GB2356989A

GB2356989A - A low-noise multi-channel radar receiver

Info

Publication number: GB2356989A
Application number: GB9923228A
Authority: GB
Inventors: Jason Hall; Bryan Stephen Rickett
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1999-10-02
Filing date: 1999-10-02
Publication date: 2001-06-06
Anticipated expiration: 2019-10-02
Also published as: GB2356989B; GB9923228D0

Abstract

In conventional multi-channel radars, each signal is converted to an IF or baseband frequency using a single coherent oscillator. The phase noise associated with each channel of the radar after conversion is correlated, so that when the radar channels are combined the phase noise adds coherently. To overcome this problem, each channel in a multi-channel radar receiver is provided with a respective local oscillator 202,204,206 phase-locked to a master oscillator 200. Phase noise outside the loop bandwidth of each VCO is uncorrelated so that the effect of this source of noise does not add when the channels are combined. Use of a single reference oscillator maintains signal coherence. Instead of using PLLs, the respective local oscillator signals may be decorrelated by using delays (figure 1). The technique may be used to provide a low phase noise test signal source (figure 3).

Description

1 NOISE REDUCTION APPARATUS The present invention relates to noise

reduction apparatus for a multi- channel system, for example, of the type used with an antenna array of a radar.

In a conventional multi-channel radar, a single coherent reference oscillator is used to convert signals in each channel of the radar to an intermediate, or baseband frequency. The use of the single reference oscillator is considered necessary, because coherence in each channel after conversion must be maintained. Consequently, phase noise components of the signals in each channel after conversion are correlated on account of the same reference oscillator being used.

During processing of the converted signals, the converted signals in each channel are combined. Since each converted signal has a component corresponding to a desired signal in addition to the phase noise component, when the signals in each channel are combined, both the desired signal components and the phase noise components add coherently. Therefore, the power of the phase noise of the combined signal rises at the same rate as the power of the desired signal component.

It is therefore an object of the present invention to obviate, or at least mitigate, the rise of the phase noise in a multi-channel system.

According to the present invention, there is provided a noise reduction apparatus for a multi-channel system comprising a reference oscillator coupled to combination means, the combination means having a plurality of input channels for receiving a plurality of respective input signals and at least one output channel, wherein the reference oscillator is coupled to the combination means via respective decorrelation means, each decorrelation means being respectively coupled to the plurality of input channels of the combination means and the reference oscillator.

Preferably, the combination means is a signal mixer.

Preferably, the decorrelation means are phased locked to the reference oscillator. Very preferably, the decorrelation means is a phased locked oscillator. Most preferably, the decorrelation means comprises a voltage controlled oscillator.

Alternatively, the decorrelation means are delay means, such as a delay line, located in a distribution path from the reference oscillator to each of the combination means.

In a preferred embodiment of the invention, a radar having a plurality of antenna elements to form an antenna array is provided comprising the noise reduction apparatus as described above.

It is thus possible to decorrelate phase noise between channels associated with a single reference oscillator whilst maintaining the required coherence provided by the reference oscillator. Consequently, as the desired signal components can be coherently combined, whilst the phase noise components add incoherently to provide a phase noise power reduction of 1 /n (where n is the number of channels) over the above described known arrangement, i.e. where the phase noise between channels is correlated.

Therefore, for radars with large numbers of channels, the effects of oscillator phase noise are significantly reduced. As a result, small targets previously hidden by the radar phase noise can be detected, provided that signals corresponding to the target are above the reduced phase noise level.

At least one embodiment of the invention will now be described,, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of an apparatus constituting a first embodiment of the invention; -3 Figure 2 is a schematic diagram of an apparatus constituting a second embodiment of the invention, and Figure 3 is a schematic diagram of an apparatus constituting a third embodiment of the invention.

Throughout the following description like identical reference numerals will be used to identify like parts.

Referring to Figure 1, although a series of n mixers are shown in Figure 1, for the purposes of simplicity and clarity of description a noise reduction circuit 100 of the following example only supports three channels.

However, it should be appreciated that many more channels can be supported by the noise reduction circuit, i.e. n channels.

In the first embodiment of the invention, the noise reduction circuit comprises a common reference oscillator 102, a first mixer 104, a second mixer 106 and a third mixer 108 having a respective first, second and third input channel CHANNEL 1, CHANNEL 2, CHANNEL 3. A respective output channel of each mixer is coupled to a respective first, second and third variable phase shifter 110, 112, 114.

A fixed delay is located in a distribution path from the local oscillator 102 to each of the first, second and third mixers 104, 106, 108. Therefore, the reference oscillator 102 is coupled to the first mixer 104 and to an input terminal of a first fixed delay 116. An output terminal of the first fixed delay 116 is coupled to the second mixer 106 and an input terminal of a second fixed delay 118, an output terminal of the second fixed delay being coupled to the third mixer 108.

In operation, the signals in each of the first, second and third channels CHANNEL 1, CHANNEL 2, CHANNEL 3 are mixed with a delayed reference signal from the reference oscillator 102. Each delay is selected so that coherence is maintained at the frequency of the reference oscillator 102 between the channels whilst phase noise between channels is incoherent.

Each signal in the respective output channel is subjected to a respective variable phase shift by the respective first, second and third variable phase shifters 110, 112, 114.

