GB2183970A

GB2183970A - Radio communication apparatus

Info

Publication number: GB2183970A
Application number: GB08530164A
Authority: GB
Inventors: Jack Levison
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1985-12-06
Filing date: 1985-12-06
Publication date: 1987-06-10

Abstract

Problems arise with co-sited radio communication apparatus on ships, for example, due to the presence of wide band noise generated by a transmitter interfering with a co-located receiver. Radio communication apparatus is provided comprising a transmitter including a wide band noise limiter positioned between an exciter/modulator (1) and a power amplifier (6), the wide band noise limiter comprising a first mixer (2) fed from the exciter/modulator, a band-pass filter (4) fed from the mixer (2) with a signal corresponding to the resonant frequency of the filter (4) and a second mixer (5) fed from the filter and arranged to feed the power amplifier (6) of the transmitter, the mixers being fed from a common local oscillator (3) the output frequency of which is selected in dependence upon the transmitted frequency so that the frequency fed to the band-pass filter (4) remains constant in the presence of output frequency variation, whereby the band-pass filter (4) is effective to suppress wide band noise which would otherwise be significant in a frequency band extending to each side of the transmitted frequency. The amplifier output is fed to an antenna 7 via a further filter 8. <IMAGE>

Description

SPECIFICATION Radio communication apparatus This invention relates to radio communication apparatus.

Problems arise with co-sited radio communication apparatus on ships, for example, due to the presence of wide band noise generated by a transmitter interfering with a co-located receiver.

It is an object of the present invention to provide radio communication apparatus in which this problem is obviated or substantially reduced.

According to the present invention, radio communication apparatus comprises a transmitter including a wide band noise limiter positioned between an exciter/modulator and a power amplifier, the wide band noise limiter comprising a first mixer fed from the exciter/ modulator, a band-pass filter fed from the first mixer with a signal corresponding to the resonant frequency of the filter and a second mixer fed from the filter and arranged to feed the power amplifier of the transmitter, the mixers being fed from a common local oscillator, the output frequency of which is selected in dependence upon the transmitted frequency so that the frequency fed to the band-pass filter remains constant in the presence of output frequency variation, whereby the bandpass filter is effective to suppress wide band noise which would otherwise be significant in a frequency band extending to each side of the transmitted frequency.

The mixers may each comprise an active balanced FET mixer which is arranged to feed a printed circuit resonant double balanced impedance transformer balun.

The mixer may also incorporate a circuit operative for image cancellation purposes.

The radio communication equipment may include a receiver which for noise suppression purposes includes a wide band noise limiter of the kind incorporated in the transmitter.

Some embodiments of the invention will now be described by way of example only with reference to the accompanying drawing, in which: Figure 1 is a block schematic diagram of a transmitter, Figure 2 is a block schematic diagram of a co-sited receiver, and Figure 3 is a block diagram of a mixer for use in the transmitter of Fig. 1 and the receiver of Fig. 2.

Referring now to Fig. 1, a transmitter comprises an exciter/modulator 1 arranged to feed a first mixer 2 fed also from a local oscillator 3. The first mixer 2 is arranged to feed a band-pass filter 4 which is coupled to a second mixer 5 which is also fed from the local oscillator 3. Output signals from the second mixer 5 are fed to a power amplifier 6 and the power amplifier 6 is arranged to feed a transmission aerial 7 via a filter 8 in accordance with standard transmission practice. In operation of this circuit, the frequency of the local oscillator 3 is chosen so as to provide an output signal on a line 9 from the first mixer 2 which corresponds to the centre frequency of the band-pass filter 4.Since the output signals from the filter on line 10 are mixed in the second mixer 5 with the signals from the local oscillator 3, an output signal from the mixer 5 is afforded on a line 11 which corresponds in frequency to the input signal fed from the exciter/modulator to the# first mixer 2 on a line 12.

