GB2095063A - Radio receiver - Google Patents

Abstract

A superheterodyne receiver comprising a surface elastic wave amplifier (12) of a parametric amplification type in the HF amplifier unit of the receiver. The antenna (1) is connected to a transducer of the surface elastic wave amplifier (12) via at least one matching circuit (15). The amplifier (12) is also connected downstream to a frequency converter (5, 6, 7) via a matching circuit (16). <IMAGE>

Description

SPECIFICATION Radio receiver The present invention relates to a radio receiver, and more particularly, to a novel and improved radio receiver superior in antiinterference performance and sensitivity.

As well known, the performances required for the radio receivers are generally the following four ones: (1) Sensitivity (2) Anti-interference performance (3) Fidelity (4) Stability Such radio receivers which meet all the above four requirements can of course be accepted as quality apparatus. Along with the ever-increasing number of broadcasting stations, however, the required performance (2) above, say, the antiinterference performance, has recently been more and more important in this field.

A general means of improving the antiinterference performance is to use narrow-band filters; Figure 1 shows the construction of a conventional superheterodyne receiver equipped with such filters. In this Figure, the reference numeral 1 denotes an antenna, 2, 4 and 7 indicate filters, 3 refers to an HR amplifier, 5 to a frequency converter, 6 to a local oscillator, 8 to an IF amplifier, 9 to a demodulator, 10 to an amplifier and the numeral 11 refers to a speaker.

The anti-interference performance of the radio receiver of a such construction will be discussed below.

The antenna 1 does not meet our expectation in respect of the frequency selectivity. Suppose that an HF signal received bythis antenna 1 is amplified as it is by the HF amplifier 3, harmful phenomena such as intermodulation or crossmodulation will be caused by the nonlinear effect of the amplifier 3. To avoid this, the filter 2 is connected downstream of the antenna 1.

The frequency converter 5 is substantially a nonlinear circuit which will incur an image frequency interference in addition to the intermodulation or cross-modulation; to avoid the above, the insufficient frequency selectivity of the filter 2 is covered by another filter 4. The numeral 6 denotes a local oscillator. The filter 7 is provided to select a signal which has been converted in frequency to IF.

As described in the above, the conventional radio receivers are usually provided with 3 filters.

Of them, the filters 2 and 4 have been as follows, in case of the received frequency being variable: (1) Their central frequency is variable; (2) They are switched to those designed for different frequencies; and (3) Wide-band filters which let pass all the signals of frequencies within the variable band.

However, the filters in (3) above are not substantially narrow-band ones, and consequently are poor in performance of anti-interference.

Although the filters in (2) above may be used to some extent for a narrow band, many such filters are necessary so that the costs and mounting space will be great. Further, with the filters in (1) above, it is difficult to provide a sufficient frequency selectivity to suppress other than a desired signal.

To obtain a sufficient frequency selectivity using the filters in (1), it is necessary to provide an increased number of filter stages, which, however, will lead to an increase of loss. More particularly, if a such filter is used as the above-mentioned filter 2, the noise factor of the radio receiver is adversely affected, and a tracking error of the filter takes place; thus, it is not possible to increase the number of filter stages at random.

To suppress the interference from adjoining channels, there is provided an IF signal selecting filter 7 downstream of the frequency converter 5.

This filter 7 maybe such that its central frequency is fixed and its subject frequency is low.

Accordingly, inexpensive filter of good selectivity can possibly be used.

It is apparent that the frequency selectivity of the filters 2 and 4 are insufficient to suppress the intermodulation or cross-modulation caused by the HF amplifier 3 and frequency converter 5. The mechanism of intermodulation occurrence will be described herebelow: Assume that the received frequency is fd and that there are interference waves fd+Afl and fd+Af adjacent to the frequency fd. There occurs in the HF amplifier the frequency fd from the tertiary nonlinear factor as below: 2(fd+Af) - (fd*Af) = fd This means the occurrence of interference from the waves adjacent to the frequency fd. This frequency varies depending upon the kind of communication being made, and is on the order of 10 kHz in some cases.

