and .phi..sub.LF are the phases of the sine waves with
the frequencies f.sub.HF and f.sub.LF, respectively. After passing
through DUT 4, the phase becomes
.phi..sub.right=k.phi..sub.HF+n.phi..sub.LF+.psi..sub.DUT(kf.sub.HF+nf.su-
b.LF). The left sideband for the harmonic number k of the high
frequency f.sub.HF at the output of DUT 4 has a frequency
kf.sub.HF-nf.sub.LF and a phase
.phi..sub.left=k.phi..sub.HF-n.phi..sub.LF+.psi..sub.DUT(kf.sub.HF--
nf.sub.LF). The phases .phi..sub.HF and .phi..sub.LF of high and
low frequency sine wave oscillators are unknown and different
during each signal acquisition. However, the high frequency phase
is identical for the right and left sidebands, and therefore the
phase difference equals
.DELTA..phi.=.phi..sub.right-.phi..sub.left=.psi..sub.DUT(kf.sub.HF+nf.su-
b.LF)-.psi..sub.DUT(kf.sub.HF-nf.sub.LF)+2n.phi..sub.LF..
[0034] The low frequency phase .phi..sub.LF creates a shift of
measured value .DELTA..phi., wherein this shift is different for
each signal acquisition. However, since the low frequency component
is always present in the signal spectrum, the value of .phi..sub.LF
is measured from the signal spectrum and compensated. After this
operation, the group delay value .tau. is calculated as
.tau.=.DELTA..phi./(f.sub.right-f.sub.left)/(2.pi.)=.DELTA..phi./(2nf.sub-
.LF)/(2.pi.). Thus, group delay values are obtained for arbitrary
frequency with arbitrary frequency steps, depending on a particular
choice of f.sub.HF and f.sub.LF. By choosing small value of the low
frequency (e.g., 2-5 MHz), any monotonic and slow changing group
delay introduced by the limiter circuit is minimized, while group
delay of DUT 4 is obtained with high frequency resolution and
accuracy.
[0035] The method of current technology can be readily simulated
using an idealized amplitude limiter and a 40 Gs/s ADC model. In
the simulation, the ADC is modeled using real phase and amplitude
frequency responses. A test signal is obtained by mixing a variable
high frequency signal in the range of 100 MHz-13 GHz with a 50 MHz
step, using a 5 MHz low frequency signal. Both high and low
frequency signals are assigned random phase values for each
frequency in the measurement range. The sum of the sine waves is
amplitude limited and each spectral component is distorted by the
frequency response functions of the ADC. When the received signal
is mixed with additive white Gaussian noise at 40 dB SNR, the
spectrum of the signal is determined using a Fast Fourier transform
and group delay is calculated as
.tau.(f)=(.DELTA..phi.-2n.phi..sub.LF)/(2nf.sub.LF)/(2.pi.). The
result of this simulation using multiple independent measurements
coincides with a model group delay within 5 ps accuracy. Different
distortions of the amplitude limiter circuit are also modeled, such
as asymmetry of positive and negative threshold levels, monotonic
group delay and frequency roll-off. None of them degraded measured
group delay.
[0036] FIGS. 7a and 7b show results of an experimental measurement
of a group delay of 40 Gs/s for an interleaved ADC using several
independent acquisition sets with different values of low frequency
(2.5, 5, 10 and 20 MHz). Each measurement set consisted of a single
low frequency value and 640 high frequency values in the range of
20 to 14000 MHz with 21.925 MHz steps. Each digitized waveform size
is set at 8 million samples in order to provide sufficient data
averaging and high resolution spectral measurement. As seen from
FIG. 7a, all data sets result in practically identical measurements
of group delay. FIG. 7b shows a zoomed region of FIG. 7a in the
range 10700 to 13300 MHz, demonstrating that different low
frequency signals (2.5 to 20 MHz) result in nearly identical group
delay values, with a maximum deviation less than 10 ps, this
accuracy in highly non-uniform high frequency region, is sufficient
for all practical purposes.
[0037] Although the foregoing description of the embodiment of the
present technology contains some details for purposes of clarity of
understanding, the technology is not limited to the detail
provided. There are many alternative ways of implementing the
technology. The disclosed embodiments are illustrative and not
restrictive.
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