JPH04343504A - Frequency conversion circuit - Google Patents

Abstract

PURPOSE:To attain gain adjustment without sacrificing impedance matching with a filter by driving an oscillator filter through a buffer circuit for inputting the output of an intermediate frequency transformer. CONSTITUTION:The intermediate frequency transformer(IFT) 2 is controlled by a frequency converter 11 in a frequency conversion IC 1. A ceramic filter 3 to be the oscillator filter is driven through a buffer circuit 12 in the IC 1 which inputs the output of the IFT 2. Since the filter 3 is driven through the circuit 12 without directly driving the filter 3 by the IFT 2, a frequency conversion circuit capable of executing gain adjustment without sacrificing impedance matching with the filter 3 and independently of the characteristics of the filter 3 and reducing the deterioration of frequency characteristics can be obtained.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a frequency conversion circuit.
In particular, the present invention relates to frequency conversion circuits used in receiver tuners.

0002

[Prior Art] A conventional frequency conversion circuit of this type is shown in FIG.
As shown in , the frequency conversion IC4 and the intermediate frequency transformer (
IFT) 2 and a ceramic filter 3. The frequency conversion IC 4 converts the frequency f of the input signal S
A frequency converter 11 is provided to convert the frequency to an intermediate frequency that is the difference or sum of the frequency S and the local oscillation frequency fL. IFT2 consists of a primary side coil L21, a secondary side coil L22, and a capacitor C21, and as a load of the frequency converter 11, a tuned circuit consisting of a coil L21 whose resonant frequency is tuned to the intermediate frequency and a capacitor C21 is used. have. The ceramic filter 3 is a well-known piezoelectric filter using a ceramic piezoelectric element, which is one of the vibrating circuit elements, and has a steep bandpass characteristic with respect to an intermediate frequency. Other examples of this type include crystal filters, mechanical filters using electromechanical vibration elements, and surface acoustic wave filters using surface acoustic wave elements.

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

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

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 the 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 coil L21 on the primary side of the IFT2 whose terminals are connected to the collectors of the transistors Q11, Q13 and Q14, Q12 of the differential circuit, respectively. Coil L21 and capacitor C21 constitute a tuning circuit for 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 coils L21 and L22, equivalent resistance RIF
is a circuit connected in parallel with the coil L21. The number of turns of coil L21 is n1, and the number of turns of coil L22 is n2.
Then, the impedance Z seen from the secondary side of IFT2 is
1 is expressed by the following formula.

[0007]

[0008] The equivalent resistance RIF is usually about several tens of kilohms. Moreover, the output impedance of the frequency converter 11 is
Since it is the output impedance of the collector of the transistor, it is usually on the order of several MΩ, and therefore, the impedance of the primary coil L21 of the IFT2 during resonance is approximately 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 side and the output side. Therefore, the input resistance RIN and output load resistance ROUT, respectively, are required to be appropriately set. Therefore, it is necessary to match so that the output impedance of IFT2, that is, the impedance Z1 seen from the secondary side, has the same value as the input resistance RIN. Therefore, the ratio of the number of turns n1 of the coil L21 of IFT2 to the number of turns n2 of the coil L22, that is, the number of turns ratio n2/n
1 will be adjusted.

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

[0011]

Further, the mutual conductance gm of the frequency converter 11 is expressed by the following equation.

[0013]

As shown in equations (1) and (2), the gain G of the frequency conversion circuit 11 is equal to the equivalent resistance RIF of the IFT2.
, the input impedance RF of the ceramic filter 3, the turns ratio n2/n1 and the mutual conductance gm of the frequency converter 11. However, the equivalent resistance RIF and the turns ratio n2/n1 can be set by the input impedance R of the IFT2
Since F is a value limited by the element structure of the ceramic filter 3, the gain G changes depending on the type of IFT 2 and the ceramic filter 3. Therefore, usually
The gain G of the frequency conversion circuit 11 is gm, that is, the resistance R1
Adjustments were made using 1.

When converting the frequency conversion circuit 11 into an IC, g
The resistor R11 that sets m is also fixed. At this time, since the gain G is generally set as the highest priority, I
The output impedance Z1 of FT2 is not necessarily matched to the same value as the input resistance RIN of the ceramic filter. If impedance matching cannot be achieved, for example, for ideal characteristics as shown in FIG. 7(B), the undesired signal fU, which is separated by Δf, will be larger than the desired signal fD shown in FIG. 7(A). This resulted in deterioration of frequency characteristics.

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 connected between the output side of the IFT 2 and the input side of the ceramic filter 3. 3
It has been practiced to insert an impedance matching circuit consisting of an input impedance RF and a parallel resistance equal to the parallel connection resistance value of this series resistance. Normally, the series resistance value is about 5Ω, and the ceramic filter 3
If the input impedance RF of is 330Ω, the value of the parallel resistance is 350Ω. However, because this impedance matching circuit attenuates the signal level by about 23 dB, it is necessary to increase the gain of the subsequent stages.