Accordingly, to prevent any interference, an HF filter is necessary which can sufficiently suppress the frequency components very near the frequency used in communication. However, there have been available so far only piezo-electric HF filters using crystal, of which the central frequency is fixed. Because of this fixed central frequency, such filters are those in (2) above.

The present invention has an object to overcome the above-mentioned drawbacks of the prior-art radio receivers by providing a radio receiver excellent in anti-interference performance and sensitivity, in which a filter small in loss due to variation of the central frequency and high in sensitivity is adopted which comprises an elastic surface wave amplifier which provides a parametric amplification.

The foregoing and other advantages will be better understood from the ensuing description made by way of example of the embodiments of the present invention with reference to the accompanying drawings in which: Figure 1 is a block diagram of a conventional superheterodyne receiver taken as example; Figure 2 is a block diagram of an embodiment of the radio receiver according to the present invention; Figure 3 is a circuit diagram showing an example of the surface elastic wave amplifier used in the embodiment shown in Figure 2; Figure 4 is a frequency characteristic curves of the surface elastic wave amplifier in Figure 3; Figure 5 is a block diagram of another embodiment of the present invention; Figure 6 is a block diagram showing a partial variation of the second embodiment shown in Figure 5;; Figure 7 is a block diagram showing a third embodiment of the radio receiver according to the present invention; and Figures 8 and 9 are block diagrams, respectively, showing partial variations of the third embodiments in Figure 7.

Referring now to Figure 2 showing a first embodiment of a superheterodyne receiver to which the present invention is applied, the reference numerals similar to those in Figure 1 indicate like elements or parts; in this Figure, the reference numeral 12 denotes a surface elastic wave amplifier, 13 refers to a dc bias voltage generator of wave amplifier 12, 14 to a pumping power generator circuit and 1 5 and 1 6 refer to matching circuits for the transducers of the surface elastic wave amplifier, which will be further described later.

Figure 3 shows an example of said surface elastic wave amplifier. In this Figure, S indicates a semiconductive substrate made of silicon (Si), and I a piezo-electric film made of zinc oxied (ZnO). I' denotes a film of silicon oxide (SiO2). The above semiconductive substrate S, silicon oxide film I' and piezo-electric film I are so stacked as to form a iamination.

The above-mentioned silicon oxide film I' acts to stabilize the surface of the semiconductive substract S.

Further, the numerals 1 2t and 122 refer to an electric signal input means and ouput means, respectively, which are composed of a surface elastic wave transducer made of a comb-shaped electrode. The electric signal input and output means are connected to said matching circuits 1 5 and 16, respectively; an electric signal is converted by the input means 12, to a surface elastic wave, while the surface wave signal is converted to an electric signal by the output means 122.

Symbol M, is an electrode for application of dc bias voltage and pumping power, and this electrode M, is disposed in the propagation path of the surface wave signal.

Symbol M2 indicates an electrode to provide an ohmic contact with the semiconductive substrate S.

The above-mentioned electrode M, is connected to the earth through a choke coil CH for suppressing of HF current and a dc power source 1 3 of which the voltage is variable and which is to apply a dc bias voltage. This electrode M, is also connected to the earth through a capacitor C for suppressing of dc current and an HF power source 14 for supplying a pumping power.

An electric signal supplied from the matching circuit 1 5 to the input means 12, is converted to a surface wave signal and is propagated over the surface of the piezo-electric film I toward the output means 1 22. Suppose that the frequency of surface wave signal thus propagated is f, and that a dc bias voltage is applied from the dc power source 13 to the electrode M, on the piezoelectric film I while a pumping power of a frequency 2f is supplied to the electrode M,. The surface wave signal is amplified under the effect of the parametric interaction due to the nonlinearity of the surface charge layer capacity at the surface of the semiconductive substrate S below the electrode M,; this amplified signal is converted by the output means 122 and taken out.