The magnitude of each of the first and second fixed delays 116, 118 is such that phase noise sidebands of signals in each of the first, second and third channel CHANNEL 1, CHANNEL 2, CHANNEL 3 are decorrelated between the channelsY the decorrelation being from the minimum Doppler frequency of interest of the radar to the bandwidth of the radar. The first, second and third variable phase shifters 110, 112, 114 compensate for any errors within the constant delay elements which may exist as a result of difficulties encountered in setting the magnitude of the delays accurately.

In order to decorrelate the phase noise in certain systems, the necessary magnitude of the first and second fixed delays 116, 118 may be too large to be practical. In such cases, the noise reduction apparatus according to the second embodiment of the invention (Figure 2) can be employed.

Again, although a series of n mixers are shown in Figure 2, for the purposes of simplicity and clarity of description a noise reduction circuit 100 of the following example only supports three channels. However, it should be appreciated that many more channels can be supported by the noise reduction circuit, i.e. n channels. The noise reduction circuit 100 comprises the reference oscillator 102 coupled to a first Phase Locked Loop (PLL) 202, a second PLL 204 and a third PLL 206. The first, second and third PLLs 2025 2041, 206 are respectively coupled to the first mixer 104, the second mixer 106 and the third mixer 108 having the respective first, second and third input channels CHANNEL 1, CHANNEL 2, CHANNEL 3.

In operation, each Voltage Controlled Oscillator (VCO) (not shown) within each of the first, second and third PLL 202, 204, 206 is locked to a multiple of the frequency of the reference oscillator 102. Due to the nature of the first, second and third PLLs 202, 204, 206, phase noise of the first, second and third PLLs 202. 204, 206 is a combination of the phase noise of the reference oscillator and the phase noise of the respective VCO.

Within a loop bandwidth of a given PLL, the phase noise of the reference oscillator 102 is predominant over the phase noise of a VCO of the given PLL, whereas outside the loop bandwidth of the given PLL the phase noise of the VCO is predominant on account of the action of the control loop of each PLL. Therefore, since the VC0s of the first, second and third PLLs 202, 204, 206 are phase locked to the single reference oscillator 102, the phase noise outside the loop bandwidth of each of the first, second and third PLLs 202. 204, 206 are uncorrelated between each of the first, second and third PLLs 202, 204, 206. However, due to the phase locking, coherence is maintained between each of the first, second and third PLLs 202, 204, 206 within the loop bandwidth.

Since each of the first, second and third channels CHANNEL 1, CHANNEL 2, CHANNEL 3 have a respective first, second and third PLL 202) 204 206 associated therewith, the coherence of desired signals in the radar can be maintained by employing the single reference oscillator 102 for all of the first, second and third channels.

An appropriate loop bandwidth is selected according to various engineering factors known in the art in order to achieve substantially l/n reduction in phase noise power within the radar outside the loop bandwidth.

In a third embodiment of the invention, a multi-channels system, for example a piece of test equipment for measuring noise, comprises the reference oscillator 102 coupled to the first, second and third PLLs 202, 204, 206. Each of the output terminals of the first, second and third PLLs 202, 2045 206 are respectively coupled to a plurality of input channels of a power combiner 300 having an output channel 302. In this example, the above described elements replace a single reference oscillator coupled to a single PLL employed in existing known test equipment.

In operation, each Voltage Controlled Oscillator (VCO) (not shown) within each of the first, second and third PLL 202 204, 206 is locked to a multiple of the frequency of the reference oscillator 102. Due to the nature of the first, second and third PLLs 202, 204, 206, phase noise of the first, second and third PLLs 202, 204, 206 is a combination of the phase noise of the reference oscillator and the phase noise of the respective VCO. Since the VCOs of the first, second and third PLLs 202, 204,206 are phase locked to the single reference oscillator 102, the phase noise outside the loop bandwidth of each of the first, second and third PLLs 202, 204 206 are uncorrelated between each of the first, second and third PLLs 202, 204, 206.

The signals received at the plurality of input channels of the power combiner 300 are combined to form a new oscillating signal at the output channel 302 having a lower level of phase noise compared to an equivalent oscillating signal generated by existing multi-channel systems.

Although throughout the above description reference has been made to PLLs, it should be appreciated that use of this term is intended to include the use of Phase Locked Oscillators (PLOs).

Claims

CLAIMS:

1. A noise reduction apparatus for a multi-channel system comprising a reference oscillator coupled to combination means, the combination means having a plurality of input channel for receiving a plurality of respective input signals and at least one output channel, wherein the reference oscillator is coupled to the combination means via respective decorrelation means, each decorrelation means being respectively coupled to the plurality of input channels of the combination means and the reference oscillator.

2. An apparatus as claimed in Claim 1, wherein the combination means is a signal mixer.

3. An apparatus as claimed in Claim 1, wherein the decorrelation means are phased locked to the reference oscillator.

4. An apparatus as claimed in any one of Claims 1 to 3, wherein the decorrelation means is a phased locked oscillator.

5. An apparatus as claimed in any one of Claims 1 to 4, wherein the decorrelation means comprises a voltage controlled oscillator.

6. An apparatus as claimed in Claim 1 or Claim 2, wherein the decorrelation means are delay means located in a distribution path from the reference oscillator to each of the combination means.

7. An apparatus as claimed in Claim 6, wherein the delay means are delay lines.

8. A radar comprising the noise reduction apparatus as claimed in any one of the preceding claims.

9. A noise reduction apparatus substantially as hereinbefore described with reference to Figure 1 or 2.