GB2380641A

GB2380641A - common channel transceiver, which can reduce the noise from the transmitter antenna, appearing on the receiver antenna by using a digital filtering stage

Info

Publication number: GB2380641A
Application number: GB0208225A
Authority: GB
Inventors: Christopher Keith Richardson
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-09-14
Filing date: 2002-04-10
Publication date: 2003-04-09
Anticipated expiration: 2022-04-10
Also published as: GB2380641B; GB0208225D0; GB0122196D0

Abstract

A common channel transceiver including first oscillator means (3, figure 1) to modulate an output audio signal to an rf transmission line, antenna means (1, figure 1) to output said modulated signal, antenna means (5, figure 1) to accept received signals, first and second mixer means (10, figure 1) for mixing an rf transmission signal with said received signal, such that one of the signals output from the first mixer means is in-phase I and signal output from the second mixer means is quadrature Q, analogue to digital converter means 16 applied to each of the outputs of the mixers so that the signal are converted to a digital signals, means 19 to digitally apply a substantially 90 degree phase shift to the digitally converted in-phase signal relative to the quadrature signal by appropriate phase shifting of one or more of these signals and means to sum the resulting signals and DAC means to provide an output signal from the summed signal. A common antenna and other enhancements are described. Such a transceiver allows simultaneous transmission reception on a common channel.

Description

Transceiver This invention relates to transceivers which simultaneously receive and transmit the same signal and/or which receive and transmit signals on the same channel. It has particular application to N-Plex transceivers which permits the reception of many concurrent FM signals whilst simultaneously transmitting on the same frequency. A simple application of such a transceiver is also as a booster for transceivers which are out of range of each other. In such an example two persons each trying to transmit and receive signals to each other may have insufficient signal strength to overcome distance and natural obstacles such as e. g. a hill.

Such a transceiver located on a hilltop would provide received signals to be rebroadcast. Such transceivers also have application where there is a desire to have simultaneous two way (transmitting and receiving) communication from a transceiver to one or more similar transceivers without having to switch to and from transmit/receive modes. In other words it enables users to communicate as and hear and speak as if they were talking in a room. The use of such transceivers in N-Plex communication is known.

UK Patent 1577514, the inventor being the same as the current application describes such a transceiver. This original N-Plex concept is described again here briefly with reference to figure 1 in order to provide the necessary background. It shows a narrow band FM transmitter combined with a direct conversion single side-band receiver, where the transmitted FM signal also forms the receive local oscillator. The transmit antenna 1, which is the output of an amplifier 2 from a local oscillator 3 which provides modulation from a microphone 4. The receiver antenna 5

also receives signals which are then amplified by amplifier 6. This is then split by splitter 7 into the in-phase 8 and quadrature 9 components. Each of these components are mixed by mixers 10 via 90 degree shifter 11 so they are shifted 900 relative to each other from the output of the microphone and local. The outputs of the mixers are fed into capacitors 35; this provides AC coupling in the I and Q paths. The outputs from the capacitors are each then fed into a low pass filters 12, which defines the channel, and then an amplifier 13. The in-phase signal is then shifted through 90 degrees by phase shifter 14 before being summed at 15 to provide the audio output. When transmitting there will be a leakage signal between the transmit and receive antennas, as shown by the dotted line on the figure, which will be typically larger than any wanted received signal.

However since the instantaneous frequency of the leakage and the local oscillator signal are identical, the output of the mixers will be zero, i. e. at a DC level.

An FM signal received using an SSB (Single Sideband Suppressed Barrier) demodulator provided the FM deviation is small (peak phase deviation is less than 90 degrees). Under such conditions the FM signal consists substantially of a carrier component and the first order pair of side-bands. Thus an SSB detector may be used to demodulate the signal.

In the N-plex receiver the phasing method of SSB detection is used in which the I and Q baseband signals are first shifted 90 degrees relative to each other, as mentioned. In this way, if multiple (N) received signals are present on the same frequency a linear superposition occurs and the composite signal may be demodulated as above. In this case the recovered audio will preserve the relative loudness of the individual signals according to their relative RF levels at the antenna and hence their respective ranges.

In such a system however, errors in the phasing or amplitude imbalance between the I and Q channel can cause distortion of the recovered audio signal. A further difficulty is the requirement for a wideband 90 degree audio phase shift network which requires very close tolerance

components when implemented as in the patent mentioned above.

Example I Figure 2 shows the basic configuration of the invention. The receiver antenna is input to an amplifier and split into I and Q components as described above and it shows all the same components with identical reference numbers. After passing through a filters, the I and Q components are put through analogue to digital (A/D) converters. The I and Q components are then passed through further high and low pass digital filters 17,18. The I component is then phase shifted by 90 degrees at 19. It is then summed digitally with the Q component and the output converted to an analogue signal by D/A converter 20. The shifting is done digitally which allows for many advantages.