This circuit arrangement acts to reduce the transmitter noise bandwidth and thereby suppress spurious emissions. Moreover, it will be appreciated that since the local oscillator 3 is arranged to track with the selected input signal the signal to the band-pass filter 4 on the line 9 remains at a constant frequency throughout the transmitted frequency spectrum whereby a single band-pass filter can be utilized throughout the frequency range of the transmitter.

By utilizing a transmitter which embodies between the exciter/modulator 1 and the power amplifier 6 a filter mixer arrangement of the kind described the transmission of unwanted spurious signals is very much reduced with the advantageous result that self-jamming problems appertaining to co-sited radio installations are significantly reduced.

Although a useful first-order improvement can be achieved with a transmitter as shown in Fig. 1, further improvements may be afforded utilizing a receiver as shown in Fig. 2.

Turning now to Fig. 2, the receiver comprises an aerial 13 which is arranged to feed an input signal filter 14, the filter 14 being coupled to a RF amplifier 15 via a mixer filter arrangement similar to that embodied in the transmitter and comprising first and second mixers 16 and 17 fed from a common local oscillator 18 and coupled by a band-pass filter 19. In this arrangement, the local oscillator 18 is arranged to track with the selected input signal frequency thereby to produce a substantially constant intermediate frequency on a line 20 at the input to the band-pass filter 19 whereby the band-pass filter 19 is effective over the whole of the operating range of the receiver. Furthermore, the frequency of signals on a line 21 feeding the mixer 16 corresponds to the frequency of signals on a line at the output of the mixer 17.It will be appreciated that although the conventional filter 14 provides some suppression to wide band signal noise, the step of including a tuned filter of the kind just before described embodying the mixers 16 and 17 and the band-pass filter 19 enables significant outer band signal suppression to be achieved. This facilitates operation of co-sited transmitter/receiver units without self-jamming problems particularly when automatic fast tuning or wide bandwidths are used such as may be necessary for frequency hopping arrangements.

The band-pass filter 4 of Fig. 1, or 19 as shown in Fig. 2, might conveniently comprise a crystal band-pass filter and the mixers 2,5,16 and 17 might be fabricated as will now be described with reference to Fig. 3.

Turning now to Fig. 3, a double balanced active FET mixer comprises an input transformer 23 to which input signals are fed on a line 24 and which is arranged to feed a pair of FET transistors 25 and 26. The transistors in turn are arranged to feed an impedance transformer 27 which might be fabricated using printed circuit techniques to define a printed circuit balun impedance transformer which provides an output signal to the band-pass filter on a line 28. The input from the local oscillator is provided on a line 29.

It will be appreciated that various modifications to the arrangements just described may be effected without departing from the scope of the invention as defined in the appended claims. For example, alternative mixer configurations may be used and it will also be appreciated that the construction of the receiver and/or transmitter may take any conventional form.

Claims

1. Radio communication apparatus comprising a transmitter including a wide band noise limiter positioned between an exciter/modulator and a power amplifier, the wide band noise limiter comprising a first mixer fed from the exciter/modulator, a band-pass filter fed from the first mixer with a signal corresponding to the resonant frequency of the filter and a second mixer fed from the filter and arranged to feed the power amplifier of the transmitter, the mixers being fed from a common local oscillator the output frequency of which is selected in dependence upon the transmitted frequency so that the frequency fed to the band-pass filter remains constant in the presence of output frequency variation, whereby the band-pass filter is effective to suppress wide band noise which would otherwise be significant in a frequency band extending to each side of the transmitted frequency.

2. Radio communication apparatus as claimed in Claim 1, in which the mixers each comprise an active balanced FET mixer which is arranged to feed a printed circuit resonant double balanced impedance transformer balun.

3. Radio communication apparatus as claimed in Claim 1 or 2, in which the mixer incorporates a circuit operative for image can cellation purposes.

4. Radio communication apparatus as claimed in any one of Claims 1 to 3, including a receiver which for noise suppression purposes includes a wide band noise limiter of the kind incorporated in the transmitter.

5. Radio communication apparatus substantially as hereinbefore described with reference to the accompanying drawing.