JPH0410718A - Filter for transmitter-receiver - Google Patents

Abstract

PURPOSE:To obtain a filter appropriate for reducing the size and weight of a transmitter-receiver by using either one of a transmitting LC filter capable of obtaining high impedance at receiving frequency on the side of transmitting/ receiving antenna and a receiving LC filter capable of obtaining high impedance at transmitting frequency on the transmitting/receiving antenna side. CONSTITUTION:At least either one of the transmitting LC filter capable of obtaining high impedance at the receiving frequency on the transmitting/ receiving antenna side and the receiving LC filter capable of obtaining high impedance at the transmitting frequency on the antenna side is arranged. For instance, the transmitting LC filter 32 is connected to the antenna 10 side of the transmitting SAW filter 22. The filter 32 having a pass band allowing transmitting frequency to pass is constituted so that the impedance at the receiving frequency on the antenna 10 side is sufficiently higher than the impedance of the transmitting system. Consequently, sufficiently effective pass band characteristics can be obtained at both the transmitting and receiving frequency bands.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a transceiver filter used in a transceiver such as a wireless communication device.

[Prior Art] Among wireless communication systems, there is a wireless communication system that simultaneously transmits and receives data using different frequencies.

A block diagram of a transceiver used in such a system is shown in FIG. A transmitter 14 is connected to a single transmitting/receiving antenna 10 via a bandpass filter 12, and a receiver @18 is connected via a bandpass filter 16. These bandpass filters 12.16
is provided to prevent the transmitted signal and the received signal from influencing each other.

Conventionally, these bandpass filters 12 and 16 are
LC filters and helical filters that utilize electrical resonance have been used.

In recent years, in response to the demand for smaller transmitters and receivers that require no adjustment, consideration has been given to using surface acoustic wave filters that utilize Rayleigh waves propagating on the surface of a piezoelectric material as bandpass filters.

[Problems to be Solved by the Invention] However, as shown in FIG. 10, it has been found that sufficient characteristics cannot be obtained when 5AW (surface acoustic wave) filters 22 and 26 are used as bandpass filters.

For example, as a single transmission SAW filter, the first
As shown in the narrow band characteristics in Figure 1 (a) and the wide band characteristics in Figure 1 (b), sufficient out-of-band attenuation and excellent in-pass band characteristics (
The receiving SAW filter alone has sufficient out-of-band attenuation and an excellent passband, as shown in the narrow band characteristics in Figure 11 (C) and the wide band characteristics in Figure 11 (d). Internal characteristics (center frequency 380MHz
), if these transmitting SAW filter 22 and receiving SAW filter 26 are directly connected as shown in FIG. 10, as shown in FIG. 12, at a transmitting frequency < 254 MHz) Passband characteristics (
Narrowband characteristics in Figure 12(a), wideband characteristics in Figure 12(b)) and pass frequency characteristics at the receiving frequency (254MHz) (narrowband characteristics in Figure 12(C), wideband characteristics in Figure 12(d)) ) deteriorates significantly.

This is due to the following reasons.

Assume that the transmitting frequency is fT, the receiving frequency is fR, and the transmitter/receiver is designed in the ROΩ system. Reception SAW filter 26 at transmission frequency fT different from reception frequency fR
The input impedance at Ro termination is capacitive (capacitance value C
R), and the equivalent circuit of FIG. 10 for the transmission frequency f7 is as shown in FIG. 13(a). Therefore, the impedance of the transmitting SAW filter 22 on the transmitting/receiving antenna 10 side is 1. / (1, /Ro+j 2πfTCR
), resulting in an impedance mismatch. Therefore, the passband characteristic at the transmission frequency f'r is as shown in Fig. 12(a).
It deteriorates as shown in (b).

Similarly, the receiving frequency f is different from the transmitting frequency fT. The input impedance of the transmitting SAW filter 22 at the Ro termination also becomes capacitive (capacitance value CT), and the receiving frequency f
The equivalent circuit of FIG. 10 for 1 is as shown in FIG. 13(b). Therefore, the impedance of the transmitting/receiving antenna 10@ of the receiving SAW filter 26 is 1/(1/
Ro+j 2πfRC7), and an impedance mismatch occurs. The passband characteristics at this diagonal reception frequency fR deteriorate as shown in FIGS. 12(c) and 12(d).

