GB2203304A - Local-oscillator system - Google Patents

Abstract

A local-oscillator system suitable for use with a frequency-agile pulse radar including spin-tuned magnetron (1). The local-oscillator system includes a fast-tuned local oscillator (in 9), the frequency (Fo) of which is adjusted by a summing amplifier in dependence on a coarse tuning signal and on a fine tuning signal. The fine tuning signal is derived by comparing the frequency of the oscillator with that of radiation in a current radar pulse, and the coarse tuning signal depends on the output of a frequency read-out system (6), associated with the magnetron of the radar, and on an error signal which is derived by a microprocessor from a plurality of fine tuning signals corresponding to earlier radar pulses. <IMAGE>

Description

LOCAL OSCIllATOR SYSTEMS This invention relates to fast tuned local oscillator systems for use in frequency-agile pulsed radar systems.

Frequency-agile pulsed radar systems require a fast-tuned local oscillator (FTLO) to follow the large frequency variations of the transmitter from pulse to pulse to enable the receiver system to detect and process signal returns. A simple local oscillator system which ideally could achieve this objective consists of a very wide-band frequency control system which, during each transmitteed pulse, detects the error in the FTLO frequency and produces a correction signal before the end of the pulse. The frequency of the FTLO would then be held constant until the next transmitted pulse. Such a system would require a very wide-band discriminator and an FTLO that could be tuned very rapidly over its entire range. Systems of this type do not provide a sufficiently accurate FTLO frequency.

The usual solution to this problem is the use of a slow AFC system to bring the FTLO to almost the correct frequency just before the pulse is transmitted, minor final frequency corrections then being made during the transmitted pulse by a fast AFC system. This method makes less stringent demands on the discriminator/mixer arrangement and on the frequency stability and accuracy of the FTLO. Examples of systems of this type are given in British Patents 1480496, 1466136, 1251270, 1076097 (equivalent to U.S. Patents 3979678, 4001825, 3611380, 3290678 respectively). In some circumstances, however, these systems fail to tune the FTLO sufficiently rapidly and accurately, especially when the transmitted pulse has a relatively short duration which is insufficient to allow the fast AFC system to provide accurate tuning of the FTLO.

One object of the present invention is to alleviate the aforementioned disadvantage.

Accordingly, a fast tuned local oscillator system suitable for use with a frequency agile pulsed radar system comprises: a variable frequency local oscillator, a first frequency control system arranged to adjust said local oscillator to a predetermined frequency related to the frequency of the next pulse to be transmitted by the associated frequency agile pulsed radar transmitter, a second frequency control system arranged to detect and correct any error in the frequency of said local oscillator set by said first frequency control system, processing means arranged to analyse a plurality of said errors and to modify said first frequency control system, thereby reducing any consistent errors in the frequencies of said local oscillator set by said first frequency control system.

In a preferred embodiment, the first (or slow) AFC system includes a pair of data stores, which each store samples of the frequency waveform of the pulsed transmitter. One of the data stores is used to adjust the local oscillator to the frequency of each successive pulse before transmission. The local oscillator frequency is further adjusted to correct for any error in its frequency detected by the second (or fast) AFC system during the period of transmission of the pulse. The errors detected in this way are analysed to check for consistent errors which are then used to update the second data store. The updated second data store is then used to adjust the local oscillator frequency while the first data store is updated in a similar way. The roles of the two data stores are reversed when necessary to ensure that the aost up-to-date information is used.

This invention enables consistent errors, such as drift in the transmitted frequencies (due to external influences), irregularities in the frequency waveform and non-linearities in the FTLO frequency response, to be reproduced in the coarse tuning signal provided by the slow AFC system, thereby enabling the FTh0 to be fine-tuned more rapidly and more accurately by the fast AFC system.

One embodiment of the invention will now be described by way of example only with reference to the accompanying drawings of which: Figure 1 shows a fast-tuned local oscillator system according to the invention Figure 2 shows a slow AFC system suitable for use in the invention Figure 3 shows a fast AFC system suitable for use in the invention and Figure 4 shows timing and phase relationships relevant to the invention.

Referring to the embodiment shown in Figure 1 by way of example, a spin-tuned magnetron 1 is operated by an EHT supply which is interrupted by a pulse modulator 2, so that pulsed EHT is supplied to the cathode of the oagnetron 1. A spinner within the magnetron is driven by the servo-sotor 3 to provide cyclical tuning of the magnetron, so that BF pulses can be provided at any frequency within the bandwidth of the tuning range of the spinner. The speed of the servo-sotor 3 can be adjusted by the motor speed controller 4 to avoid reduction in the number of frequencies transmitted as a result of synchronism between the cyclical tuning of the magnetron and the pulse repetition frequency of the IF pulses.The instantaneous position of the spinner may be identified by means of markings around the spinner. This information is transmitted by a fibre optic link 5a to the read out system 6.

A suitable arrangement is described in our co-pending European Patent Application No. 86300940.3 (Publication No. 195509).

The operating temperature of the magnetron may also be measured and transmitted via the link 5b to the readout system 6.

Information from the pulse modulator 2 and the readout system 6 is supplied to the slow AFC system 7, which provides a coarse tuning signal at an input 8 of the fast AFC system 9, as described in more detail hereinafter with reference to Figure 2. The fast AFC system 9 tunes the FTLO by means of the coarse tuning signal and an error signal, the error signal being derived by comparison of the FTLO output frequency with the frequency of the current r.f. pulse provided by the magnetron 1, as described in more detail hereinafter with reference to Figure 3. The error signal is also fed back to the slow AFC circuit 7, so that any appropriate corrections can be made to the coarse tuning signal. The fast AFC provides signals to the radar receiver at the frequencies L01 and L02 which act as the first and second local oscillators respectively.

