Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Reexamination Certificate
1999-06-17
2002-06-25
Le, N. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
Reexamination Certificate
active
06411076
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to Electronic Surveillance Measurement (ESM) receivers such as Instantaneous Frequency Measurement (IFM) receivers, and more particularly relates to the selection of delays in IFM receivers to minimize the quantity of delay lines required to achieve a given accuracy.
DESCRIPTION OF THE PRIOR ART
Electronic Surveillance Measurement (ESM) receivers commonly require that frequency calculations be performed on an input signal from targets of interest. The frequency of the input signal is often measured using an Instantaneous Frequency Measurement (IFM) receiver as illustrated in FIG.
1
. The IFM receiver uses a difference in phase between a delayed and a non-delayed version of the input signal to calculate the frequency of the input signal.
The IFM receiver is the simplest, most mature technique for obtaining accurate pule-by-pulse frequency information over a broad frequency band. Accurate frequency information is required for a variety of purposes in ESM receivers. For instance, it is useful as a sorting parameter for de-interleaving multiple emitters in a dense environment, being second only to angle-of-arrival in the hierarchy of sorting parameters. An additional role is in active power-managed ECM systems to define the band of jamming response, thereby optimizing utilization of jamming power and maximizing jamming effectiveness. Further, IFM receivers are useful in the detection and display of frequency-agile and pulse compression radar emitters.
The IFM receiver illustrated in
FIG. 1
includes a receptor element or antenna
10
, a power divider
22
, a reference delay line
12
, one or more differential delay lines
14
, an N-channel phase measurement receiver
16
, and a phase-to-frequency decoder
18
. An input signal
20
is received and split into two or more constituent signals by the power divider
22
. One constituent signal is applied to the reference delay line
12
, and the remaining constituent signals are applied to the differential delay lines
14
. The delayed signals are then applied to separate channels of the N-channel phase measurement receiver
16
. The difference in delay between the reference delay line
12
and the differential delay lines
14
causes a frequency dependent phase shift, which is measured by the N-channel phase measurement receiver
16
. The frequency of the input signal
20
is determined from this phase shift by the phase-to-frequency decoder
18
.
Phase measurement receivers are alternatively referred to as phase discriminators, phase correlators or quadrature mixers. Further detail regarding phase measurement receivers can be found in a product specification catalog entitled,
Anaren RF
&
Microwave Components,
February 1997, distributed by Anaren Microwave, Inc., 6635 Kirkville Road, East Syracuse, New York 13057, the pertinent portions of which are incorporated herein by reference.
The phase shift or difference &psgr; radians (relative to the path through the reference delay line
12
), which is created by a delay difference &tgr; seconds (relative to the same path through the reference delay line
12
) at a frequency f hertz, is given by equation (1) as follows:
&psgr;=2
&pgr;&tgr;f,
(1)
and thus
f=&psgr;/
2&pgr;&tgr;, (2)
where &tgr;=&Dgr;
1
/c is the delay difference in seconds &Dgr;
1
is a length differential between the delay lines, and c is a velocity of propagation in the delay lines.
A measured phase difference &phgr;
meas
will differ from the phase difference &psgr; due to measurement error and the inability to measure phase values outside a range of 2&pgr;. For example, assume that the phase is measured with a phase receiver having a random phase measurement error &egr; degrees at a particular signal-to-noise ratio, and that bias error is removed by calibration. The measured phase difference &phgr;
meas
that results is given by equation (3) as follows:
&phgr;
meas
=MOD
2&pgr;
(2
&pgr;&tgr;f+E
), (3)
where E is a random variable with a standard deviation equal to the phase measurement error &egr; and a mean of zero.
Using a large value for the delay difference &tgr; produces a large phase difference slope with frequency, which minimizes the phase measurement error &egr; caused by E. This translates to a more accurate determination of frequency. Specifically, with a phase receiver having the phase measurement error &egr; degrees, a frequency error &Dgr;f hertz, standard deviation, is given by equation (4) as follows:
&Dgr;f
=(&egr;/360)(1/&tgr;). (4)
For example, with the delay difference &tgr; equal to 5 nanoseconds, and the phase measurement error &egr; equal to 7.2 degrees, the IFM receiver is capable of a frequency measurement with the frequency error &Dgr;f equal to 4 MHZ.
However, &phgr;
meas
, and thus a measured frequency f
meas
, is ambiguous for values of &tgr;f which are greater than unity because phase can only be determined nonambiguously within a range of 2&pgr;. In fact, multiple ambiguities will be spaced at intervals of frequency equal to 1/&tgr;. For the example above, &tgr;f is greater than unity for f>0.2 Ghz, and ambiguities occur every 0.2 Ghz. In other words, the output of the phase receiver is identical for input frequencies of 0.05 Ghz, 0.25 Ghz, 0.45 Ghz, and so forth.
To measure frequency nonambiguously with one differential delay, a nonambiguous delay &tgr;
nonamb
must be chosen such that it is equal to or less than 1/f
max
, where f
max
is a maximum frequency to which the phase receiver will be subjected. To reduce the number of ambiguities in a practical multi-channel IFM receiver, as shown in
FIG. 2
, the input signal
20
is translated to an IF frequency band by a frequency converter
24
to limit f
max
. Such a configuration operates on input signals
20
in one frequency band at a time, and switches local oscillators within the frequency converter
24
to change bands. A typical IF band is about 2-4 Ghz, and thus f
max
is typically about 4 Ghz.
To resolve ambiguities, IFM receivers commonly perform a second nonambiguous measurement using delay lines configured in binary ratios such as 1, 2, 4 and 8 as shown in FIG.
3
. The nonambiguous delay &tgr;
nonamb
is the shortest delay used in such a Binary IFM receiver (BIFM), and it produces unambiguous estimates for the most significant frequency bit. The next differential delay is twice the nonambiguous delay &tgr;
nonamb
. This next differential delay produces a frequency estimate with twice the accuracy, which is used for the next most significant frequency bit, and it can have one ambiguity which can be resolved with the prior nonambiguous delay &tgr;
nonamb
. This process can be continued in binary fashion by adding additional differential delays, each twice as long as the preceding one, so that the length of the ith delay is given by equation (5) as follows:
&tgr;
BIFM
i
=2
(i−1)
/(
f
max
), (5)
and a frequency estimation error &Dgr;f
BIFM
of the ith delay for the BIFM is given by equation (6) as follows:
&Dgr;f
BIFM
i
=f
max
(&egr;/360)(½
(i−1)
), (6)
where
i=
1+ceil {log(&egr;
f
max
/360
&Dgr;f
BIFM
)}/log(2), (7)
and ceil refers to a “ceiling” function which outputs the next integer greater than its operand. For instance, ceil (5.2)=6, ceil (11.3)=12, ceil (10.5)=11, and so forth.
Thus, for the frequency error &Dgr;f of no more than 5 MHZ with a phase receiver having a phase measurement error &egr; equal to 7.2 degrees and a maximum frequency equal to 4 Ghz, i must be at least 5. That is, the BIFM must have at least a six channel phase receiver with one reference delay line and five differential delay lines.
BIFM receivers have also been implemented with delay lines differing in length by a factor of 4 rather than 2 as described above. Such receivers require fewer delay lines. For instance, using the example above, only 3 differential delay lines are require
AIL Systems Inc.
Hoffman & Baron LLP
Le N.
LeRoux Etienne P
LandOfFree
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