In order to prevent such deterioration of the passing frequency characteristics, the first
As shown in FIG. 4, bandpass filters 30 are also inserted between the SAW filters 22 and 26 and the antenna 10, respectively. However, since the bandpass filter 30 also uses an LC filter or a helical filter that does not utilize electrical resonance, the number of components is large, making it difficult to reduce the size and weight.

An object of the present invention is to provide a transmitter-receiver filter suitable for use in a transmitter-receiver that simultaneously transmits and receives data at different frequencies and is suitable for reduction in size and weight.

[Means for Solving the Problem] The above object is a transmitter-receiver filter used in a transmitter-receiver that simultaneously transmits and receives signals at mutually different transmitting and receiving frequencies from a common transmitting and receiving antenna by a transmitting section and a receiving section. , a transmission surface acoustic wave filter connected to the transmitting/receiving antenna side of the transmitting section and having the transmitting frequency as the passband; and a receiving surface acoustic wave filter connected to the transmitting/receiving antenna side of the receiving section and having the receiving frequency as the passband. A transmitting/receiving filter having a transmitting/receiving surface acoustic wave filter that connects the transmitting/receiving antenna and the transmitting surface acoustic wave filter, has a pass band including the transmitting frequency, and has a passband including the transmitting frequency, and the transmitting/receiving antenna is connected to the transmitting/receiving surface acoustic wave filter. A transmitting LC filter that has a high impedance when viewed from the side, and a transmitting/receiving antenna connected to the receiving surface acoustic wave filter, having a passband including the receiving frequency, and having a pass band including the receiving frequency from the transmitting/receiving antenna side at the transmitting frequency. This is achieved by a transmitter/receiver filter characterized by having at least one of the receiving LC filters that has a high impedance when viewed from the outside.

[Function] A transmitting LC filter that has a passband that includes the transmitting frequency and has a high impedance when viewed from the transmitting and receiving antenna side at the receiving frequency, and has a passband that includes the receiving frequency.
Since at least one of the receiving LC filters is provided with a high impedance at the transmitting frequency when viewed from the transmitting and receiving antenna side, it is possible to achieve sufficiently good passband characteristics at both the transmitting frequency and the receiving frequency using the LC filter with a simple configuration. Obtainable.

[Example] The filter for a transmitter/receiver according to the first example of the present invention is
This will be explained using FIGS. 3 to 3. Components that are the same as those of the conventional transmitter/receiver filter shown in FIG. 9 are designated by the same reference numerals, and description thereof will be omitted.

In this embodiment, the antenna 10 of the transmitting SAW filter 22
It is characterized in that a transmitting LC filter 32 is provided on the side. The reception SAW filter 26 is not provided with an LC filter.

The transmission LC filter 32 has a passband that passes 7 at the transmission frequency from the transmitter 14.

Also, the transmitting LC filter 32 and the transmitting SAW filter 22 seen from the antenna 10 side at the receiving frequency ftt.
The transmitting LC filter 32 is configured such that the combined impedance is sufficiently higher than the impedance of the transmission system. The antenna 10f is connected to the transmitting SAW filter 22 side of the transmitting LC filter 32 without terminating it with a capacitor CT.
The configuration is such that the impedance from Ill to Ill is sufficiently higher than RO. That is, the configuration is such that the capacitive component CT of the transmitting SAW filter 22 and the transmitting LC filter 32 are combined to resonate in parallel at the receiving frequency fR.

By providing such a transmission LC filter 32, the signal at the transmission frequency fR from the transmitter 14 is transmitted from the antenna 10 with low loss. At the same time, even if a signal at the receiving frequency f7 is received from the antenna 10, the input impedance to the transmitting LC filter 32 is sufficiently high, so that reception is possible without being affected by the circuit on the transmitter 14 side.

The transmitting LC filter 32 is constituted by, for example, a T-type low-pass filter including inductances L1 and L2 and capacitance C1 as shown in FIG. Inductance Ll, L2 is 56'n) (, -q Japacitance C is 10
It was set as pF. The cutoff frequency will be approximately 320 MHz. As the transmitting SAW filter 22 (FIG. 11 fa)
A filter with the characteristics shown in FIG. 11(c) and (d) was used as the reception SAW filter 26.