The above-mentioned amplification is a function of the length of the electrode M1 in the propagating direction of the surface wave, nonlinear strength 5 at the surface of the semiconductive substrate S, and of the frequency of the pumping power; thus, the amplification may be changed by changing the values of the abovementioned factors. The above-mentioned strength g depends upon the nonlinearity of the surface charge layer capacitor of the semiconductive substrate S, which is determined by the value of dc bias voltage, and the magnitude of pumping power. In the practical operation, the above 2 kinds of parameter are changed to adjust the amplification.

The surface elastic wave amplifier 12 depends in amplification upon the nonlinearity of surface charge layer capacity at the surface of the semiconductive substrate S made of silicon, etc.

Since the effect of this nonlinearity is rather great as compared with the piezo-electric body itself as having been illustrated and explained in connection with the conventional apparatus, the pumping power can effectively be reduced for a predetermined amplification.

Because the parametric amplification by the surface elastic wave amplifier 1 2 is a kind of positive feedback amplification, it is necessary to increase the electric Q in order to increase the amplification. Figure 4 shows an example frequency response of the amplification of the amplifier 12 with the change of Q. As seen from this Figure, as Q increases, the frequency bandwidth narrows while the amplification A is larger. Since the frequency bandwidth can be changed in this way with the change of amplification A, the amplifier 12 can be further equipped with a function of amplification within a variable bandwidth.

As described in the above, the surface elastic amplifier 12 can be sued as an HF amplifier with great gain, high selectivity and variable tuning by supplying an appropriate dc bias voltage and pumping power (for variable tuning, it suffices to use a pumping power frequency two times higher than a received frequency), this surface elastic wave amplifier 1 2 may be substituted for the conventional filters 2 and 4, and HF amplifier 3.

Because of the high selectivity of the amplifier 12, it is possible to suppress frequency components near the received frequency and which could not be suppressed by the conventional variable tuning type amplifier, and consequently prevent harmful phenomena such as intermodulation or crossmodulation which have been experienced in the conventional HF amplifier and frequency converter, from occurring. Further, since the mechanism of amplification of the surface elastic wave amplifier is a parametric amplification, the amplifier is substantially of low noise and of an increased sensitivity.

The filter 7 used in the IF stage may not by a high-performance one.

Figure 5 shows a second embodiment according to the present invention, in which the pumping power source of circuit 13 is not composed of an independent oscillator, but a pumping power is obtained from the local oscillator 6 of the receiver as in the following.

In Figure 5, the numeral 1 7 refers to a frequency doubler, 1 8 to a filter, 1 9 to a local oscillator, 20 to a frequency converter, 21 to a filter, and 22 to a buffer power amplifier circuit. The frequency converter unit composed of the local oscillator 19, frequency converter 20 and filter 21 may be disposed upstream of the frequency doubler 17.

Assume here that the received frequency is fs and the intermediate frequency (IF) is fi. The oscillation frequency of the local oscillator is selected to fs + fi (or fs - fi). The output of this frequency from the local oscillator 6 is doubled in the frequency doubler 1 7 to provide a frequency signal of 2(fs + fi).A fundamental harmonic (fs + fi) and frequency components more than 3 times higher than the received frequency are removed from this frequency signal by the filter 1 8. As the local oscillator 9 delivers at the output thereof a fixed-frequency signal of 3fi, the output from the frequency converter 20 will be composed of the following: 2(fs + fi) + 2fi = 2fs ( 1 ) 2(fs + fi) + 2fi = 2fs +3fi () Namely, the frequency signals of 2fs, 2fs + 3fi (when the oscillator frequency of the local oscillator 6 is selected to fs + fi) and 2fs - 3fi (when the oscillation frequency is selected to fs - fi) are delivered from the frequency converter 20.Since the frequency component of 2fs f 3fi of the frequency signals is unnecessary, it is removed by the filter 12, and only the component of 2fs is amplified by the buffer power amplifier circuit 22 and supplied to the surface elastic wave amplifier 12.

