JPH0354426Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial field of application] The present invention relates to a frequency modulation circuit suitable for use in a frequency modulation (hereinafter abbreviated as FM) transceiver with a relatively simple configuration such as a portable communication device. It is.

[Conventional technology]

The modulation method used when transmitting with an FM transmitter or MF transceiver is to use a crystal-controlled primary oscillator to generate an FM wave, which is then multiplied to obtain the necessary transmission radio waves in medium-sized or larger stationary equipment, but the equipment is complicated. Because of its large size, portable FM transceivers that aim to be compact and simple use a voltage-controlled capacitance diode in the oscillation tuning circuit of a self-excited oscillator, and also generate FM waves by superimposing a modulation signal on the bias voltage. However, it had the disadvantage of poor stability.

[The problem that the idea aims to solve]

As shown in Figure 2 as an example of the configuration of a simple FM transceiver, the receiving section uses PLL control to control the local oscillator injected into the superheterodyne mixer stage.
A VCO is used to perform tuning operation with good frequency stability and frequency accuracy, and when transmitting, a modulation signal is added to the VCO to directly create an FM wave at the transmission frequency, which is then power amplified and sent to the antenna. It has a streamlined configuration. In this circuit, the reception frequency and local oscillation frequency differ only by the intermediate frequency, so in order to receive and transmit at the same frequency, the oscillation frequency of the VCO, which is the FM source oscillator, must be shifted by the intermediate frequency during transmission. must be the same as the receiving frequency.

An example of a VCO circuit used for the above purpose is shown in FIG. In the figure, a tuning coil 31 and a voltage-controlled capacitor diode 32 form an oscillation circuit in combination with a negative resistance circuit, and the voltage-controlled capacitor diode 32
A control voltage 12 is applied from the PLL circuit to lock the oscillation frequency to a predetermined value. The standard intermediate frequency of an FM receiver is 10.7MHz, and the local oscillation frequency is usually set to a lower value that is easier to oscillate, so in order to lower the VCO frequency by 10.7MHz when transmitting than when receiving, capacitor 33 is used. When switching the transmitting and receiving frequencies in order to separate the frequency from the oscillation tuning circuit during transmission, use a switching diode 34 that can be used for both the transmitting and receiving circuits and can have a switching effect using a bias voltage. Tuning coil 31 and voltage controlled capacitance diode 32
A capacitor 33 is connected in parallel to an oscillation resonant circuit consisting of a switching diode 3 so that the capacitor 33 can be inserted into the circuit or the circuit can be interrupted.
4 are connected in series and grounded. During reception, the switching diode 34 is made conductive and the capacitor 33 is inserted into the circuit. During transmission, a reverse bias of 10 is applied to the switching diode 34, and the voltage on one side of the switching diode 34 is The capacitance between the electrodes of the switching diode 34 is several pF, and the capacitance value also changes depending on the reverse bias voltage value, so the reverse bias voltage is set to 10. An FM wave is obtained by superimposing the modulation signal 11.

Since small deviation FM of ±10KHz or more is used for current communications, the switching diode 34
It is often possible to make do with FM due to the change in capacitance of
If the amount of deviation is insufficient, a voltage controlled capacitance diode may be used.

In the circuit system shown in Fig. 3, when the amount of frequency deviation is a problem, the control voltage 12 is set over a wide range and measured in order to average the output and equalize the oscillation intensity. If the control voltage is increased, the capacitance of the voltage controlled capacitance diode 32 will decrease, so if the switching diode 34 is also used as a modulation diode, the amount of frequency deviation due to the change in capacitance will be large, and if the control voltage 12 is made small, the voltage Since the capacitance of the control diode 32 increases, the amount of frequency deviation may become smaller even if the capacitance change of the modulating diode remains the same.

As a conventional example of countermeasures against changes in the amount of frequency deviation, as shown in FIG. A voltage controlled capacitor diode 35 similar to the voltage controlled capacitor diode 32 as shown in FIG.
At the same time as the modulation signal 11, resistor 3 is added.
There is a method of automatically correcting the change in frequency deviation due to a change in the control voltage 12 by applying the control voltage 12 through the control voltage 6, but all of these methods involve adding parts and increasing the number of circuits. This goes against the purpose of simplifying the circuit configuration by sharing the switching diode 34 for modulation, and a simple correction method has been desired.

[Means to solve the problem]

A capacitor is connected to an oscillation tuning circuit including a voltage-controlled variable capacitance diode, the other end of the capacitor is connected to the anode of a switching diode, and the cathode of the switching diode is connected to the collector of a common-emitter switching transistor, oscillation frequency shifting means that connects a circuit that supplies a positive voltage during reception to the anode of the switching diode and the base of the switching transistor to ground the capacitor; and a modulation signal and a reverse bias voltage to the cathode of the switching diode. and a frequency modulation means that connects a circuit that supplies the oscillation frequency during transmission, and modulates the oscillation frequency by cutting off the switching diode during transmission;
A deviation stabilizing means is added that connects a resistor between the oscillation tuning circuit including the voltage-controlled variable capacitance diode and the reverse bias circuit to stabilize the deviation of frequency modulation through the synergistic effect of the control voltage and the reverse bias voltage. This is the configuration.

〔Example〕

FIG. 1 is a schematic diagram of the present invention. This circuit is
This relates to the local oscillator of the transmitting/receiving circuit shown in FIG. 2, which oscillates at different frequencies for reception and transmission, and directly performs frequency modulation in the oscillator during transmission.

