Digital channelized IFM receiver

An instantaneous frequency measurement receiver (IFM) is used at each output of a digital channelized receiver. A Fast Frequency Transform (FFT) is used to form an instantaneous frequency measurement (IFM) receiver at every frequency component. These IFM receivers can perform frequency coding and produce fine frequency measurement.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the field of instantaneous 
frequency measurement (IFM) receivers with digital processing, and more 
particularly to a digital channelized IFM receiver. 
A related paper by T. W. Fields, D. L. Sharpin and J. B. Tsui titled 
Digital Channelized IFM Receiver, was presented at the IEEE MTT-S 
International Microwave Symposium, May 24-26, 1994 at San Diego Calif., 
and published in the Digest of the Symposium. A copy of the paper is 
included with this application as filed, and is hereby incorporated by 
reference. 
The conventional IFM receiver is a radio frequency receiver used primarily 
in electronic warfare. Its basic function is to measure the frequency of 
pulsed signals radiated from hostile radar. Generally, it may be said that 
IFM receivers measure the frequencies of incoming RF signals utilizing 
interferometric techniques by detecting the phase shift magnitudes 
produced in multiple, calibrated delay lines. For instance, the received 
RF signal is divided and simultaneously introduced into a non-delayed path 
and a delay line of known length. Since the delay and non-delayed receiver 
paths are functions of the input signal frequency, conversion of the phase 
difference signals to video signals provides signals whose amplitudes are 
related to phase delay. These signals typically take the form of sin 
.omega..tau. or cos .omega..tau., where .omega. is the angular frequency 
or the processed input signal, and .tau. is the known delay time. The sin 
.omega..tau. and cos .omega..tau. signals are delivered to an encoding 
network which makes amplitude comparisons of the signals, determines the 
numerical value of .omega., and generates the digital frequency 
descriptive word. 
An IFM receiver has many attractive features necessary for electronic 
warfare applications, such as small size, light weight, wide instantaneous 
bandwidth, and fine frequency resolution. In a digital receiver, the 
incident radiation is mixed with a local oscillator signal and down 
converted to an intermediate frequency (IF). This IF signal is discretely 
sampled and further processing is done using digital techniques. The 
frequency of the incident radiation may be determined by performing a 
discrete Fourier transform on the sampled signal. 
The following United States patents are of interest. U.S. Pat. Nos. 
4,612,545--Asendorf et al 
4,633,516--Tsui 
4,649,536--Kninock 
5,198,748--Tsui et al 
5,109,188--Sanderson et al 
5,214,708--McEachern 
5,235,287--Sanderson et al 
SUMMARY OF THE INVENTION 
An objective of the invention is to provide an improved frequency 
measurement receiver. Another objective is to provide finer frequency 
resolution than can be obtained from a channelized receiver. 
According to the invention, an instantaneous frequency measurement receiver 
(IFM) is used at each output of a digital channelized receiver. The IFM 
receiver can have two possible applications. First, it can be used to 
determine the output frequency from the channelized receiver. Second, it 
can generate finer frequency information than obtained from the 
channelized receiver.

DETAILED DESCRIPTION 
An objective of the invention is to build an instantaneous frequency 
measurement (IFM) receiver at each output of a digital channelized 
receiver. It can have two possible applications. First, it can be used to 
determine the output frequency from the channelized receiver. Second, it 
can generate finer frequency information than obtained from the 
channelized receiver. 
FIG. 1 is a block diagram showing a frequency measurement receiver system 
having an instantaneous frequency measurement receiver (IFM) at each 
output of a digital channelized receiver. Signals received at an antenna 
10 are supplied to block 12 which comprises RF circuits and a down 
converter. The IF signals from block 12 are supplied to an 
analog-to-digital converter 14, and the digital signals are channelized in 
a digital filter bank 16. At block 18, a digital IFM receiver is formed 
for the output of each digital channel. The digital output signals from 
block 18 are supplied to a signal processing block 20 to encode the 
various parameters including the frequencies of the input signals. 
Using fast Fourier transform (FFT) or its related techniques to build a 
digital channelized receiver is probably the most promising approach. The 
most straight forward way is to use a properly selected weighting function 
to condition the data. A short time FFT operation is used to perform the 
channelization as shown in FIG. 1a, which shows a simple way to build the 
digital filter bank 16 for the digital receiver of FIG. 1. 
The conventional thought is to compare the amplitude of the outputs from 
different channels to determine the center of the radio frequency (RF) of 
the input signal. This approach was experimented many times in analog 
receiver designs. The results were usually poor. This approach generates 
many spurious responses, if the dynamic range is high i.e. over 25 dB. The 
major difficulty is that the gains of all the channels can not be made 
equal in an analog receiver. The approach may generate better results in a 
digital receiver, because all of the channels in the receiver are better 
balanced through the FFT operation. 
To improve the frequency resolution from a channelized receiver, IFM 
receivers are added to the output of every channel in analog channelized 
receiver design. In the digital channelized receiver, an IFM receiver can 
be built at each output with no additional hardware. The only requirement 
in design is to process the output from the channels differently. 
Invention Description 
This invention can work with the arrangement shown in FIG. 1a and other 
possible digital channelized receiver designs with FFT to perform the 
channelization. The only requirement is that the channel output contains 
the RF information. This information can be either in real or complex 
form. In the FFT outputs, the channel outputs are complex which makes the 
IFM receiver very simple. A complex form has in phase (I) and quadrature 
(Q) components. 
Let us use an example to demonstrate this idea. The input contains 1024 
data points. In the data points 600 points (from 212 to 812) contain a 
sine wave. A hanning window is used to modify the input data and 128 point 
short FFT is used to perform the channelization. Because the input data 
are real, a 128 point FFT will produce 64 channels. This short FFT is 
overlapped 127 points. In other words, it is a one point sliding FFT. The 
output of each channel can be written as I.sub.i (t.sub.j) and Q.sub.i 
(t.sub.j) where i=1 to 64 representing the output channel number and j=1 
to 897 (1024-128+1) representing the output time. From these outputs one 
can find the phase of the output signal as 
##EQU1## 
The phase difference of each channel can be found as 
EQU .delta..THETA..sub.i (t.sub.j)=.THETA..sub.i (t.sub.j+1)-.THETA..sub.i 
(t.sub.j) (2) 
The frequency of the output signal can be found from 
##EQU2## 
This is the conventional approach to find the frequency through an IFM 
receiver. 
A. Application to frequency encoding 
In the above example, the input signal is shown in FIG. 2, which is a graph 
with time as the horizontal axis and amplitude as the vertical axis. The 
amplitude of the output channels can be found from 
##EQU3## 
which is shown in FIG. 3. In this figure only 10 channels are plotted. The 
center channel contains the signal. The transients at both the leading and 
trailing edges are usually referred to as the rabbit ears. The frequency 
domain plot at t=400 is shown in FIG. 4. In this figure there are 64 
output channels. The result is the same as a regular FFT output. The 
conventional approach is to compare the amplitudes between channels after 
the transient to determine the center frequency. The problem is that the 
amplitude outputs from channels far away from the signal frequency is very 
low. It is difficult to compare two small quantities without generating 
error information. In this approach the frequencies of all the channels 
are measured. The results are shown in FIG. 5 and it contains the same 10 
channels. At the leading and trailing edges of the pulse the frequency is 
close to the centers of the filters. After the leading edge transient the 
frequency is equal to the signal frequency, if there is only one signal. 
FIG. 5 shows this result, all ten outputs have the same value. If there 
are two signals in one channel and their amplitudes are close, more signal 
processing is required to resolve them which will not be discussed here. 
If the frequency measured by the IFM receiver matches the center of the 
channel, that channel contains a signal. If the frequency measured by the 
IFM receiver does not match the center of the channel, that channel does 
not contain a signal. Using this measurement, the channels containing 
signals can be identified. 
B. Application to fine frequency measurement 
If the channels containing signals can be determined, the IFM receiver can 
be used to find fine frequency resolution. The frequency resolution of a 
pulsed signal depends on the length of the pulse which is very desirable 
for EW application. Fine frequency resolution can be obtained from long 
delay line. This result can be obtained from the following relation. 
EQU .delta..THETA..sub.i (t.sub.mj)=.THETA..sub.i (t.sub.j+m)-.THETA..sub.i 
(t.sub.j) (5) 
The phase difference is obtained from the phase m samples away in time, 
rather that the adjacent ones in the time domain. The corresponding 
frequency of the output signal can be found from 
##EQU4## 
However, this equation does not show the improvement in frequency 
resolution explicitly. The improvement comes from the phase angle 
measurement. The phase angle is limited to 360 degrees. If the phase 
difference between m samples is measured, there is ambiguity but the 
frequency resolution is good and this is a common approach to improve the 
resolution from IFM receiver. The ambiguity problem can be solved by 
properly choosing the m value. 
It is understood that certain modifications to the invention as described 
may be made, as might occur to one with skill in the field of the 
invention, within the scope of the appended claims. Therefore, all 
embodiments contemplated hereunder which achieve the objects of the 
present invention have not been shown in complete detail. Other 
embodiments may be developed without departing from the scope of the 
appended claims.