Suppose that the lower limit of receivable frequency is fsmin and upper limit is fsmax- The frequency range of the pumping power is 2fsmin to 2fsmax. In this case, the outputs corresponding to the above equation (2) are: 2fsmn ~ 3fi to 2fsmax +3fi . ..... . .... ... (3) 2fsmin + 3fi in (3) may take a value very near 2fsmax or it may be less than 2fsmaxS or2fs max may take a value very near 2fsmin or it is more than 2fsmin; in this case, such unnecessary components cannot be removed by the filter 21 alone.

Accordingly, a plurality of filters 21 is provided as 21 r, 212, . . . as illustrated in Figure 5, and they are used by appropriately selecting them by means of the switches SW1 and SW2 according to a received frequency.

In the above case, the frequency converter 20 may be of an image rejection type instead of providing the plurality of filters 21. Since the image rejection type frequency converted does not substantially deliver the component shown in the equation (2) above corresponding to an image frequency, the filter 21 may be eliminated or may be a simpler one by using a frequency converter of a such image rejection type.

Figure 7 shows a further embodiment of the radio receiver according to the present invention.

In this embodiment, the local oscillator 6 and pumping power source or circuit 1 3 have the form of a PLL circuit composed, respectively, of a buffer circuit 27 (30), voltage-controlled oscillator 28 (29), programmable divider 24 (31), and a phase comparator 23 (32), and further a divider 26, common to a reference oscillator 25, which divides the oscillation output of the latter.

In the above embodiment, the pumping power circuit and local oscillator of the surface elastic wave amplifier have a structure of PLL circuit; so, the superheterodyne receiver is extremely stable in operation. The pumping power circuit is made of an independent oscillator. This oscillator must be sufficiently stable to make the most of the narrow-band characteristic of the surface elastic wave amplifier. Because of the variable frequency and stability, the PLL circuit is optimum for supply of pumping power; in the present invention, the PLL circuit for the local oscillation and that for supply of pumping power share the reference oscillator and divider. Therefore, the circuit configuration is simplified and the manufacturing costs can be reduced.

The programmable divider 24(31) may have provided upstream thereof a pre-scaler 33 or a mixer 34 and a local oscillator 35, as shown in Figure 8.

Claims (9)

1. A receiver, comprising an HF amplifier composed of a surface elastic wave amplifier of a parametric amplification type, in which a received HF signal is amplified by said surface elastic wave amplifier and the signal thus amplified is delivered to the circuit downstream thereof.

2. A superheterodyne receiver, comprising: a surface elastic wave amplifier of parametric amplification type; a first matching circuit; a second matching circuit' said first matching circuit being connected between an antenna of the receiver and the input of said surface elastic wave amplifier, while the second matching circuit is connected between the output of said surface elastic amplifier and a frequency converter of said receiver.

3. A receiver as set forth in one of Claims 1 and 2, in which said surface elastic wave amplifier is provided with a means to apply a pumping power and the frequency of said pumping power is set to a double of a received frequency of said receiver.

4. A receiver as set forth in Claim 2, further comprising a circuit to feed as pumping power to said surface elastic wave amplifier the oscillation output from the local oscillator IF conversion of said receiver.

5. A receiver as set forth in Claim 4, in which said circuit comprises a frequency doubler circuit, a mixer circuit for frequency conversion of doublefrequency signal from said frequency doubler circuit, and a filter circuit to extract only frequency components two time higher than the received frequency from the frequency-converted output from said mixer circuit.

6. A receiver as set forth in Claim 5, in which said filter circuit is composed of a plurality of filters and so constructed that said filters are selectively used according to a received frequency.

7. A receiver as set forth in Claim 3, in which said pumping power circuit is composed of a first PLL circuit.

8. A receiver as set forth in Claim 7, in which said local oscillator for IF conversion of said receiver is composed of a second PLL circuit.

9. A receiver substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.