JP2972379B2 - Frequency conversion circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency conversion circuit,
In particular, the present invention relates to a frequency conversion circuit used for a tuner of a receiver.

[0002]

2. Description of the Related Art A conventional frequency conversion circuit of this type is shown in FIG.
As shown in (1), the frequency conversion IC 4, the intermediate frequency transformer (IFT) 2, and the ceramic filter 3 were included. The frequency conversion IC 4 includes a frequency converter 11 that converts the frequency fS of the input signal S and the local oscillation frequency fL into an intermediate frequency that is a difference or sum frequency. The IFT 2 includes a primary coil L21, a secondary coil L22, and a capacitor C21. As a load of the frequency converter 11, a tuning circuit including a coil L21 whose resonance frequency is tuned to an intermediate frequency and a capacitor C21 is provided. Have. The ceramic filter 3 is a well-known piezoelectric filter using a ceramic piezoelectric element, which is one of the vibration circuit elements, and has a steep band-pass characteristic with respect to an intermediate frequency. Other examples of this type include a mechanical filter using a crystal filter or an electromechanical vibration element, and a surface acoustic wave filter having a surface acoustic wave element.

FIG. 4 is a circuit diagram showing an example of the frequency converter 11 of the frequency conversion IC 4. In FIG. 4, the frequency converter 11 is constituted by a well-known Gilbert multiplication circuit using two differential circuits including transistors Q11 to Q16, current sources I11 and I12, and a resistor R11.

Next, the operation of the conventional frequency conversion circuit will be described.

[0005] The input signal S and the local oscillation signal L are multiplied by a frequency converter 11 which is a multiplication circuit, and converted into an intermediate frequency fI which is a difference between the frequency fS and the local oscillation frequency fL. The intermediate frequency fI, which is the output signal of the frequency converter 11, is output to the primary coil L21 of the IFT 2 whose terminals are connected to the collectors of the transistors Q11, Q13 and Q14, Q12 of the differential circuit. The coil L21 and the capacitor C21 form a tuning circuit for the intermediate frequency fI. The primary coil L21 and the secondary coil L22 are inductively coupled.

FIG. 5 is a diagram showing an equivalent circuit of IFT2. IFT2 is a capacitor C2 which is an actual circuit element.
1 and the coils L21 and L22, the equivalent resistance RIF
Is a circuit connected in parallel with the coil L21.
The number of turns of the coil L21 is n1, and the number of turns of the coil L22 is n2.
Then, the impedance Z seen from the secondary side of IFT2 is
1 is represented by the following equation.

[0007]

The equivalent resistance RIF is usually on the order of several tens of KΩ. The output impedance of the frequency converter 11 is
Since this is the output impedance of the collector of the transistor, it is usually on the order of several MΩ. Therefore, the impedance at the time of resonance of the coil L21 on the primary side of the IFT 2 is substantially determined by the equivalent resistance RIF.

FIG. 6 is a diagram showing an equivalent circuit of the ceramic filter 3. As is well known, the frequency characteristics of the ceramic filter 3 are critical to changes in impedance on the input and output sides. Therefore, an appropriately set input resistance RIN and output load resistance ROUT are required. Therefore, the output impedance of IFT2, that is, the impedance Z1 viewed from the secondary side is
It is necessary to perform matching so as to have the same value as the input resistance RIN. Therefore, the ratio of the number of turns n1 of the coil L21 of the IFT2 to the number of turns n2 of the coil L22, that is, the turns ratio n2 /
n1 will be adjusted.

On the other hand, the gain G of the frequency conversion circuit 11 is expressed by the following equation, where gm is the transconductance of the frequency converter 11 and RF is the input impedance of the ceramic filter 3.

[0011]

The transconductance gm of the frequency converter 11 is expressed by the following equation.

[0013]

As shown in the equations (1) and (2), the gain G of the frequency conversion circuit 11 is equal to the equivalent resistance RI of the IFT 2.
F, the input impedance RF of the ceramic filter 3;
The turn ratio n2 / n1 and the mutual conductance gm of the frequency converter 11 can be set. However, the equivalent resistance RIF, the turn ratio n2 / n1 is limited to the IFT2, and the input impedance RF is limited to the element structure of the ceramic filter 3, respectively. Since the value is a value, the gain G changes depending on the type of the IFT 2 and the ceramic filter 3. Therefore, normally, the gain G of the frequency conversion circuit 11 is adjusted by gm, that is, the resistor R11.

When the frequency conversion circuit 11 is formed into an IC, g
The resistor R11 for setting m is also fixed. At this time, since the gain G is generally set to the highest priority,
Matching is not always performed so that the output impedance Z1 of the IFT 2 is equal to the input resistance RIN of the ceramic filter. When the impedance cannot be matched, for example, the ideal characteristic shown in FIG. 7B is smaller by Δf than the desired signal fD shown in FIG.
In this case, the frequency characteristics of the undesired signal fU which is farther away are deteriorated.

In order to prevent such deterioration of the frequency characteristics, a series resistor having a resistance sufficiently higher than the output impedance Z1 of the IFT 2 and a ceramic filter are provided between the output side of the IFT 2 and the input side of the ceramic filter 3. 3
In this case, an impedance matching circuit including an input impedance RF and a parallel resistance equal to the parallel connection resistance of the series resistance is inserted. Typically, the value of the series resistance is about 5 K Omega, if the input impedance RF ceramic filters 3 and 330 ohms, the value of the parallel resistance becomes 350 ohms. However, since the signal level is attenuated by about 23 dB by the impedance matching circuit, it is necessary to increase the gain of the next and subsequent stages.

[0017]

In the conventional frequency conversion circuit described above, the gain is set to the highest priority.
Are not always matched so that the output impedance of the ceramic filter becomes equal to the input resistance of the ceramic filter, so that the bandpass characteristic of the ceramic filter is deteriorated. Further, when a matching circuit including a series resistor and a parallel resistor network is inserted in order to avoid deterioration of the bandpass characteristic of the ceramic filter, there is a disadvantage that the gain is reduced.

[0018]

According to the present invention, there is provided a frequency conversion circuit for converting a frequency of a high frequency signal into an intermediate frequency which is a difference or a sum of a first frequency and a second frequency . an intermediate frequency transformer is connected as a load on the output side of the frequency converter and tuned to the intermediate frequency, and a vibration circuit elements filter using an oscillating circuit element is a band pass filter for the intermediate frequency, the intermediate
The oscillator circuit element responsive to the supply of the output of the frequency transformer.
A frequency conversion circuit comprising a buffer circuit for driving the filter , wherein the buffer circuit has an emitter follower type.
An input circuit and one input is driven by this input circuit
A differential amplifier output signal is fed back to the input of the differential
A current mirror circuit constituting the load of the amplifier, the difference
Emitter-follower type output circuit that outputs the output of a dynamic amplifier
And adjusting the input impedance of the oscillation circuit element filter.
It is provided with a combined resistor .

[0019]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the frequency conversion circuit of the present invention.

As shown in FIG. 1, the frequency conversion circuit of this embodiment includes a frequency conversion IC 1, an intermediate frequency transformer (IFT) 2 having coils L21, L22 and a capacitor C21.
And a ceramic filter 3.
The frequency conversion IC 1 is a frequency converter 1 similar to the conventional example.
1 and a buffer circuit 12. The other components are the same as those described in the above-described example of the related art, and the description thereof will not be repeated here.

FIG. 2 is a circuit diagram showing an example of the frequency converter 11 and the buffer circuit 12 of the frequency conversion IC 1. In FIG. 2, the frequency converter 11 is the same as that of the conventional example, and includes transistors Q11 to Q16 and a current source I1.
It comprises a well-known Gilbert multiplication circuit using two differential circuits comprising I1, I12 and a resistor R11.
The buffer circuit 12 receives the output of the IFT 2 via the capacitor 1 and drives the ceramic filter via the resistor R26, and includes a transistor Q21 and a resistor R21,
An input circuit 121 including an R22, a power supply V21, and a current source I21; transistors Q22 and Q23;
2 and I23, a current mirror circuit 123 including transistors Q24 and Q25 and resistors R23 and R24 and serving as a load of the differential amplifier, a transistor Q26, resistors R25 and R26, and a current source I24. And an output circuit 124.

Next, the operation of this embodiment will be described.

First, the frequency converter 1 of the frequency conversion IC 1
1 and the operation of IFT2 are performed by the coil L2 on the secondary side of IFT2.
2 is the resistance R of the input circuit 121 of the buffer circuit 12.
Other than 21 is common to the above-mentioned conventional example, and the description will be omitted here because it is duplicated.

Next, the turns ratio n2 / n1 of the coils L21 and L22 of the IFT2 and the equivalent resistance of the primary side of the IFT2 are expressed by RI
F, the input impedance of the ceramic filter 3 is R
F, the output resistance of the output circuit 124 of the buffer circuit 12 is represented by a resistor R26, and the mutual conductance gm of the frequency converter 11
Then, the gain Gt of the entire frequency conversion circuit is expressed by the following equation.

