Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
330
BACK~:ROUND OF THE INVENTION:
This inventicn relates to single channel duplex communication
systems, and more particularly to portable radio transmitter/receiver equip-
ments.
Many communication systems, especially those connected with the
switched telephone network, require a full duplex link. This has traditionally
presented problems when a radio link is needed. Solutions either require
frequency division of the two paths together with duplexing filters for "2 to 4
wire" splitting, or TDM which can only be easily implemented in systems which
are already digital. An immediate application where this problem exists is for
so-called "cordless telephones".
SUMMARY OF THE INVENTION:
According to the present invention, there is provided a
transmitter/receiver for a single channel duplex communication system which
includes a local oscillator whose frequency of which is the center frequency of
the single channel signals. The output of the local oscillator is modulated with a
first audio frequency signal, the output of the local oscillator output is mixed in
phase quadrature with received signals frequency modulated with a second audio
frequency signal through the first and second mixing means which have an
attenuation of the mixed signals of not less than -lSdB relative to the local
oscillator output. The mixed outputs of the first and second mixing means are
filtered by a low pass filter and amplified to a constant level by an amplifier.
Each of the outputs of the amplifier is differentiated and each differentiated
output is multiplied with its input to provide the demodulated second audio
frequency signal. A proportion of the first audio frequency signal is subtracted
from the demodulated signal and the output of the local oscillator is fed in
combination with a phase quadrature output of the local oscillator to an aerial
via which signals frequency-modulated with the second audio frequency are
received.
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B~IEF DESCRIPTION OF THE DRAWING:
Embodiments of the invention will now be described with reference to
the accompanying drawing in which:
Fig. I illustrates the general arrangement of a single channel duplex
transmitter/receiver; and
Figs. 2 and 3 illustrate alternative arrangements for the transmitter
portion of Fig. 1.
D~TAILED DESCRIPTION OF THE DRAWING:
In the arrangement of Fig. 1, the receiver portion makes use of a so-
called "zero IF" or "direct conversion" method of demodulation. Such a method is
described in British Specification S.N. 1,530j602 published November 1, 1978 to
l.A.W. Vance. Briefly, local oscillator signals in phase quadrature at the carrier
frequency are each separately mixed with the incoming audio modulated si~nal.
The resulting signals have zero l.F. with the two sidebands folded over on each
other at the baseband and extending in frequency from DC to the single sideband
width of the original signal. The mixer outputs are low pass filtered and then
amplified to a standard level. After amplification, the two signals are separately
differentiated. Each differentiated signal is then multiplied by the input to the
other differentiator and one of the multiplier outputs is subtracted from the other.
In the present invention, the local oscillator is also used for the trans-
mitter. To achieve this, it is necessary to provide a dual splltting/combining
network with some directional properties, as shown in Fig. 1. The local oscillator 10
feeds two hybrid circuits 11, 12 which have isolation between the local oscillator
ports and the output ports. E~y means of the inherent non-perfect balance of such
hybrids, or by deliberate reflections at the input port, a fraction of the oscillator
signal is passed into the receiver. If, for example, 20 dB of isolation is achieved,
and the oscillator power is 200 mW in each arm, then 2 m~;V will pass into each side
of the receiver. The remaining power is then equally split between the input port
and the load. The two inputs are coupled to the aerial via another hybrid 13 which
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I.A.W. Vance- 4
(Rev)
has the property of isolating the output of hybrid 11 from that of hybrid 12 and of
performing a 90 phase shift of one with respect to the other. The oscillator signal
thus arrives at the aerial at a level of 100 mW (100 mW each from 11 and 12, added in
quadrature) and has been frequency modulated with the transmit information from
microphone 14. Recei~ed signals pass through hybrid 13 suffering no attenuation,
and enter 11 and 12 in quadrature. With 3 dB loss, they join the -20 dB local
oscillator signal and are mixed in two single ended mixers 15, 16. The difference
signals are filtered out in filters 25 and 26 and processed in the processing block 17
in the manner outlined in the British Specification previously cited. The signal
output from block 17 will now consist of both the incoming modulation and that of
the local oscillator. The latter is cancelled in a differential audio output amplifier
18 with a local link (or indeed in the demodulator or elsewhere). A balance of say,
20 dB should be adequate and easily obtainable.
There clearly exists a potentially regenerative acoustic feedback path
19 in this system (or any duplex system) but, if the levels and sensitivities are set to
be the same as, for example, in a normal wired telephone system, then the acoustic
isolation of the handset must be sufficient to ensure stability. That ls, the
conditions at the handset are unchanged.
A large variety of alternative configurations exist for the
oscillator/aerial/receiver connection and splitting circuits. Two possiblities are
shown in Figures 2 and 3. In Figure 2, circulators 20, 21 are used to avoid the 3 dE~
loss in the hybrids 12 and 11 while Figure 3 shows a lossier but much cheaper system
using resistive splitting networks 30, 31. In this case, as the isolation is small, some
additional phase shift of the two local oscillator channels is implied as shown in 3(b)
and 3(c). This is not a problem with a system such as the zero IF receiver where
exact phase quadrature is not a necessity. There are clearly some limitations on
the performance obtainable with this type of system. The transmit power is
limited by the isolation of the splitting system together with the power required by
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the mixer. This limit is of the order of lmK to 1 watt. The receiver sensitivity is
also limited as an RF amplifier cannot be used, and there is some loss in the
splitting/combining arrangements. However, the mixer can be low loss and the
following circuits sensitive. Overall, a noise figure of say, 15 dB should be
obtainable.
A great many applications lie within these limitations. For example,
cordless telephones operating within a subscriber's premises only need a few
milliwatts of transmitted power together with receivers of moderate sensitivity.It may be noted that in this type of application, low power is also a
necessity to enable the re-use of the same frequency at the minimum distance.
For the method described, the peak frequency deviation out of the
mixers is the sum of the peak local oscillator and input signal deviations. Thus, the
low pass filters and the receiver pre-detection noise bandwidth are required to be s
twice as large as a non-duplex receiver. Therefore, more bandwidth than one
channel would need to be allocated, eventhough the transmitted signal only
occupies the standard spectrum bandwidth.
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