In the slow AFC system shown in Figure 2, by way of example, the input from the frequency readout system 6 (Figure 1) is fed to a su=ing amplifier 22 via a phase correction circuit 21, which corrects the current frequency reading to the frequency expected at the centre of the net transmitted pulse. An error signal is added to the phrase-corrected frequency readout signal at the summing amplifier 22 and the output signal is fed to the sample and hold circuit 23, which is activated for a short period of time by the slow AFC window generated by the monostable multivibrator 24 under the control of a trigger pulse provided by the pulse modulator 2 (Figure 1) The output from the sample and hold circuit 23 is amplified in the buffer amplifier 25 to provide the coarse tuning input to the fast AFC system. The error and window signals from the fast AFC system are fed to the demodulation circuit 26, which holds the error level of the preceding error pulse until the next error pulse is received. The output from the demodulation circuit 26 is fed to the microprocessor 27, which also receives the frequency readout signal. The microprocessor 27 includes 2 data stores which contain the corrections required to each frequency readout signal over the full spinner cycle. The microprocessor updates one of the two stores, whilst the other store is used to provide the appropriate error signal to the summing amplifier 22, thereby to increase the accuracy of the coarse tuning signal input to the fast AFC system fro the FTLO. The updated store then provides the error signals while the other store is being updated.

In the fast AFC system shown in Figure 3, by way of example, the coarse tuning input from the slow AFC system is fed to the sunning amplifier 31, which adds on the fine tuning signal, the corrected signal is then fed to a sample and hold circuit 32 which tunes the FTL0 33. The output of the FTLO is split by the power divider 34, one part being mixed at the mixer 35 with a sample of the r.f. transmitted pulse after amplification to a constant amplitude at the level control 36.

The mixer output is passed to the discriminator 37, which provides the fine tuning input to the summing amplifier 31.

The sample and hold circuit 32 is controlled by the fast AFC window generator 38, which generates a window covering the period when the transmitted pulse has a relatively constant amplitude. The window is derived from the front edge of the r.f. pulse provided by the level control 36 and the pulse width data provided by the pulse modulator 2. Outputs from the discriminator 37 and the fast AFC window generator 38 provide respectively the fast AFC error and fast AFC window inputs to the demodulator circuit 26 of the slow AFC system (Figure 2).

The power divider 34 also provides an input to the mixer 39 at the FTLO frequency, which is equal to the transmitted frequency of the preceding pulse. The oscillator 40 provides the other input to the mixer 39, comprising a signal at the frequency F1 equal to the first intermediate fequency of the radar receiver. The output of the mixer 39 after filtering at 41 and aiplification by the buffer amplifier 42 provides an input to the radar receiver which acts as the first local oscillator. A further oscillator 43 provides a signal at the frequency F1 - F2, where F2 is equal to the second IF frequency. After amplification by the buffer amplifier 44, this signal provides an input to the radar receiver which acts as the second local oscillator. The two oscillators 40 and 43 are synchronised by the synch circuit 45.

As shown in Figure 4, there is a delay between the trigger pulse and the transmitter pulse generated by the pulse modulator 2 (Figure 1). The slow AFC window provided by the monostable multivibrator 24 (Figure 2) is wider than the transmitted pulse to ensure that the coarse tuning signal is held constant over the whole of the duration of the transmitter pulse. The fast AFC window generated at 38 (Figure 3) is limited in duration to the period during which the amplitude of the transmitted pulse is relatively constant, so that the fine tuning signal is accurately derived. At the end of the fast AFC window the FTL0 frequency is held constant during the listening time of the radar receiver.

The FTLO frequency is held constant before the front edge of the transmitted pulse, but the FTLO frequency should equal the frequency of the centre of the transmitted pulse. Furthermore, there may be a frequency difference between the digital frequency readout from the magnetron and the actual magnetron frequency. Thus it is necessary to apply a phase correction in the frequency cycle to ensure that the FTh0 frequency is correct at the time of the transmitted pulse. This phase correction is made by the phase correction circuit 21 and is illustrated in the upper part of Figure 4.

It will be appreciated by those skilled in the art that modifications could be made to the above-described procedure while still using the fast AFC system to increase the accuracy of the coarse tuning of the FTLO by the slow AFC systems in accordance with the invention.

The increase in accuracy of the coarse tuning of the FTLO by the slow AFC system, in accordance with this invention, enables the operating range of the fast AFC correction to be reduced and therefore a narrow fast AFC window and narrower transmitted pulses to be used.

Claims (4)

1. A local oscillator system suitable for use with a frequency-agile pulse radar, the local oscillator system comprising, a variable frequency oscillator circuit, means for adjusting said local oscillator circuit in dependence on a control signal and on a comparison signal, said comparison signal being derived by comparing the frequency of said local oscillator circuit with that of radiation in a current radar pulse, wherein said control signal depends on an operational parameter of the associated radar, the operational parameter being indicative of the expected frequency of radiation in the current radar pulse, and depends also on an error signal derived from comparison signals corresponding to a succession of earlier radar pulses.

2. A local oscillator system according to Claim 1 including first and second, operatively interchangeable data stores for storing said comparison signals, and processing means for up-dating one of said data stores and for deriving said error signal from comparison signals held in the other of said data stores.

3. A local oscillator system according to Claim 1 or Claim 2 including means to confine a said adjustment to an interval of time when the amplitude of a current radar pulse is substantially constant.

4. A local oscillator system substantially as hereinbefore described by reference to and as illustrated in the accompanying drawings.