In FIG. 1, 1 is a tuning coil, 2 is a voltage-controlled variable capacitance diode, and the oscillation frequency is varied by a control voltage 12. 3 is a capacitor that shifts the oscillation frequency during reception, and when receiving, a forward bias 13 is applied to the circuit, which can be used for both the transmission and reception circuits, and the bias voltage conducts the switching diode 4, which has the function of a switch. By doing so, capacitor 3 is added to the oscillation circuit. The switching diode 4 is supplied with a reverse bias 10 so as to be non-conductive during transmission, and furthermore, a modulation signal 11 is superimposed on this reverse bias 10 and is supplied via the switching diode 4 for frequency modulation.

A resistor is connected between the reverse bias 10 and the oscillation tuning circuit that controls the voltage-controlled variable capacitance diode 2 in the circuit configured as described above, and the effect of changes in the control voltage 12 is also applied to the switching diode 4 that operates as a modulation element. This configuration reduces the change in the amount of frequency deviation caused by the magnitude of the control voltage 12 applied to the voltage-controlled capacitance diode 2.

The actual circuit has the configuration of the embodiment shown in FIG. Portions with the same reference numerals in FIG. 6 and FIG. 1 are the same parts. The negative resistance circuit part has a Colpitts circuit configuration using FETs. switching transistor 6
When transmitting, there is a switching diode 4 at the base.
A common forward bias is applied to conduct between the collector and emitter, thereby forming a return path for the switching diode 4. During transmission, the base is at zero bias, so the collector and emitter are open, and the modulation signal 11 and resistor 7 are connected. A reverse bias 10 passed through the circuit is applied to the cathode side of the switching diode 4 to perform frequency modulation. By applying a control voltage 12 to this circuit through a resistor 5, the FM due to changes in the control voltage 12 is applied to the switching diode 4. This reduces changes in the frequency deviation of waves.

In actual measurements using the circuit constants shown in Figure 6, when the control voltage 12 is set to 2.5V and the oscillation frequency is set to 152MHz, the oscillation frequency is approximately 1V when the control voltage 12 is 1V.
140MHz, control voltage 12 is 4V, oscillation frequency is approx.
162MHz, and the modulation signal 11 is set so that the FM frequency deviation is ±3.5kHz when the control voltage 12 is 2.5V.
Set the level of control voltage 12 only to 2V and 4V
The frequency deviations are ±3.2kHz and ±3.3kHz, respectively, and the maximum deviation is ±0.3kHz, which is 10
% or less. For comparison, the results of a conventional circuit in which resistor 5 is removed from the circuit in Figure 6 are shown. At control voltage 12 of 1V and 4V, the frequency deviations are ±2.6kHz and ±3.9kHz, respectively, and the maximum deviation is ± 1.3k
Hz, with a rate of change of approximately 37%.

The above results are shown graphically in FIG. 7, where the results for the circuit of the present invention are shown by the solid line A, and the results for the conventional circuit are shown by the dotted line B. The effects of the circuit of the present invention are clear.

[Effect of idea]

The present invention has the effect of stabilizing changes in the frequency deviation of FM waves due to changes in control voltage by an extremely simple improvement of the circuit configuration of a conventional frequency modulation circuit by simply adding one resistor. This is clear from the actual measurement results shown in FIG. 7 in the embodiment of the present invention.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the frequency modulation circuit of the present invention, Figure 2 is a circuit diagram of a simple FM transceiver such as a portable type, and Figure 3 is a circuit diagram of the local oscillator shown in Figure 2.
An example of a frequency modulation circuit suitable for an FM modulator, FIG. 4. FIG. An example of the invention, Fig. 7 shows the measured values of control voltage versus frequency deviation of the circuit implementing the invention shown in Fig. 6, and the actual measured values of control voltage versus frequency deviation of a conventional circuit to which the invention is not applied. This is the graph to display. 1, 31... Tuning coil, 2, 32, 35...
Voltage controlled variable capacitance diode, 3, 33... Capacitor, 4, 34... Switching diode,
5, 7, 35...Resistor, 6...Switching transistor, 10...Reverse bias, 11...Modulation signal, 12...Control voltage, 13...Forward bias.

Claims (1)

[Scope of utility model registration request] In the frequency modulation circuit of a voltage controlled oscillator, which applies a frequency modulation signal directly to the oscillation circuit during transmission and shifts the oscillation frequency during reception, a capacitor is connected to the oscillation tuning circuit including the voltage controlled variable capacitance diode of the voltage controlled oscillator. The other end of the capacitor is connected to the anode of a switching diode, the cathode of the switching diode is connected to the collector of a switching transistor whose emitter is grounded, and the anode of the switching diode and the base of the switching transistor are connected to each other. An oscillation frequency shift means that connects a circuit that supplies a positive voltage during reception to ground the capacitor, and a circuit that supplies a modulation signal and a reverse bias voltage during transmission to the cathode of the switching diode, a frequency modulation means for modulating the oscillation frequency by blocking the switching diode;
A deviation amount stabilizing means is added to stabilize the deviation amount of frequency modulation by connecting a resistor between the control voltage circuit and the reverse bias circuit of the voltage-controlled variable capacitance diode to stabilize the deviation amount of frequency modulation by the synergistic effect of the control voltage and the reverse bias voltage. A voltage controlled oscillator frequency modulation circuit featuring: