GB2104746A - Radio frequency transmitter - Google Patents

Abstract

An R.F. transmitter has a digital power phase shifter (13) located between the output (11) of an R.F. power generator (10) and an antenna (12). The power generator produces power at a predetermined frequency and constant phase. The phase of the power radiated by the antenna can be digitally coded by a code generator (24) controlling the phase shifter (13). The phase modulator comprises three delay lines (14, 15, 16) in parallel with RF power switches (17, 18, 19) such as PIN diode switches. The arrangement obviates the need for amplifying the coded output. <IMAGE>

Description

SPECIFICATION Radio frequency transmitter The present invention is concerned with radio frequency transmitters, particularly such transmitters with phase code modulation.

Phase code modulation and, particularly, digital phase code modulation, is well known. In known systems, the output of a radio frequency oscillator is switched between two or more preset phases in accordance with a predetermined binary code. The phase code modulated signal is then amplified to produce radio frequency power for feeding to and driving a transmitting antenna.

Such techniques have been used in radar apparatus for pulse compression wherein each transmitted radar pulse is internally phase code modulated. The received echo pulses can then be compressed in time to improve the range definition of the radar. A pulse compression ratio corresponding to the number of phase code bits per radar pulse can be achieved.

According to the present invention, a radio frequency transmitter with phase code modulation comprises a transmitting antenna, R.F. power generator means for generating radio frequency power at a predetermined frequency and constant phase and suitable for driving said transmitting antenna, a digital power phase shifter connected between the output of said power generator means and said antenna and switchable to shift the phase of the R.F. power fed to the antenna between at least two predetermined phases, and phase coding means operative to switch said digital phase shifter in accordance with a predetermined code. With this arrangement, the radio frequency power signal is itself phase shifted prior to being fed to the transmitting antenna. No amplifier is thus required between the phase shifter and the antenna.This can represent a considerable saving in cost since such amplifiers must commonly be fairly wide band linear amplifiers so as to preserve the phase code modulation of the radio frequency signal. Furthermore, with the present arrangement, a high power R.F. oscillator can be employed directly with optimum efficiency to produce the R.F. power for transmission. R.F.

power oscillators, such as magnetrons and the like, are considerably cheaper than combinations of low power R.F. oscillators and high power linear amplifiers.

Said phase shifter may comprise at least one delay line connected in series with the feed to the antenna and an R.F. power switch in parallel with the delay line and controlled by the phase coding means. There may be a plurality of said delay line and switch combinations connected in series and the digital code from the phase coding means may then have a corresponding plurality of bits.

The or each said R.F. power switch may be a PIN diode switch.

In one embodiment, the transmitter is employed as an automatic beacon, for example, a distress beacon for boats or shipping. Then, the R.F. power generator means may comprise an R.F.

power amplifier and a surface acoustic wave (SAW) delay line connected to oscillate at the predetermined frequency. Such a power generator means may not have a frequency stability comparable to that of other power oscillators such as magnetrons. However, the amplifier and SAW generator arrangement can be much cheaper. Furthermore, where the R.F. power generator means has a predetermined frequency stability whereby the actually generated frequency falls within a predetermined range about the precise intended frequency, the phase shifter and phase coding means may be arranged to spread the spectrum of the R.F. power fed to the antenna at least as much as said predetermined range so as to embrace said intended frequency.In this way, even though the R.F. power generator frequency stability may be such that the transmitted centre frequency is outside the passband of a highly stable receiver, the phase code modulation of the transmitted frequency ensures that the transmitted spectrum extends into the receiver passband.

In another embodiment of the invention, the radio frequency transmitter of the invention is employed as the radar transmitter of pulse radar apparatus so that each radar pulse is phase code modulated. Then, the radar receiver may have pulse compression apparatus responsive to said phase code modulation in received radar echo pulses to compress said received pulses in time.

Examples of the present invention will be described with reference to the accompanying drawings in which~ Figure 1 is a schematic diagram illustrating a radio frequency transmitter embodying the present invention; Figure 2 illustrates an alternative embodiment of R.F. transmitter embodying the present invention; and Figure 3 is a schematic block diagram of a radar apparatus embodying the present invention.

Figure 1 shows an R.F. power generator 10 producing radio frequency power at an output 11.

The output power from the generator 10 has a predetermined frequency and constant phase. The output power is fed to a transmitting antenna 12 via a phase modulator 13. The phase modulator 13 comprises a series of three switchable delay lines 14, 15 and 16. Each delay line has an R.F.

power switch 17, 18, 19 connected in parallel with it which when closed effectively shortcircuits the respective delay line. Delay line 14 is arranged to provide a time delay corresponding, with regard to the frequency of the signal from the generator 10, to a phase shift n. Delay line 15 provides a time delay corresponding to a phase shift 7r/2 and delay line 16 provides a delay corresponding to a phase shift of n/4. Thus, depending on the states of the R.F. power switches 17, 18 and 19, the modulator 13 can be arranged to produce any phase shift in units 7r/4 between 0 and 7 7r/4.

