GB2261345A

GB2261345A - Transceiver having a feedback loop

Info

Publication number: GB2261345A
Application number: GB9123809A
Authority: GB
Inventors: Christopher Nigel Smith; Julian Robert Hildersley
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1991-11-08
Filing date: 1991-11-08
Publication date: 1993-05-12
Also published as: DE4237614A1; GB9123809D0

Abstract

The receiver forms part of a feedback loop, and generates a feedback signal which is applied to the transmitter. To generate a GMSK signal at 1.8 GHz, as required for PCN, a low frequency vector modulator output must be frequency translated. A preferable technique for translation is to use a phase locked loop incorporating a downconversion mixer within the loop to define an upconversion transmitter. In the present invention, the downconversion mixer in the receiver 'front-end' is also used as the transmitter phase looked loop mixer. The additional cost of, power consumption of, and space occupied by a separate mixer is thus avoided. <IMAGE>

Description

1 A TRANSCEIVER The present invention relates to a transceiver for use in

communication apparatus.

The transceiver may be used in the Personal Communication Network (PCN) environment, and may also find utility in linear modulation schemes where both amplitude and phase (or I and Q) feedback is required.

In a PCN transmitter there is a requirement for a Gaussian Minimal Shift Keying (GMSK) signal of high phase accuracy to be generated at the output of the transmitter. This may be achieved by using direct modulation of a frequency synthesi zer, direct signal synthesis, or vector (I/Q) modulation. Each of these techniques have their own advantages and disadvantages, but with modern integrated circuit technology, the vector modulation technique offers the best compromise between performance, complexity and power consumption, particularly if it is implemented at a fixed relatively low frequency.

To generate a GMSK signal at 1.8 GHz, the low frequency vector modulator output must be frequency translated. Although mixing and filtering could be used to achieve this translation, a preferable technique is to use a phase locked loop (PLL) incorporating a downconversion mixer within the loop, as shown in Figure 1. The inphase and quadrature phase signals are received by a vector modulator 2, which also receives a signal generated from a synthesizer circuit 4 at an intermediate frequency fi. The intermediate frequency output signal generated from the vector 2 modulator 2 is applied to a phase sensitive detector 6, and the output of the detector 6 is fed through a loop filter 8 to a voltage controlled oscillator 10. The final output from the oscillator 10 is applied to a driver circuit 12, the output of which is passed through a power amplifier 14 which generates a final output signal fo which is applied to the antenna 16. The output of the power amplifier 14 is fed back to a downconversion mixer 18 which receives a synthesized signal at frequency fo-fi from the synthesizer 20. The signal fi represents the GMSK signal. The output of the mixer 18 is applied to an intermediate frequency amplifier 22, the output of which is fed back to an input of the phase sensitive detector 6. It will be seen from Figure 1 that the phase locked loop comprises the detector 6, loop filter 8, voltage control oscillator 10, driver circuit 12, power amplifier 14, the downconversion mixer 18 and the intermediate frequency amplifier 22.

The use of a phase locked loop has many benefits, it avoids the requirement for the extremely selective band pass filtering needed in the mixing concept for imaging carrier frequency rejection. Since the mixer in the phase locked loop is downconverting, it does not have to be balanced. The transmitter power amplifier is incorporated in the loop so that any phase distortion introduced by the amplifier (eg. due to power ramping) will be corrected by the negative feedback action. Any incidental amplitude modulation on the vector modulator output is removed by the loop. Any phase distortion produced by reflected power arising from gross antenna mismatch (eg. when the antenna is retracted) will be removed by the loop.

3 One of the disadvantages with any receiver or transceiver arrangement is that of size, cost and complexity. Therefore, it is always advantageous to attempt to reduce cost, size and complexity wherever possible.

An object of the present invention is to provide a transceiver in which the above mentioned advantages of the receiver are incorporated in the transceiver in a manner which reduces cost, size and complexity.

According to the present invention there is provided a transceiver comprising a receiver, and a transmitter controlled by a feedback loop, characterised in that the receiver is connected in the feedback loop, and generates a feedback signal which is applied to the transmitter.

According to a feature of the invention, the feedback signal is an intermediate frequency (IF) signal or a base band signal.

According to another feature of the invention, the feedback loop comprises a phase locked loop.

Embodiments of the present invention will now be described with reference of the accompanying drawings in which; Figure 2 shows a transceiver using a receiver as a feedback element, and; Figure 3 shows a phase locked loop upconversion transceiver.

Referring to Figure 2, there is shown a transmit signal generator/control loop 24 which receives a transmitter input signal, TI. The output of the circuit 24 is connected to a driver circuit 26 which in turn is connected to a power amplifier.28. The power amplifier is connected to a duplexer or transmit/receive switch 30.

4 The duplexer or switch 30 is connected to the antenna 32 for transmission and reception purposes. The duplexer or switch 30 is also connecied to the receiver 34 which is connected to a frequency synthesizer 36. The receiver generates an output signal OP, and also generates a feedback signal FS which is applied to the transmit signal generator/control loop 24. The feedback signal may be an intermediate frequency signal or a base band signal.

It will be seen from Figure 2 that the receiver 34 is connected directly in the feedback loop between the duplexer or switch 30 and the transmit signal generator/control loop 24. This has the advantage of reducing the number of components required in the 0 transceiver, because the transmit side of the transceiver which comprises the transmit signal generator/control loop 24, the driver 26 and the power amplifier 28 would require a further feedback path in order to control the operation and accuracy of the transmitter. This would inevitably mean that duplication of components would be necessary in the feedback path and therefore it can be readily appreciated that the cost of such a transceiver, size thereof and complexity is therefore reduced.

The above type of transceiver could be used in a phase feedback system such as would be required in a PCN application, it could also be used in linear modulation schemes where both amplitude and phase (or I and Q) feedback is required.

A particular embodiment of the present invention will now be described with reference to Figure 3 which shows a phase locked loop upconversion transceiver. In Figure 3 a vector modulator 38 is arranged to receive the inphase and quadrature phase input signals lin and Qin respectively. A synthesizer 40 is connected to the vector modulator and generates a signal at frequency fi which is applied to the vector 'modulator 38. The vector modulator 38 is connected to a phase sensitive detector 42. The detector 42 is connected to a loop filter 44 which has an output connected to a voltage controlled oscillator 46. The output of the oscillator 46 is connected to a driver circuit 48, the output of which is connected to a power amplifier 50.

The output of the power amplifier is connected to a first input of a duplexer 52, 54, a first output of which is connected to an antenna 56. The antenna 56 is also connected to a second input of a duplexer 52, 54, and a second output of the duplexer 52, 54 is connected to a low noise amplifier 58. The output of the amplifier 58 is connected to a downconversion mixer 60, the mixer 60 is also connected to a frequency synthesizer 62 which receives a signal at a frequency fo-fi therefrom. The output of the mixer 60 is connected to an intermediate frequency amplifier 66, the output of which is connected to an intermediate frequency filter 68. The output of the filter 68, signal IFO represents the output of the receiver part of the transceiver. The output of the mixer 60 is further connected to an input of an amplifier 64, the output of which is connected to a further input of the phase sensitive detector 42.

1 It will be appreciated that the receiver of the transceiver comprises the antenna 56, the part 54 of the duplexer 52, 54 the low noise amplifier 58, the downconversion mixer 60, the synthesizer 62, the intermediate frequency amplifier 66 and the intermediate frequency filter 68. It will further be appreciated that the transmitter. part of the transceiver comprises the synthesizer 40, the 6 vector modulator 38, the phase sensitive detector 42, the loop filter 44, the voltace controlled oscillator 46, the driver 48, the power amplifier 50, the part 52 of the duplexer 52, 54, the antenna 56 and the amplifier 64. It will also be appreciated that the mixer 60 forming part of the receive side of the transceiver is used as part of the feedback path for the transmit side of the transceiver, therefore obviating the need for two mixers. The feedback path also comprises the part 54 of the duplexer 52, 54 and low noise amplifier 58. In operation a certain amount of leakage occurs between the duplexer part 52 and the duplexer part 54 thus maintaining the feedback path during transceiver operation. An additional feedback path 70 is provided between the oscillator 46 and the mixer 60 so that when the power amplifier 50 is switched off, leakage is provided from the oscillator 60.

The operation of the phase locked loop upconversion transceiver shown in Figure 3 will now be described. The voltage controlled oscillator 46 runs at a final output frequency fo and feeds a signal to the antenna 56 via the driver 48, power amplifier 50 and the part 52 of the duplexer 52, 54. Leakage through the duplexer parts 52, 54 is amplified by the low noise amplifier 58 and then converted down to an intermediate frequency fi in the mixer 60. The signal at fequency fl is amplified by the amplifier 64 and is fed back to the phase sensitive detector 42. The vector modulator 38 and the synthesizer 40 together operate on the received inpliase and quadriture phase signals, I and Q respectively to produce a GMSK modulated output- signal which is applied to the phase sensitive detector 42. The amplified signal from the amplifier 64 is phase 7 compared with the GMSK modulated vector modulator signal generated from the modulator 38 by the phase sensitive detector 42. The detector 42 generates an output error signal which is amplified in the loop filter 44, and used to control the voltage controlled oscillator 46 thereby closing the loop. The oscillator 46 is thus phase locked to both the synthesized local oscillator at a frequency of fo-fi and the GMSK signal at a frequency fi. By a suitable choice of loop gain and bandwidth, the transmitter output phase can therefore be made to closely approach that of the vector modulator output. The output of the oscillator 46 is fed by a driver 48 to a power amplifier 50 and through the part 52 of the duplexer 52, 54 to the antenna 56.

It will readily be appreciated by those skilled in the art that the described embodiments may be used in any transceiver employing angle modulation. The PCN application may be operated in frequency division duplex or time division duplex modes. Also, the mixing to the intermediate frequency need not necessarily be a downconversion, but could be an upconversion to an intermediate frequency above the output frequency, eg. in a high frequency application.

Provided that the feedback signal from the transmitter does not over load the receiver 'front-end', and the transmitter receive intermediate frequencies are different, then the technique could be used for other frequency division duplex applications.

8

Claims

CLAIMS:

A transceiver comprising a receiver, and a transmitter controlled by a feedback loop, characterised in that the receiver is connected in the feedback loop, and generates a feedback signal which is applied to the transmitter.
2 A transceiver as claimed in claim 1, wherein the feedback signal is an intermediate frequency signal or a base band signal.
3 A transceiver as claimed in claim 1 or claim 2, wherein dulpexing means is provided in the transmitter and the receiver.
4 A transceiver as claimed in claim 3, wherein the feedback loop is a phase locked loop.
A transceiver as claimed in claim 4, wherein the receiver includes mixer means which is connected as part of the phase locked loop, and is arranged to generate an intermediate frequency which is applied to a phase sensitive detector.
6 A transceiver as claimed in claim 5, wherein the phase sensitive detector is further arranged to receive a modulated vector output signal, and is arranged to generate an error signal which is used to control a voltage controlled oscillator which is connected as part of the phase locked loop.

9 7 A transceiver as hereinbefore described with reference to Figure 2 of the accompanying drawings.

c 8 A transceiver as hereinbefore described with reference to Figure 3 of the accom anying drawings.

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