JP3149615B2 - AM receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AM receiver, and more particularly to an AM receiver having an automatic search function and having a high search speed during a search operation.

[0002]

2. Description of the Related Art A having a conventional search sensitivity switching function
The M receiver will be described with reference to FIG.

In FIG. 4, reference numeral 1 denotes an antenna for receiving an AM signal from a broadcasting station; 2, a high-frequency amplifier that amplifies the AM signal input from the antenna 1 and outputs the amplified signal to a mixer 3; A mixer 3 mixes the output of the high-frequency amplifier 2 and the output of the local oscillator 4 to generate an intermediate frequency signal, and outputs the signal to the intermediate frequency amplifier 20. Reference numeral 4 denotes a local oscillator, and a PLL circuit 1
The oscillation frequency is continuously varied in accordance with the DC voltage input from the mixer 9, and the oscillation output is output to the mixer 3 and the PL.
Output to L circuit 19. Reference numeral 20 denotes an intermediate frequency amplifier, which amplifies an intermediate frequency signal input from the mixer 3 by continuously varying the degree of amplification according to a DC voltage input from an IFAGC circuit (intermediate frequency automatic gain control circuit) 6. . Reference numeral 13 denotes a detector, which performs envelope detection on the AM output of the intermediate frequency amplifier 20 and outputs the result to the amplifier 14 and the level detector 18.

[0004] The amplifier 14 amplifies the output of the detector 13 and outputs it to the speaker 15. The level detector 18 extracts a DC component of the output of the detector 13 using an LPF including a resistor 16 and a capacitor 17 and outputs the DC component to the IFAGC circuit 6. The IFAGC circuit 6 includes an arbitrary DC voltage source (voltage V ₆ ) therein. When the DC voltage output of the level detector 18 becomes larger than the arbitrary voltage V ₆ , the DC voltage applied to the intermediate frequency amplifier 20 is increased. (AGC voltage) to decrease the gain of the intermediate frequency amplifier 20, and the level detector 1
8 is maintained at the same potential as the voltage V ₆ , the DC voltage applied to the intermediate frequency amplifier 20 when the DC voltage output of the level detector 18 is smaller than the voltage V ₆ , and the DC voltage of the level detector 18 When the output is higher than the voltage V ₆ , the difference voltage from the DC voltage applied to the intermediate frequency amplifier 20 is output to the signal meter circuit 7. The signal meter circuit 7 adds the output of the detector 13 and the output of the IFAGC circuit 6 and outputs the result to the comparator 10 as a signal meter output.

[0005] Reference numerals 8 and 22 denote reference voltage generating circuits, which respectively generate DC voltages V ₈ and V ₂₂ that can be set freely and output them to the search sensitivity switch 21. Search sensitivity switch 21
Selects either one of the DC voltages V ₈ and V ₂₂ input from the reference voltage generation circuits 8 and ₂₂ by a search sensitivity switching signal from the outside, and outputs it to the comparator 10. The comparator 10 compares the voltage output of the signal meter circuit 7 with the voltage output of the search sensitivity switch 21 and, when the voltage output of the signal meter circuit 7 is larger, outputs a search stop command signal to a high level voltage or It is given to the frequency sweeper 11 as a low level voltage. The frequency sweeper 11 starts sweeping the oscillation frequency in response to a search start command signal externally given as a low level voltage or a high level voltage, and stops sweeping the oscillation frequency in response to a search stop command signal from the comparator 10. While maintaining the oscillation frequency at that time, the oscillation output is output to the PLL circuit 19. PL
The L circuit 19 controls the local oscillator 4 so that the oscillation frequency of the local oscillator 4 becomes equal to the oscillation frequency of the frequency sweeper 11.
Adjust the DC voltage applied to.