The switches 17, 18 and 19 are controlled by switching pulses on lines 20, 21 and 22 respectively. In the illustrated example, the switches 17, 18 and 19 are PIN diode switches which can be switched between low impedance and high impedance states by application of an appropriate DC bias across the diode. The switch driving pulses on lines 20, 21 and 22 supply this bias voltage and the lines themselves are decoupled from the radio frequency power signal by means of inductors 23.

A code generator 24 is arranged to generate the switch driving pulses on the lines 20, 21 and 22 so as to apply the required digital phase code modulation to the radio frequency power signal transmitted from the antenna 12. Thus, the generator 24 supplies a three-bit binary code on the lines 20, 21 and 22 which applies corresponding phase shift to the radio frequency power signal. The phase of the power signal is modulated in accordance with variations in the three-bit word produced by the generator 24.

In this arrangement, the R.F. power generator 10 can be a relatively simple high power R.F.

oscillator such as a magnetron. There is no need for a linear amplifier between the output of the modulator 13 and the antenna 12 since the phase modulation is performed on the high power output of the generator 10.

In one particular application, a transmitter embodying the present invention is used in a distress beacon for use on a boat or ship and arranged to be activated automatically to transmit a distress signal if the boat or ship gets into difficulties, or sinks. The beacon is arranged to transmit a signal which can be received by satellites circling the earth for relay to a fixed ground station on the earth. The position of the distress beacon can then be identified.

For such an application it is desirable that the radio frequency transmitter be compact, simple, cheap, yet of sufficient power and frequency stability to meet the required performance.

Furthermore, for satellite communications, a relatively high frequency transmission is required.

A transmitter embodying the present invention can be made to meet the required parameters for such a beacon. Figure 2 illustrates an example.

The radio frequency power generator of the transmitter in Figure 2 comprises an R.F. power amplifier 30 and a surface acoustic wave (SAW) delay line 31 connected together to form a regenerative loop oscillator. The delay of the SAW delay line 31 is arranged so that the loop oscillates at the required radio frequency of transmission. A microwave coupler 32 couples R.F. power from the output of the amplifier 30 and delivers the R.F. power to an antenna 33 via a switchable delay line 34. A PIN diode device 35 is connected in parallel with the delay line 34 and arranged to be switched to form a short-circuit around the delay line in accordance with switching pulses on control lines 36 from a phase code generator 37. The control lines 36 are isolated from the R.F. power by means of inductors 38.

The delay line 34 is arranged to produce a predetermined phase shift in the radio frequency power supplied to the antenna 33. In a preferred example, the phase shift is arranged to be n radians. This phase shift is switched "in and out" in accordance with a one-bit binary signal on lines 36 as generated by the generator 37. The generator 37 may be arranged to produce a pseudo-random or fixed series length code, e.g.

from a digital shift register with a selected feedback connection.

It will be appreciated that for a distress beacon as described above, it is important that the emitted radio frequency falls within the passband of the intended receiver. The receiver would normally have a highly stable local oscillator. To ensure that the beacon transmitter has a correspondingly stable transmission frequency may be excessively expensive. However, with the arrangement of Figure 2, the phase code generator 37 may be arranged to apply phase code modulation to the transmitted signal so as to spread the spectrum of the transmitted signal to ensure that it embraces the passband of the intended receiver.Thus, if the radio frequency power generator of the beacon transmitter has a known frequency stability, i.e. the actual transmitted frequency will fall within a predetermined range about the intended frequency, then the phase code generator 37 can be arranged to spread the spectrum of the transmitted signal to produce a corresponding transmitted signal band width.

In Figure 3, a radar apparatus has a rotating aerial 40 supplied by an R.F. circulator 41 arranged to couple transmitted R.F. power on a line 42 to the antenna 40 and received radar signals from the antenna 40 to a line 43. A trigger generator 44 generates radar triggers in accordance with the radar p.r.f. Radar triggers are supplied to a pulse modulator 45 which generates pulses corresponding to the envelopes of the radio frequency pulses to be transmitted by the radar. The pulse modulator 45 controls a radio frequency generator 46 which generates the radar pulses. The generator 46 may, for example, be a Trapatt device.

