JP2827307B2 - PLL receiver - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to receiving a plurality of reception bands with a single reception circuit.
It relates to a PLL receiver.

Conventional technology In recent years, PLL receivers have very good frequency accuracy and can store the desired receiving frequency, so they are very easy to use and can be electronically controlled. Is coming.

Hereinafter, an example of the above-described conventional PLL receiver will be described with reference to the drawings. Figures 2 and 3 show conventional PLLs
FIG. 3 is a circuit diagram of a receiver and a graph showing tuning frequency versus varicap diode voltage. In FIG. 2, 1 is RF
An amplifier 2, a mixer 3, and an IF amplifier 3 convert a received high-frequency signal into a low-frequency signal. An audio amplifier 4 amplifies a low frequency signal. A speaker 5 converts a low-frequency signal into a sound. 6 is a local oscillator. 7,8
Is a coil. 9 is a varicap diode,
It is composed of eight coils and a resonance circuit and is locally oscillated by six local oscillators. 10 is a diode, 11 is a capacitor, 10
By applying a voltage to the diode, the coil 8 is short-circuited and the receiving band is switched. Reference numeral 12 denotes a local oscillation circuit which switches the reception band. A frequency divider 13 divides the frequency oscillated by the local oscillation circuit. A reference oscillator 14 oscillates a reference frequency of the PLL receiver. A phase comparator 15 detects a phase difference between the output of the frequency divider 13 and the output of the reference oscillator 14, and outputs a phase difference output. 16 and 17 are transistors, 18 is a resistor, 19 is a capacitor, and 20 is a resistor, constituting a low-pass filter 21. Reference numeral 22 denotes a controller for switching the frequency division ratio of the frequency divider 13 and the reception band of the local oscillator 12.

The operation of the PLL receiver configured as described above will be described below. First, the high-frequency signal amplified by the RF amplifier 1 is input to a mixer 2 and converted into an IF signal by a difference from a frequency of 12 local oscillation circuits described later. IF
The signal converted into a signal is amplified by the IF amplifier 3 and converted into a low-frequency signal. Next, it is converted into sound by five speakers amplified by the four audio amplifiers. Next, the local oscillation circuit 12 that determines the reception frequency will be described. First, in the first reception band in which the voltage is not applied to the diode 10 from the control controller 22, the local oscillation circuit 7 connected to the local oscillator 6 is used.
A parallel resonance circuit is oscillated by the inductance of the directly connected coil 8 and the capacitance of the varicap diode 9 connected in parallel to the coils 7, 8. The frequency is determined by the voltage applied to the varicap diode 9.

Next, a system for determining the above-mentioned frequency to a desired reception frequency will be described. The signal oscillated by the 12 local oscillation circuits is divided by the control controller 22 by setting the division ratio of the divider 13 corresponding to the reception frequency. The frequency-divided signal is input to the phase comparator 15 and compared with the frequency of the reference oscillator 14 to output a phase difference output. Its signal is the transistor 16,1
7 is input to the low-pass filter 21 connected to Darlington. The cutoff frequency of this low-pass filter is determined by the time constant of the capacitor 19 and the resistor 20, and converts the phase difference signal into a DC voltage. The converted DC voltage is fed back to the varicap diode 9 to fix the oscillation frequency to a desired frequency. In the first receiving band, the control controller 22 can control the frequency from 76 MHz to 108 MHz, and as shown in the graph of FIG.
The varicap diode voltage changes up to 6.5V.

Next, a description will be given of the case of the second reception band in which the reception controller 10 applies the reception band switching voltage to the diode 10.
When a voltage is applied to the diode 10, a current flows through the coil 8. As a result, the diode 10 is short-circuited, and the coil 8 is short-circuited in an alternating manner through the capacitor 11. That is, the oscillation frequency of the local oscillation circuit 12 is
And a varicap diode 9 and a parallel resonance circuit, the inductance is lower than in the first reception band, the oscillation frequency is higher, and the second reception band can be selected. Hereinafter, the description of the operation will be omitted because it is the same as that of the first reception band, but this second reception band is 175 MHz to 220 MHz as shown in the graph of FIG.
Until the varicap diode voltage is 3V
~ 7V.

As described above, the tuning frequency is changed by changing the varicap diode voltage.As a design point of the varicap diode voltage, the tuning voltage must be designed within the minimum voltage and the maximum voltage of the loop filter 21. No.

In this configuration, the minimum voltage is the sum of the V _CE voltage of the transistor 17 and the V _BE voltage of the transistor 18 and is about 0.7 V,
This is point a in the graph of FIG. The maximum voltage is determined by V _CC applied to the loop filter 21, and in this example, it becomes 8.5V at the point c in the graph of FIG. 3, which indicates that the tuning voltage is covered.

However, in the above configuration, when the second reception band is received, the coil component of the oscillation circuit is small, the capacity of the varicap diode 9 is large, and Q is reduced. In addition, the diode 10 and the capacitor 11 enter into the resonance circuit loop and are dumped, so that the Q is further reduced and the oscillation is hardly performed. When the reception band is switched in such a state, if the output voltage of the loop filter 21 drops to the minimum voltage, the capacity of the varicap diode 9 becomes extremely large, the Q value decreases, and oscillation may stop. In this state, there is a quality problem that it is often impossible to return to the desired frequency again, when there is no signal for comparing the phases.

The present invention has been made in view of the above problems, and provides a PLL receiver in which a decrease in Q of an oscillation circuit is reduced and a quality problem that reception becomes impossible due to oscillation stop is eliminated.

Means for Solving the Problems To solve the above problems, the PLL receiver of the present invention is:
A switching circuit in which a constant voltage element is inserted into a ground terminal of a loop filter and a switching circuit is provided in parallel with the constant voltage element, and the switching circuit is switched by a control controller in synchronization with switching of a local oscillation circuit. It is.

Operation The present invention, with the above-described configuration, the constant voltage element enters the ground terminal of the loop filter, so that the minimum voltage of the loop filter rises, the voltage applied to the varicap diode does not decrease unnecessarily, and the capacity does not become too large. That is, the Q of the oscillation circuit is not reduced unnecessarily.

Embodiment Hereinafter, a PLL receiver according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a PLL receiver according to the present invention, and FIG. 3 is a graph showing frequency versus varicap diode voltage. In FIG. 1, 1 is an RF amplifier, 2 is a mixer, and 3 is an IF amplifier, which converts a received high-frequency signal into a low-frequency signal. An audio amplifier 4 amplifies a low frequency signal. A speaker 5 converts a low-frequency signal into a sound. 6 is a local oscillator. 7, 8 are coils. Reference numeral 9 denotes a varicap diode, which includes coils 7 and 8 and a resonance circuit, and locally oscillates with a local oscillator 6. Reference numeral 10 denotes a diode, and reference numeral 11 denotes a capacitor. When a voltage is applied to the diode 10, the coil 8 is short-circuited to switch the receiving band. Reference numeral 12 denotes a local oscillation circuit which switches the reception band. A frequency divider 13 divides the frequency oscillated by the oscillation circuit. 14
Causes a reference oscillator of a PLL receiver to oscillate. 1
Reference numeral 5 denotes a phase comparator which detects a phase difference between the output of the frequency divider 13 and the output of the reference oscillator 14, and outputs a phase difference output. 16,17
Is a transistor, 18 is a resistor, 19 is a capacitor, and 20 is a resistor, constituting a low-pass filter 21. 25 is a ground terminal of the low-pass filter. A constant voltage element 26 is connected between the ground terminal of the low-pass filter and the ground. A transistor 27 is connected in parallel with the constant voltage element 26 to form a switching circuit. Reference numeral 22 denotes a control controller for switching the frequency division ratio of the frequency divider 13 and the reception band of the local oscillator 12 and controlling the switching of the switching circuit.

The operation of the PLL receiver configured as described above will be described below. First, the high-frequency signal amplified by the RF amplifier 1 is input to a mixer 2 and converted into an IF signal by a difference from a frequency of a local oscillation circuit 12 described later. The signal converted into the IF signal is amplified by the IF amplifier 3 and converted into a low frequency signal. Next, it is amplified by the audio amplifier 4 and converted into sound by the speaker 5. Next, the local oscillation circuit 12 that determines the reception frequency will be described. First, in the first reception band in which no voltage is applied to the diode 10 from the control controller 22, the series connection of the 7, 8 connected to the local oscillator 6 is performed. The inductance of the connected coil and its coil 7,
A parallel resonance circuit is oscillated by the capacitance of the varicap diode 9 connected in parallel to 8. The frequency is determined by the voltage applied to the varicap diode 9.

Next, a system for determining the above frequency to a desired frequency will be described. The signals oscillated by the 12 local oscillator circuits are
The frequency divider 13 corresponding to the reception frequency by the control controller 22
Is set and the frequency is divided. The frequency-divided signal is input to the phase comparator 15 and compared with the frequency of the reference oscillator 14 to output a phase difference output. The signal is applied to transistors 16, 17
Is input to the low-pass filter 21 connected to Darlington. The cutoff frequency of this low-pass filter is determined by the time constant of the capacitor 19 and the resistor 20, and converts the phase difference signal into a DC voltage. The converted DC voltage is fed back to the varicap diode 9 to fix the oscillation frequency to a desired frequency. In the first reception band, the voltage of the transistor 27 connected to the ground terminal 25 of the low-pass filter is applied to the base of the transistor 27 by the voltage from the control controller 22, and the transistor 27 is turned on. Therefore, the voltage of the constant voltage element 26 is
_CE voltage is fixed to 0.1V or less. That is, the output voltage of the low-pass filter 21
V _BE voltage of transistor 17 and V _CE voltage of transistor 17 and transistor 27
In about 0.7V is the sum of V _CE voltage, maximum voltage is V _CC voltage 8.5V being applied to the low-pass filter 21, a voltage of 1V at 76MHz~108MHz the first reception band of the graph of Figure 2 Can cover up to 6.5V.

Next, a description will be given of the case of the second reception band in which the reception controller 10 applies the reception band switching voltage to the diode 10.
When the voltage of the diode 10 is applied, a current flows through the coil 8. As a result, the diode 10 is short-circuited, and the coil 8 is short-circuited in an alternating manner through the capacitor 11. That is, the oscillation frequency of the local oscillation frequency circuit 12 becomes a parallel resonance circuit of the coil 7 and the varicap diode 9, and the inductance is lower than that of the first reception band, the oscillation frequency becomes higher, and the second reception band can be selected. it can. Hereinafter, the description of the operation is omitted because it is the same as that of the first reception band.
In the second reception band, no voltage is applied from the control controller 22 to the switching circuit of the transistor 27, and the transistor 27 is in the OFF state. Therefore, the voltage of the constant voltage element 26 connected to the low-pass filter ground terminal 25 is directly generated between the low-pass filter ground terminal 25 and the ground. That is, the output voltage of the low-pass filter 21 is the sum of the minimum voltage of 1.3 V of the constant voltage element 26, the V _{BE of} 0.6 V of the transistor 16 and the V _{CE of} 0.1 V of the transistor 17, and 2 V at the point b in the graph of FIG. It is set higher than in the first reception band. Maximum voltage is determined by 8.5V point c in the graph of V _CC voltage second view are applied to low pass filter 21. As described above, the varicap diode voltage in the second reception band of 175 MHz to 220 MHz can cover the range from d to e in the graph of FIG. 3 from 3 V to 7 V, and the minimum required voltage for reception. The minimum required voltage of the low-pass filter 21 that does not lower the voltage more than necessary from the point d of 3 d in the graph of FIG. 3 can be set by the constant voltage element 26.

The reason why the lowest receiving voltage in the second receiving band is set higher than the lowest receiving voltage in the first receiving band is that the capacitance of the varicap diode 9 of the oscillation circuit is made as small as possible, the inductance of the coil 7 is made large, and oscillation is performed. This is because it is desired to increase the Q of the circuit as much as possible.

As described above, according to the present embodiment, the constant voltage element is inserted into the ground terminal of the low-pass filter, the switching transistor connected in parallel with the constant voltage element is provided, and the diode is switched by the control controller to switch the local oscillation circuit. By switching the switching transistor in synchronization with the minimum output voltage of the low-pass filter by switching the switching transistor, the minimum output voltage of the low-pass filter that is optimal for the required varicap diode minimum voltage of each receiving band can be set. Can be. Therefore, it is possible to solve the problem that the minimum output voltage of the low-pass filter is reduced more than necessary with respect to the minimum voltage required for reception and the capacity of the varicap diode becomes very large, thereby reducing the Q of the local oscillation circuit and stopping the oscillation.

Effect of the Invention As described above, the present invention inserts a constant voltage element into the ground terminal of the low-pass filter, provides a switching circuit connected in parallel with the constant voltage element, and synchronizes with the switching of the local oscillation circuit by the controller. By switching the minimum output voltage of the low-pass filter by switching the switching circuit, it is possible to set the minimum output voltage of the low-pass filter optimal for the required varicap diode minimum voltage of each reception band. Therefore, when the minimum output voltage of the low-pass filter is reduced more than necessary with respect to the minimum voltage required for reception, the capacity of the varicap diode becomes very large and the Q of the local oscillation circuit
, The oscillation is stopped, the input signal of the phase comparator is lost, the output of the phase difference is lost, and the signal cannot be received again, so that the quality problem can be solved and the quality can be stabilized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a PLL receiver according to an embodiment of the present invention,
FIG. 2 is a circuit diagram of a conventional PLL receiver, and FIG. 3 is a graph showing tuning frequency versus varicap diode voltage. 1 ... RF amplifier, 2 ... Mixer, 3 ... IF amplifier, 4
... audio amplifier, 5 ... speaker, 6 ... local oscillator, 7, 8 ... coil, 9 ... varicap diode, 10 ... diode, 11 ... capacitor, 12 ... local oscillation circuit, 13 ... Frequency divider, 14 …… Reference oscillator, 15… Phase comparator, 16,17 …… Transistor, 18,20 …… Resistance, 19
... capacitor, 21 ... low-pass filter, 22 ... controller for control, 25 ... low-pass filter ground terminal, 26 ... constant voltage element, 27 ... transistor.

Claims (1)

(57) [Claims] 1. A PLL receiver comprising a local oscillation circuit capable of switching a plurality of reception bands to oscillate, a control controller, and a low-pass filter, wherein a constant voltage element is inserted into a ground terminal of the low-pass filter. A switching circuit connected in parallel with the constant voltage element is provided, and the minimum output voltage of the low-pass filter is switched by switching the switching circuit in synchronization with the switching of the local oscillation circuit by the control controller. PLL receiver characterized by doing.