Example 2 The figure 3 embodiment shows a further preferred refinement. Again it includes all the integers of the previous examples as before. The microphone output is duplicated by also being input to an inverter 21 and converted to a digital signal by A/D converter 22; this digital signal is then used to frequency modulate a further oscillator 23, which is a numerically controlled oscillator (NCO) which provides a frequency controlled digital output. The output from the oscillator is fed through a time delay T to give sine and cos + (+ is an the arbitrary phase angle and this illustrates that these two outputs are orthogonal) before being

input via a complex multiplier 24 along with the output from A/D Converters 16. This delay element compensates for the delay through the analogue signal path.

Example 4 Figure 4 shows a yet further refinement. This embodiment is similar to that described in example 3; however the audio input is fed directly into A/D converter 22 and modulated by oscillator (NCO). It is to be noted that the inverter is not present as a module, as long as the inverter function is performed at some stage e. g. digitally with other functions.

The sine and cosine output 30,31 output are input to the time delay.

They are also converted into analogue signals using two digital to analogue converters 25, which after passing through low pass filters 26 the I and Q components (transmit signals) are then mixed by mixer 27 with the output of a fixed local oscillator at the frequency of the carrier (fc) via phase shifter 26 and summed at 28 producing the desired FM signal at the output frequency. The local oscillator before mixing with the I and Q signals 32,33 has applied a 90 degree phase shift by shifter 2d.

Effectively the audio in from the microphone is fed through two oscillators compared to the figure 3 embodiment, in such a way as to be input into VCO (Local Oscillator) only after having digital FM modulation previously. The advantage of this arrangement is that the unwanted FM may be generated much more accurately since it is not subject to variabilities and tolerances of a directly modulated oscillator.

Novel Antenna for Use in N-Plex Transceiver The inventor has also developed a novel antenna arrangement which obviates the need for two antennas for transmitting and receiving

arrangement N-plex system will normally be used in hand held systems where separate receive and transmit antennas are not practicable. In a further embodiment a simple means of realising a single antenna mixer for use in N-plex systems is shown in figure 5. The antenna 30 is connected to one side of a 45 degree delay line 31 and the transmitter to the other via resistor 32. The signals at both sides of the delay line are rectified by diodes 33 connecting to two further potentiometers 34 as shown. The transmitter output is also rectified and a portion of the rectified signal is effectively subtracted from the other two outputs. Any AM noise on the transmitter signal is nulled by adjustment of the two potentiometers.

The outgoing signal is delayed by 45 degrees but in the opposite direction as it is travelling in the opposite direction through the delay line. This give a relative phase shift of 90 . The product of the received and transmitted signals compared between the two rectifier/detector outputs is therefore at the desired phase quadrature. The circuit of figure 5 thus conveniently and neatly performs all the functions within the dotted line of figures 2.

The resistor 32 series with the transmitter output buffers the transmitter form the signal products.

Claims

Claims 1. A common channel transceiver including: a) first oscillator means to modulate an output audio signal to an rf transmission line ; b) antenna means to output said modulated signal; c) antenna means to accept received signals; d) first and second mixer means for mixing an rf transmission signal with said received signal; such that one of the signals output from the first mixer means is in-phase and signal output from the second mixer means is quadrature; e) analogue to digital converter means applied to each of the outputs of the mixers so that the signal are converted to a digital signals; f) means to digitally apply a substantially 90 degree phase shift to the digitally converted in-phase signal relative to the quadrature signal by appropriate phase shifting of one or more of these signals; g) means to sum the resulting in-phase and quadrature signals from step f) and DAC means to give provide an output signal from the summed signal.
2. A transceiver as claimed in claim I also including band-pass filter means such applied to the output from the mixers.
3. A transceiver as claimed in claims 1 or 2 including a high pass and low pass digital filers located after the A/D converters.
4. A transceiver as claimed in 1 to 4 additionally including: a) inverter means so that the output audio signal is inverted;

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b) means to convert said output from a) from digital to analogue; c) second oscillator means to modulate the output from step b) to provide two output signals sine and cosine, at 900 relative to each other; d) time delay means to provide a time delay to said signals from step c), e) means to be mixed with signals from step e) in claim 1.
5. A transceiver as claimed in claim 6 wherein said means in step (e) is a complex multiplier.
6. A transceiver as claimed in claim 4 or 5 having: a) additional means to convert the signals from step c) of claim 3 to analogue signals to provide second pair of in phase and quadrature signals: b) mean to apply low pass filtering to the signals from step a) above; c) means to apply a 900 phase shift to the output of the first oscillator; d) further mixer means to mix each of the in-phase and quadrature signals from step (b) with the output from step (c), thus performing the task of step a) of claim 1 ; e) summing means to sum the outputs of step d) to provide the transmission signal.
7. A transceiver as claimed in any preceding claim wherein a common aerial is used to both transmit and receive.
8. A transceiver as claimed in claim 7wherein said common aerial is connected via a 45 degree delay and rectifying circuit to the rf

transmission line, I and Q outputs, so as to perform the tasks of d) claim 1.

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9. A method of transmitting and receiving rf signals employing a transceiver as claimed in any of claims Ito 8.