The radar pulses are supplied to the circulator 41 on line 42 via a switchable delay line 47 in parallel with a R.F. power switch 48 controlled by a one-bit binary signal on a line 49 from a code generator 50. The delay line 47 has a path length providing a predetermined phase shift at the radio frequency of the transmitted radar pulses. By way of example, the phase shift may be or radians. The code generator 50 also receives trigger pulses from the trigger generator 44 in response to which the generator 50 produces a fixed length series pseudo-random binary code on the line 49.

The bit rate of the code on the line 49 is arranged such that at least one complete series of the code is generated during a single radar pulse.

A time base 51 also triggered from the trigger generator 44 drives a PPl display 52. Received radar echo signals from the circulator on line 43 are fed via a T/R protection cell 53 which protects the receiver from any transmitter power leaking through the circulator 41, to a mixer 54 where the received signals are mixed with the output of a local oscillator 55. The Intermediate Frequency (IF) signal from the mixer 54 is amplified in IF amplifier 56 and fed to a dispersion compressor 57. The operation of the compressor 57 will be described in more detail later.

The compressed pulses from the compressor 57 are fed to a radar video detector 58, typically a threshold detector, and thence to a video amplifier 59 and via a video processor 60 to the PPI display 52.

The majority of the above described components of the radar apparatus, except for the delay line and switch 47 and 48, the code generator 50, and the dispersion compressor 57, are commonplace radar components and will not be described here in any further detail. A unit 61 provides power supply and controls for the radar apparatus.

A problem with radar apparatus is the conflict between the requirement to have a relatively short radar pulse, to provide sufficient range definition, and the requirement to have a relatively high transmitted mean power. If the p.r.f., and the transmitted pulse length are constant, then an increase in the mean power can be achieved only by increasing the peak power of the transmitted pulses. This makes inefficient use of the transmission components.

A well established technique for improving radar mean power is to use transmitted pulses which are relatively long, but to modulate the transmitted pulses so that on reception received pulses can effectively be time-compressed.

Compression ratios of 1000:1 and higher can be achieved with known systems.

The arrangement illustrated in Figure 3 provides a convenient pulse compression system for a relatively simple radar apparatus. The binary phase coding applied by the generator 50, the delay line 47 and switch 48 can be used for compressing the received echo signals. Thus, the dispersion compressor 47 may form a filter matched to the known phase coding of the transmitted pulses. Instead, the compressor 57 may comprise a correlator arranged to correlate received pulses with a time delayed version of the transmitted pulse. The compressor 57 may operate in analogue fashion on the analogue received IF signals, or alternatively, the received IF can be phase decoded to reproduce the binary code applied during transmission and the received codes can then be compared digitally with the transmitted code.

Claims (9)

Claims

1. A radio frequency transmitter with phase code modulation, comprising a transmitting antenna, R.F. power generator means for generating radio frequency power at a predetermined frequency and constant phase and suitable for driving said transmitting antenna, a digital power phase shifter connected between the output of said power generator means and said antenna and switchable to shift the phase of the R.F. power fed to the antenna between at least two predetermined phases, and phase coding means operative to switch said digital phase shifter in accordance with a predetermined code.

2. A radio frequency transmitter as claimed in claim 1 wherein said phase shifter comprises at least one delay line connected in series with the feed to the antenna and an R.F. power switch in parallel with the delay line and controlled by the phase coding means.

3. A radio frequency transmitter as claimed in claim 2 wherein there are a plurality of said delay line and switch combinations connected in series and the digital code from the phase coding means has a corresponding plurality of bits.

4. A radio frequency transmitter as claimed in claim 2 or claim 3 wherein the or each said R.F.

power switch is a PIN diode swtich.

5. A radio frequency transmitter as claimed in any preceding claim wherein said R.F. power generator comprises an R.F. power amplifier and a surface acoustic wave (SAW) delay line connected to oscillate at the predetermined frequency.

6. A radio frequency transmitter as claimed in any preceding claim wherein said R.F. power generator means has a predetermined frequency stability whereby the actually generated frequency falls within a predetermined range about the precise intended frequency, and the phase shifter and phase coding means are arranged to spread the spectrum of the R.F. power fed to the antenna at least as much as said predetermined range so as to embrace said intended frequency.

7. Pulse radar apparatus having, as the radar transmitter, a radio frequency transmitter as claimed in any of claims 1 to 4, so that each radar pulse is phase code modulated, and, in the radar receiver, radar pulse compression apparatus responsive to said phase code modulation in received radar echo pulses to compress said received pulses in time.

8. A radio frequency transmitter substantially as hereinbefore described with reference to and as illustrated in Figures 1 or 2 of the accompanying drawings.

9. Pulse radar apparatus substantially as hereinbefore described with reference to and as illustrated in Figure 3 of the accompanying drawings.