FIG. 5 shows a conventional signal meter input / output characteristic. The horizontal axis represents antenna (ANT) input. When the antenna input exceeds V _AGC, the output voltage of the level detector 18 and the output of the detector 13 are kept constant. (A)
Shows the voltage input from the detector 13 to the signal meter circuit 7, (b) shows the voltage input from the IFAGC circuit 6 to the signal meter circuit 7, and (c) shows the signal meter output.

To set the search sensitivity to V ₁ and V ₂ , the generated voltages V ₈ and V _{22 of the} reference voltage generating circuits 8 and _{22 are used.}
May be set to the same voltages as V _S1 and V _{S2 in} FIG. 5, respectively.

[0008] In automatic search, while sweeping the reception frequency by the frequency sweeper 11, and signal meter output reference voltage V ₈ is selected in the search sensitivity switcher 21 of the reference voltage generating circuit 8, 22 or the compared by the comparator 10 and V _22, the antenna input increases, the signal meter output voltage is greater than the output voltage of the search sensitivity switching unit 21, the instruction signal from the comparator 10 is output a frequency sweeper 11 sweep Stop and fix the tuning frequency. That is, switching of the search sensitivity is done by selecting one of the voltage V ₈ or V ₂₂ by the search sensitivity switching device 21.

[0009]

[SUMMARY OF THE INVENTION However, when performing a search by setting a search sensitivity voltage V ₂ of Figure 5,
Since the level detector 18 has a time constant (τ = CR) determined by the resistor 16 and the capacitor 17, if the sweep speed of the frequency sweeper 11 is too fast, the level detector 18 will respond to a sudden change in the antenna input. As a result, the voltage applied from the IFAGC circuit 6 to the signal meter circuit 7 in FIG. 5 (b) does not rise, and accurate search cannot be performed. For this reason, the upper limit of the sweep speed is determined by the time constant τ of the level detector 18, which is restricted in increasing the sweep speed.

[0010]

SUMMARY OF THE INVENTION The present invention provides an IFAGC circuit for automatically adjusting the output of an intermediate frequency amplifier, a signal meter circuit for producing a signal meter output from the output level of the intermediate frequency amplifier and the AGC voltage of the IFAGC circuit, In an AM receiver equipped with a comparator that compares the meter output with a reference voltage, an auto search function that uses the output of the comparator, and a search sensitivity switch, the open gain of the intermediate frequency amplifier is used when the search sensitivity is switched. Is provided, and the search sensitivity is switched by switching the reference voltage and the open gain of the intermediate frequency amplifier.

[0011]

The search sensitivity can be set to a high input level without using the AGC voltage of the IFAGC circuit by lowering the gain of the intermediate frequency amplifier at the time of search, so that the search speed can be increased.

[0012]

FIG. 1 is a block diagram showing an AM receiver according to an embodiment of the present invention.

In FIG. 1, reference numeral 9 denotes a reference voltage generation circuit.
The DC voltage V ₉ is generated and output to the search sensitivity switch 12. The search sensitivity switch 12 selects one of the DC voltages V ₈ and V ₉ supplied from the reference voltage generating circuits 8 and 9 in response to an external search sensitivity switching signal, and outputs a signal to the comparator 10. And a control signal for switching the open gain to the intermediate frequency amplifier 5.
The intermediate frequency amplifier 5 receives the control signal from the search sensitivity switch 12 and switches to a value in which the open gain is set, so that the output curve of the signal meter 7 is shifted by the gain change. The intermediate frequency amplifier 5 is connected to the IFAG
The amplification degree is continuously varied in accordance with the DC voltage input from the C circuit 6, and the intermediate frequency signal input from the mixer 3 is amplified and output to the detector 13.

In this embodiment, elements having the same functions as those of the embodiment shown in FIG. 4 are denoted by the same reference numerals.

FIG. 2 is a diagram showing the input / output characteristics of the signal meter in this embodiment. The horizontal axis is the antenna (AN
T) represents input.

The voltage curve input from the detection circuit 13 to the signal meter circuit 7 in response to an external search sensitivity switching signal shifts from (a) to (e), and is input from the IFAGC circuit 6 to the signal meter circuit 7. voltage curve shifts from (b) to (f), and the shift signal meter output curve from (c) to (d), also the output voltage and detector of the level detector 18 with the antenna input V _AGC or The output of the level detector 18 and the output of the detector 13 are kept constant when the output of the detector 13 is kept constant but the antenna input is _{equal to} or more than _VAGC2 .
Therefore, by the search sensitivity switching signal to select the signal meter output reference voltage generation circuit 9 (d), when setting the search sensitivity V ₂ in Figure 2, the output voltage V ₉ of the reference voltage generating circuit 9 What is necessary is just to set it to the same voltage as _VS3 . This lowers the gain of the intermediate frequency amplifier 5 during the search,
By setting the search sensitivity to a high input level without using the AGC voltage by the IFAGC circuit 6, the search speed is increased.

FIG. 3 shows an example of the configuration of the intermediate frequency amplifier according to the present embodiment.
2 is a switch that is switched by the control from the second control.

If the antenna input in FIG. 1 is sufficiently low,
The arbitrary voltage V ₆ of the IFAGC circuit 6 is higher than the output of the level detector 18, and the voltage applied from the IFAGC circuit 6 to the base of the transistor 25
When it is sufficiently lower than the voltage applied to the base of 6,
In the differential circuit including the transistors 25 and 26, when the transistor 26 is on,
Is turned off. Thus, most of the collector current of the transistor 29 is supplied from the emitter of the transistor 26. In this state, when the switch 23 is set to the ground side, the diode 28 is applied with a reverse voltage and becomes high impedance, and the collector load of the transistor 29 is only the resistor 27. At this time, the AC gain is the ratio of the resistance values of the resistors 30 and 27. Also, when the switch 23 is on the + V _CC side with the antenna input being sufficiently low, the forward voltage is applied to the diode 28 and the impedance is low, so that the resistance of the resistors 27 and 24 is smaller than the impedance of the diode 28. Is sufficiently high, the collector load of transistor 29 becomes a parallel resistance by resistors 27 and 24. At this time, the AC gain is the ratio of the resistance value of the resistor 30 to the parallel resistance value of the resistors 24 and 27.
That is, the open gain can be switched by the switch 23.

[0019]

As described above, according to the present invention,
Even if the search speed is increased, the search sensitivity can be set to a high level without being affected by the time constant of the level detector.

[Brief description of the drawings]

FIG. 1 is a block diagram of an AM receiver according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram of a signal meter of an AM receiver according to one embodiment of the present invention.

FIG. 3 is a circuit diagram showing an example of a configuration of an intermediate frequency amplifier according to one embodiment of the present invention.

FIG. 4 is a block diagram of an example of a conventional AM receiver.

FIG. 5 is a characteristic diagram of a signal meter of a conventional AM receiver.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Antenna 2 High frequency amplifier 3 Mixer 4 Local oscillator 5 Intermediate frequency amplifier 6 IFAGC (Intermediate frequency automatic gain control) circuit 7 Signal meter circuit 8 Reference voltage generating circuit 9 Reference voltage generating circuit 10 Comparator 11 Frequency sweeper 12 Search sensitivity switching Detector 13 Detector 14 Amplifier 15 Speaker 16 Resistance 17 Capacitance 18 Level detector 19 PLL circuit

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03J 5/00-7/18 H04B 1/16-1/26

Claims (1)

(57) [Claims] An IF for automatically adjusting an output of an intermediate frequency amplifier
AGC circuit, output level of IF amplifier and IFAG
An AM having a signal meter circuit for generating a signal meter output from the AGC voltage of the C circuit, a comparator for comparing the signal meter output with a reference voltage, an auto search function using the output of the comparator, and a search sensitivity switcher. An AM receiver, comprising: means for switching an open gain of an intermediate frequency amplifier when a search sensitivity is switched.