JPH021972Y2 - - Google Patents

Description

[Detailed description of the invention] (Technical field to which the invention pertains) This invention relates to the improvement of the second mixing (or demodulation) circuit of a receiver. Moreover, the present invention is characterized in that it provides a high input resistance type second mixing circuit suitable for applications requiring a particularly low demodulation distortion rate.

(Prior Art) It is very difficult to perform demodulation with a low distortion rate in a receiver used in an electric field where the received input fluctuates over a wide level range, especially in a narrowband FM mobile receiver. The reason for this is as follows.

Figure 1 is a block diagram of an example of the circuit configuration of a general conventional receiver. ANT in the figure is an antenna input terminal;
1 is a wideband filter for suppressing image spurious frequencies, and 2 is a first frequency converter or mixer (hereinafter referred to as mixer) with a mixing or conversion gain of 0 dB or more. LO ₁ is a first local signal (local oscillation frequency) input, and LO 3 is a narrowband filter for obtaining intermodulation characteristics, which is assumed to be a phase linear filter here. 4 is a second mixer, LO ₂ is a second local signal input, 5 is a narrow band filter to obtain adjacent channel selectivity, and 6 is a high gain intermediate frequency amplifier.
IFA, 7 is a detector, and 8 is an audio frequency amplifier AFA.

In the receiver shown in Figure 1, the received wave input from the antenna is demodulated through paths 1 to 8, but in a receiver that requires low demodulation distortion, the phase characteristics are linear within the band. The phase linear filter becomes 3
and 5. However, in order to obtain this phase linear characteristic, it is necessary to properly match the input and output impedances of the circuit. If the output of the first and second mixers is linear with respect to the input, that is, the input and output impedance of each mixer is constant, then 3 and 5
Since the matching of the phase linear filter does not change, it is possible to maintain a low demodulation distortion rate. However, the received wave voltage P ₀ (dB) input from the antenna is
Since the input voltage of the second mixer 4 is _amplified with a gain G (dB) at
The mixer leaves the linear characteristic region earlier than the first mixer, and the bandpass filters BPF3 and BPF5 are mismatched, making it impossible to obtain the desired characteristics in terms of phase linearity and amplitude characteristics.

Even if conventional mobile radio receivers have the above problems, they do not have much of an effect because they are used in areas where the distortion rate is high enough to allow calls, and conventionally, the second
A simple second mixer circuit, an example of which is shown in the figure, is used. In FIG. 2, 3 and 5 are the same phase linear filters as in FIG. 1, _C4 is a capacitor for inputting the second local signal, 4a is a matching circuit, and 4b is a grounded emitter mixer circuit. As shown in the figure, the second local signal LO ₂ is input to 4a through a small capacitance capacitor C ₄ , but since C ₄ is small, the local signal cannot be input sufficiently, so the linearity of the second mixer circuit is poor and it cannot be used as a high input type mixer. I can't hope to become one. If the capacitor _C4 is changed to a larger capacitance, not only will the received input that has passed through the filter 3 flow toward LO2 on the second local oscillator side, causing a loss, but the first filter ₃ will have a high center frequency and If the capacitance of _{C 4} is increased, the filter 3 will not be able to match and the characteristics will deteriorate, which is a disadvantage, so C ₄ needs to be small.

Note that the SCPC-FM method (one radio carrier wave
In mobile communications using single communication (single communication that transmits two pieces of information), BPF, which has strict receiver selectivity and a steep attenuation slope, is generally used to avoid interference from adjacent channels. It is well known that the demodulation distortion rate of such a BPF deteriorates due to linear distortion of the phase and amplitude of the transmission characteristics within the passband. For this reason, BPFs 3 and 5 are used that take the above requirements into consideration, but these BPFs have characteristics that are sensitive to the values of input and output load impedance.

(Specific Purpose of the Invention) The present invention was made to eliminate the above-mentioned drawbacks, and the second mixer can be controlled by sufficiently applying the input LO ₂ from the second local oscillator without lowering the gain of the second mixer. The linearity of the 2nd mixer was expanded and the input resistance of the second mixer was increased.

(Structure and operation of the invention) FIG. 3 is a diagram showing an example of the structure of a second mixer circuit implementing the invention. In this figure, numerals 3 and 5 are phase linear BPFs similar to those shown in FIG. Also, 9 is a 2-distributor, 4a
and 4b are the same matching circuit and emitter ground mixer as in FIG.

Since the present invention is characterized by the use of a two-way divider 9 compared to the one shown in FIG. 2, the two-way divider 9 will be explained first.