[0017]

[Problems to be Solved by the Invention] The conventional frequency conversion circuit described above sets the gain as the top priority, so the IFT
Since the output impedance of the ceramic filter is not necessarily matched to the same value as the input resistance of the ceramic filter, there is a drawback that the bandpass characteristics of the ceramic filter deteriorates. Another disadvantage is that when a matching circuit consisting of a series resistor and a parallel resistor network is inserted to avoid deterioration of the bandpass characteristics of the ceramic filter, the gain decreases.

[0018]

[Means for Solving the Problems] The frequency conversion circuit of the present invention includes a frequency converter that converts a first frequency, which is the frequency of a high-frequency signal, and a second frequency, respectively, into an intermediate frequency that is the difference or sum of the second frequency. A frequency conversion circuit comprising an intermediate frequency transformer connected as a load on the output side of the frequency converter and tuned to the intermediate frequency, and a resonant circuit element filter using a resonant circuit element that is a bandpass filter for the intermediate frequency, It is configured to include a buffer circuit that inputs the output of the intermediate frequency transformer and drives the resonant circuit element filter.

[0019]

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

FIG. 1 is a block diagram showing an 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 IC1, 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 example of the prior art mentioned above, and the explanation will be omitted here because the explanation will be redundant.

FIG. 2 is a circuit diagram showing an example of the frequency converter 11 and buffer circuit 12 of the frequency conversion IC 1. In FIG. 2, the frequency converter 11 is the same as the conventional example, and includes transistors Q11 to Q16 and a current source I1.
1, I12 and a resistor R11, which is a well-known Gilbert multiplication circuit. The buffer circuit 12 inputs the output of the IFT2 via the capacitor 1 and drives the ceramic filter via the resistor R26, and includes a transistor Q21, a resistor R21,
Input circuit 121 consisting of R22, power supply V21, and current source I21, transistors Q22, Q23, and current source I22
, I23, a current mirror circuit 123 that is a load of the differential amplifier and includes transistors Q24, Q25, and resistors R23, R24, and a current source I24 that includes transistors Q26, resistors R25, R26, and current source I24. It is composed of an output circuit 124.

Next, the operation of this embodiment will be explained.

First, the frequency converter 1 of the frequency conversion IC 1
1 and IFT2 are operated by coil L2 on the secondary side of IFT2.
The load of 2 is the resistance R of the input circuit 121 of the buffer circuit 12.
21 is the same as the above-mentioned conventional example, and the explanation will be omitted here since it will be redundant.

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

[0029]

As is clear from equation (5), even when impedance matching is achieved with the ceramic filter 3, the frequency conversion circuit of this embodiment has the same number of turns and resistance of the coils L21 and L22 on the primary and secondary sides of the IFT2. The gain can be freely set with the resistance value of 21. Therefore, frequency converter 1
Even if the mutual conductance gm of 1 is fixed, the gain can be adjusted without sacrificing impedance matching with the ceramic filter 3, so deterioration of frequency characteristics can be prevented.

Furthermore, since there is no need for a matching circuit consisting of a series resistor and a parallel resistor network for impedance matching, there is no loss of gain due to this.

[0032]

As explained above, the frequency conversion circuit of the present invention has a buffer circuit that inputs the output of the intermediate frequency transformer and drives the oscillating circuit element filter.
Since the gain can be adjusted without sacrificing impedance matching with the resonant circuit element filter, there is an effect that deterioration of frequency characteristics can be prevented. Furthermore, there is an effect that the gain can be adjusted regardless of the characteristics of the resonant circuit element filter.

[Brief explanation of the drawing]

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

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

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

[Figure 4] Frequency conversion IC in a conventional frequency conversion circuit
FIG. 2 is a circuit diagram showing an example of a frequency converter.

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

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

FIG. 7 is a diagram showing ideal and degraded frequency characteristics of a ceramic filter, respectively.

[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 to I23 Current source L21
, L22 Coil Q11~Q16, Q21~Q26 Transistor R11, R21~R26 Resistor

Claims (2)

[Claims] 1. A frequency converter for converting a first frequency, which is the frequency of a high-frequency signal, and a second frequency, respectively, into an intermediate frequency that is the difference or sum of the second frequency; In a frequency conversion circuit that includes an intermediate frequency transformer tuned to an intermediate frequency and a resonant circuit element filter using a resonant circuit element that is a bandpass filter for the intermediate frequency, the output of the intermediate frequency transformer is inputted and the vibration A frequency conversion circuit comprising a buffer circuit that drives a circuit element filter. 2. The buffer circuit includes an emitter follower type input circuit, a differential amplifier whose one input is driven by this input circuit, loads a current mirror circuit, and whose output signal is fed back to the other input; 2. The frequency conversion circuit according to claim 1, further comprising an emitter follower type output circuit for outputting the output of the amplifier, and a resistor for input impedance matching of the oscillating circuit element filter.