The characteristics in this case are shown in FIG. Figure 3(a) shows the narrowband characteristics on the transmitting side, Figure 3(b) shows the wideband characteristics on the transmitting side, Figure 3(C) shows the narrowband characteristics on the receiving side, and Figure 3(d) shows the narrowband characteristics on the receiving side. It is wide-band carrying/W. The characteristics within the passband were significantly improved compared to the prior art shown in FIG. 12.

The fourth embodiment of the transmitter/receiver filter according to the second embodiment of the present invention is
This will be explained using FIGS. 6 to 6. Components that are the same as those of the transmitter/receiver filter according to the first embodiment are given the same reference numerals, and description thereof will be omitted.

In this embodiment, the antenna 10 of the transmitting SAW filter 22
Along with the transmitting LC filter 32 on the side, a receiving LC filter 36 is also provided on the antenna 10 side of the receiving SAW filter 26.
It is distinctive in that it is provided with

Similar to the first practical rejection example, the transmission LC filter 32 is
It has a passband that passes the transmission frequency f'r from the transmitter 14, and is configured such that the impedance of the transmission LC filter 32 is sufficiently high from the antenna 10 side at the reception frequency fR. Similarly, the newly provided reception LC filter 36 also has a pass band that passes the reception frequency fR from the antenna 10, and the reception LC filter 36 seen from the antenna 10 side at the transmission frequency fT.
The structure is such that the impedance is sufficiently high. That is, at the transmission frequency fT, the capacitive component CR of the receiving SAW filter 26 and the receiving 1-C filter 36 are configured to resonate in parallel in a combined state.

By providing such a reception LC filter 36, the signal at the reception frequency fR from the antenna 10 is received by the receiver 18 with low loss. At the same time, even if the transmitter 14 outputs a signal with the transmission frequency f7, the input impedance to the reception LC filter 36 is sufficiently high, so the reception m1
Transmission is possible without being affected by the circuit on the 8 side.

The transmitting LC filter 32 is, for example, as shown in FIG. The LC filter 36 is connected to the inductance L5 (82
nH) and an L-type bypass filter (cutoff frequency - about 100 MHz) with a capacitance C3 (38 pF). SAW filters 22 and 26 having the characteristics shown in FIG. 11 were used.

The characteristics in this case are shown in FIG. Figure 6(a) shows the narrowband characteristics on the transmitting side, Figure 6(b) shows the wideband characteristics on the transmitting side, fc) shows the narrowband characteristics on the receiving side, and Figure 6(d) shows the wideband characteristics on the receiving side. It is a characteristic. The characteristics within the passband were significantly improved compared to the prior art shown in FIG. 12.

A seventh embodiment of the transmitter/receiver filter according to the third embodiment of the present invention
This will be explained using figures. Components that are the same as those of the transmitter/receiver filters according to the first and second embodiments are given the same reference numerals, and their explanations will be omitted.

This example is characterized in that only the receiving SAW filter 26 is provided with a receiving LC filter 36, and the transmitting SAW filter 22 is not provided with an LC filter.

Similar to the second embodiment, the reception LC filter 36 has a passband that passes the reception frequency fR from the antenna 10, and the impedance of the reception LC filter 36 seen from the antenna 10 side is sufficient at the transmission frequency fT. It is configured to be high. That is, at the transmission frequency fT, the capacitive component C8 of the receiving SAW filter 26 and the receiving LC filter 36 are configured to resonate in parallel in a combined state.

By providing such a reception LC filter 36, the signal at the reception frequency fR from the antenna 10 is received by the receiver 18 with low loss. At the same time, even if a signal with a transmission frequency fT is output from the transmitter 14, the receiving LC filter 3
Since the input impedance to 6 is high enough, receiver 18
Transmission is possible without being affected by the side circuit.

An eighth embodiment of the transmitter/receiver filter according to the fourth embodiment of the present invention
This will be explained using figures.

The present practical example is characterized in that an inductance L6 is inserted in parallel to the antenna 10 side of the receiving SAW filter 26 (or the transmitting SAW filter 22). Inductance I = 6, transmission frequency f7 is reception frequency fR
It is desirable to select a value that resonates in parallel with the input capacitance of the SAW filter 22, 26 in the vicinity. Transmission frequency f
This is effective when T and reception frequency fR are close to each other.

As described above, according to this embodiment, good transmission characteristics can be achieved with a minimum number of component constants.

The present invention is not limited to the above-mentioned embodiments, and various modifications are possible.

For example, in the first embodiment, the transmitting LC filter is a low-pass filter, but if the transmitting frequency is higher than the receiving frequency, it may be a bypass filter. Further, in the second embodiment, the transmitting LC filter and the receiving LC filter are bypass filters, but they may be low-pass filters. Furthermore, in the third embodiment, for transmission,
Although the reception LC filter is a bypass filter and is a C filter or a low-pass filter, the transmission LC filter may be a hyper-C filter and the reception LC filter may be a low-pass filter. In short, if it is a transmitting LC filter, it is sufficient that the transmitting frequency is included within the passband, and if it is a receiving LC filter, it is sufficient that the receiving frequency is included within the passband.Is it a low-pass filter? It is not essentially important whether it is a bypass filter or not.

Furthermore, in the above embodiments, the transmission frequency may be lower than the reception frequency, or conversely, the transmission frequency may be lower and the reception frequency may be higher.

[Effects of the Invention] As described above, according to the present invention, there is provided a transmitting LC filter that has a passband that includes the transmitting frequency and has a high impedance when viewed from the transmitting and receiving antenna side at the receiving frequency, and a transmitting LC filter that has a passband that includes the receiving frequency. However, the receiving L has high impedance when viewed from the transmitting and receiving antenna side at the transmitting frequency.
Since at least one of the C filters is provided, it is possible to obtain sufficiently good passband characteristics at both the transmitting station wave number and the receiving frequency with the simple configuration of the LC filter. It is possible to realize a filter for

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a transceiver in which a transceiver filter according to the first embodiment of the present invention is used; Fig. 2 is a circuit diagram showing the transceiver filter; and Fig. 3 is a circuit diagram showing the transceiver filter. FIG. 4 is a block diagram showing a transmitter/receiver in which the transmitter/receiver filter according to the second embodiment of the present invention is used; FIG. 5 is a circuit diagram showing the same transmitter/receiver filter; FIG. is a graph showing the characteristics of the transmitter-receiver filter, FIG. 7 is a block diagram showing a transmitter-receiver in which the transmitter-receiver filter according to the third embodiment of the present invention is used, and FIG. 8 is a graph showing the transmitter-receiver filter according to the third embodiment of the present invention. FIG. 9 is a block diagram of a conventional transmitter-receiver, FIG. 10 is a circuit diagram showing a conventional transmitter-receiver filter, and FIG. 11 is a single SAW filter. Figure 12 is a graph showing the characteristics of a conventional transmitter/receiver filter, Figure 13 is a circuit diagram showing the equivalent circuit of the conventional transmitter/receiver filter during transmission and reception, and Figure 14 is another conventional filter. FIG. 2 is a block diagram of a transceiver. 18... Receiver 22... SAW filter for transmission 26... SAW filter for reception 30... Band pass filter 32... LC filter for transmission 36... LC filter for reception

Claims (1)

[Scope of Claims] 1. A transmitter-receiver filter used in a transmitter-receiver that simultaneously transmits and receives signals at mutually different transmitting and receiving frequencies from a common transmitting and receiving antenna by a transmitting section and a receiving section, which Connected to the transmitting and receiving antenna side,
A transceiver filter comprising: a transmitting surface acoustic wave filter having a passband equal to the transmitting frequency; and a receiving surface acoustic wave filter connected to the transmitting/receiving antenna side of the receiving section and having a passband equal to the receiving frequency. , a transmitting LC filter that connects the transmitting/receiving antenna and the transmitting surface acoustic wave filter, has a pass band including the transmitting frequency, and has a high impedance at the receiving frequency when viewed from the transmitting/receiving antenna side; connecting a transmitting/receiving antenna and the receiving surface acoustic wave filter, and having a passband including the receiving frequency;
A filter for a transmitter/receiver, comprising at least one of a receiving LC filter having a high impedance when viewed from the transmitting/receiving antenna side at the transmitting frequency.