The 2-way divider 9 has three ports as shown in the figure.
, the signal transmission between → and → is attenuated by 3 dB each, and are separated and are not affected by each other, and the impedance seen inside is the terminal impedance of or. is the same as the characteristic impedance of the two-way divider, regardless of There is also a commercially available two-way divider with such characteristics, which is well known as a hybrid transformer.

Now, when a two-way divider with the above characteristics is used as shown in Figure 3, the input signal passing through BPF3 is
The signal that enters the port of the distributor 9 and is attenuated by 3 dB is output to the port. On the other hand, the second local signal LO ₂ enters the port and is attenuated by 3dB and output to the port.
Since the common frequencies of both signals are close, matching with the second mixer 4b is achieved by the common matching circuit 4a. 4b
The second mixer of 2 passes enough DC current, and the 2nd mixer 9
It becomes possible to input a sufficient amount of the second local signal LO ₂ from the port of LO 2, thereby increasing the gain and linearity of the mixer. (Conventionally, the linear characteristics were short because the second local signal could not be sufficiently input as described above.) By improving the linearity as described above, the input and output impedances of the port and the second mixer 4b are reduced. is constant regardless of the input level, and BPF3
Since matching of BPF5 can be established, the wave characteristics (phase and amplitude) of both BPFs can be utilized.
A receiver that generates a demodulated signal with high input resistance and a low demodulation distortion factor can be obtained.

When the second mixer circuit in Figure 3 is used, the input signal passes through the 2-way divider 9 and is attenuated by 3 dB, resulting in a 3 dB deterioration in the noise figure, but in the case of the receiver with the configuration in Figure 1, the receiver is The noise figure F is expressed by the following equation.

F=1/l ₁ (F ₂ +1/l ₃・1/l ₉ -1/G ₂ +F ₄ -1/G ₂ l ₃ l ₉ +...) Here, l ₁ is the attenuation of BPF1, F ₂ is the noise figure of the first mixer alone, _l3 is the attenuation of BPF3, _l9 is the attenuation of the two-way divider = approximately 3 dB, _F4 is the noise figure of the second mixer alone, and _G2 is the gain of the first mixer. do.

The above equation for F becomes F=1/l ₁ F ₂ if G ₂ >>1, so there is no effect of the two-way divider.

Note that even if the reflection coefficient seen from the port of the two-way divider 9 to the second mixer is somewhat bad, the reflection coefficient seen from the port is improved, so there is an advantage that matching of the filter 3 becomes easier. This is proved as follows. Now, if the input voltage of the port is a, the output voltage of the port or the input voltage of mixer 4b is b, the reflection coefficient of the mixer is Γb, the reflected voltage of mixer 4b is c, and the reflected voltage of the port is d, then b=a/
√2 (due to 3dB attenuation with 2 dividers), c=Γ _b b,
d=c/√2, and the reflection coefficient Γ _a at the port
is Γ _a = d/a = Γ _b /√2・a/√2・1/a=
Γ _b /2, that is, by connecting the second mixer to the distributor, the reflection coefficient of the single mixer is half (return loss improved by 6 dB), and matching of the filter 3 becomes easier.

(Effect of the invention) By using the second mixer circuit of the invention, the input and output impedances of the mixer do not change even when a high electric field is input, so the performance of the phase linear filter, which is sensitive to termination conditions, can be brought out as is. As a result, we were able to create a receiver with high input resistance and low demodulation distortion. Such a receiver has a wide range of applications, such as a car radio receiver where electric field strength changes significantly.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a configuration example of a general conventional receiver, FIG. 2 is a diagram showing a configuration example of a conventional second mixer circuit, and FIG. 3 is a diagram showing a configuration example of a second mixer circuit according to the present invention. 1... Broadband filter, 2... First mixer (mixer), 3, 5... Narrowband linear phase filter,
4...Second mixer, 6...IFA, 7...Detector,
8...AFA, 9...2 distributor, 4a...Matching circuit, 4b...Emitter grounding mixer, LO ₁ ,
LO ₂ ...Input from local oscillator 1 and 2.

Claims (1)

[Scope of utility model registration request] A second frequency mixing circuit of a receiver having phase linear bandpass filters at the front and rear stages is configured such that the input supplied from the front-stage bandpass filter and the local oscillation voltage are two independent inputs, and the impedance seen from the respective input terminals. is constant regardless of the terminal impedance of each of the two inputs, and each input is attenuated by approximately 3 dB and is output by a two-way divider, followed by a matching circuit and a grounded-emitter transistor mixed circuit. A second frequency mixing circuit of the receiver.