[0026]

As described above, in order to achieve impedance matching with the ceramic filter 3, it is necessary to satisfy the following condition.

R26 = RF (5) Therefore, when impedance matching is performed, the gain of the entire frequency conversion circuit is obtained. GT is represented by the following equation.

[0029]

As is apparent from the equation (5), the frequency conversion circuit of the present embodiment has the number of turns and the resistance of the primary and secondary coils L21 and L22 of the IFT 2 even when the impedance is matched with the ceramic filter 3. With the resistance value of 21, the gain can be set freely. Therefore, the frequency converter 1
Even if the mutual conductance gm is fixed, the gain can be adjusted without sacrificing the impedance matching with the ceramic filter 3, so that the deterioration of the frequency characteristic can be prevented.

Further, since a matching circuit including a series resistor and a parallel resistor network for impedance matching is not required, the gain does not decrease.

[0032]

As described above, the frequency conversion circuit of the present invention includes the buffer circuit that receives the output of the intermediate frequency transformer and drives the oscillation circuit element filter.
Since the gain can be adjusted without sacrificing impedance matching with the oscillation circuit element filter, there is an effect that deterioration of frequency characteristics can be prevented. Further, there is an effect that the gain can be adjusted independently of the characteristics of the oscillation circuit element filter.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of a frequency conversion circuit of the present invention.

FIG. 2 is a circuit diagram illustrating an example of a frequency converter and a buffer circuit of the frequency conversion IC in the frequency conversion circuit of the present embodiment.

FIG. 3 is a block diagram illustrating an example of a conventional frequency conversion circuit.

FIG. 4 is a frequency conversion IC in a conventional frequency conversion circuit.
FIG. 3 is a circuit diagram showing an example of the frequency converter of FIG.

FIG. 5 is a circuit diagram showing an equivalent circuit of the IFT.

FIG. 6 is a circuit diagram showing an equivalent circuit of the ceramic filter.

FIG. 7 is a diagram showing ideal and deteriorated frequency characteristics of a ceramic filter.

[Explanation of symbols]

 1,4 Frequency conversion IC 2 IFT 3 Ceramic filter 11 Frequency converter 12 Buffer circuit 121 Input circuit 122 Differential circuit 123 Current mirror circuit 124 Output circuit C1, C21 Capacitor I11, I12, I21-I23 Current source L21, L22 Coil Q11 To Q16, Q21 to Q26 Transistor R11, R21 to R26 Resistance

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03D 7/00-9/06

Claims (3)

(57) [Claims] 1. A frequency converter for converting an intermediate frequency, which is a difference or a sum between a first frequency and a second frequency, which are frequencies of a high-frequency signal, respectively, and a primary converter connected to an output side of the frequency converter. Side coil,
A capacitor connected between both ends of the primary coil;
And the secondary coil inductively coupled to the primary coil.
And providing a signal based on the intermediate frequency to the secondary coil.
An intermediate frequency transformer for outputting to a first node via a first node, a first resistor element connected between the first node and a power supply
And an output signal of the intermediate frequency transformer connected to the first node.
For outputting a drive signal responsive to a signal to a second node
A circuit and an impedance connected between the second and third nodes.
A second resistance element for inductance matching, and one end connected to the third node,
Driven and a band-pass characteristic with respect to the driving signal
And a filter having the following . 2. The impedance of said second resistance element is
Note that it is the same as the input impedance of the filter.
The frequency conversion circuit according to claim 1, wherein 3. An input circuit comprising a first power supply line and a fourth power supply line.
And the control terminal is connected to the first node.
A first transistor of one conductivity type continued and
A first current connected between the node and a second power line
Source and a connection between the first power supply line and a fifth node
And a control terminal connected to the fifth node and having a second conductivity type.
The second transistor, the first power supply line and the sixth
And the control terminal is connected to the fifth node.
A third transistor of the second conductivity type continued,
A control terminal connected between the fifth node and the common node,
A fourth transistor of the one conductivity type connected to a fourth node;
Connected between the sixth node and the common node.
The control terminal is connected to the third node.
Tiger Njisuta of 5, the second power source La and the common node
A second current source connected between the first current source and the first power source.
A control terminal connected between the source line and the third node
The sixth conductor of the one conductivity type connected to the sixth node
Transistor, the third node, and the second power supply line.
And a third current source connected between
3. The frequency conversion circuit according to claim 1, wherein: