JPH07336244A - Fm transmission circuit - Google Patents

Abstract

PURPOSE:To provide a FM transmission circuit with high frequency stability at a low cost. CONSTITUTION:This circuit is provided with an FM modulation circuit 6 whose resonance circuit consists of a coil, a capacitor and a variable reactance element, with an oscillation circuit 11 oscillating an oscillating signal SLO with a stable frequency, a mixer circuit 12 frequency-converting the output signal STX of the FM modulation circuit 6 into an FM signal SIF with a carrier frequency lower than that of the output signal STX, an FM demodulation circuit 13 discriminating the frequency of the FM signal SIF and a low pass filter 14 extracting a DC voltage VAFC whose level changes with frequency fluctuation of the output signal STX from the output signal of the FM demodulation circuit 13. An audio signal is fed to the variable reactance element of the FM modulation circuit 6 as its control voltage to conduct FM modulation. The VAFC is fed to the variable reactance element of the FM modulation circuit 6 as its control voltage to apply AFC control to the carrier frequency of the output signal STX to be a specified value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM transmitter circuit.

[0002]

2. Description of the Related Art As a headphone stereo (headphone type stereo cassette player), a system in which a wireless system is provided between the headphone stereo and the headphone is considered.

That is, in this wireless headphone stereo,
The reproduced audio signal has a carrier frequency
It is converted into a 238 MHz FM signal, and this FM signal is transmitted to a dedicated receiver. Then, in the dedicated receiver, the original audio signal is demodulated from the transmitted FM signal, and the demodulated audio signal is supplied to the headphones.

In this case, since the dedicated receiver can be half or less the size of a business card, for example, when listening to a cassette on a commuter train, put the headphone stereo in a bag or bag. In addition, you can put the dedicated receiver in your chest pocket and use it, and the headphone cord will not get in the way.

[0005]

However, when the resonance circuit of the FM modulation circuit is composed of the coil and the capacitor, the frequency deviation can be increased to increase the modulation degree, but the frequency stability is poor. turn into.

Further, when the resonance circuit of the FM modulation circuit is composed of a crystal oscillator, the frequency stability can be increased, but since the frequency deviation is reduced and the modulation degree is reduced, the receiver is used. S / N of audio signal obtained
Will get worse.

Further, if FM modulation is performed by a PLL,
Although the problems of frequency stability and modulation degree can be solved, in this case, the configuration becomes complicated, the cost increases, and it is disadvantageous for downsizing and low voltage.

There is also a method in which the output signal of the FM modulation circuit is frequency-converted (up-converted) to a target carrier frequency by the oscillation signal of the crystal oscillation circuit. However, in the case of this method, since the output signal of the FM modulation circuit is frequency-converted by the oscillation signal of the crystal oscillation circuit and transmitted, a large amount of spurious components are included in the transmission signal, which adversely affects others. Will end up. For example, when the headphone stereo is designed to be remotely controlled by radio waves from a dedicated receiver, the reception circuit of the remote controller may malfunction due to spurious components.

The present invention is intended to solve the above problems.

[0010]

For this reason, in the present invention, when the reference numerals of the respective parts correspond to the embodiments described later, the resonance circuit is formed of an FM modulation circuit 6 including a coil, a capacitor and a variable reactance element. The output signal STX of the FM modulation circuit 6 and the oscillation circuit 11 that forms the oscillation signal SLO having a stable frequency conversion frequency are converted into the FM signal SIF having a carrier frequency lower than the output signal STX by the oscillation signal SLO. The mixer circuit 12 for converting, the FM demodulation circuit 13 for discriminating the low frequency FM signal SIF output from the mixer circuit 12, and the output signal of the FM modulation circuit 6 from the output signal of the FM demodulation circuit 13 And a low-pass filter 14 for extracting the DC voltage VAFC whose level changes in response to frequency fluctuations. The audio signal is supplied as a control voltage to the variable reactance element of the FM modulation circuit 6 to perform FM modulation, and the DC voltage VAFC extracted by the low pass filter 14 is supplied to the variable reactance element of the FM modulation circuit 6. The carrier frequency of the output signal STX of the FM modulation circuit 6 which is supplied as the control voltage is adjusted to the specified value by AF.
C control is performed.

[0011]

The oscillation circuit 11, the mixer circuit 12, the FM demodulation circuit 13 and the low-pass filter 14 form an AFC loop, and the AFC loop stabilizes the carrier frequency of the FM signal STX from the FM modulation circuit 6 to a specified value. To be done.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, an audio signal is reproduced from a cassette tape 1 by a reproducing head 2, and this audio signal is transmitted through a signal line of a reproducing equalizer circuit 3 → pre-emphasis circuit 4 → adding circuit 5 to an FM modulation circuit 6
To the FM signal STX.

In this case, although not shown, the resonance circuit of the modulation circuit 6 is composed of, for example, a coil, a capacitor, and a variable capacitance diode, and the output signal of the addition circuit 5 receives the control voltage at the variable capacitance diode. Supplied as. Further, the carrier frequency of the FM signal STX is a target transmission frequency, for example, 238 MHz.

Then, this FM signal STX is transmitted through the output amplifier 7 and the band pass filter 8 to the transmitting antenna 9
And transmitted to a dedicated receiver (not shown).

Further, the FM signal STX from the modulation circuit 6 is
The crystal oscillator circuit 1 is supplied to the mixer circuit 12 as well.
At 1, the oscillation signal SLO having a stable frequency is formed,
This oscillation signal SLO is supplied to the mixer circuit 12. In this way, the FM signal STX supplied to the mixer circuit 12 is frequency-converted (down-converted) into the FM signal SIF having a carrier frequency of the difference between the signal STX and the signal SLO, for example, 10.7 MHz by the oscillation signal SLO. .

The FM signal SIF is supplied to the FM demodulation circuit (frequency discrimination circuit) 13. In this case, in the demodulation circuit 13, the phase shift element for frequency discrimination is composed of, for example, a ceramic filter, and its center frequency is the carrier frequency of the FM signal SIF when the carrier frequency of the FM signal STX is the correct frequency, That is,
It is set to 10.7 MHz.

Then, the output signal of the demodulation circuit 13 is supplied to the low-pass filter 14, a DC voltage VAFC whose level changes in accordance with the frequency fluctuation of the FM signal SIF is taken out, and this voltage VAFC is passed through the addition circuit 5. It is supplied to the variable capacitance diode of the FM modulation circuit 6 as its control voltage.

With such a configuration, the carrier frequency of the FM signal STX from the FM modulation circuit 6 is AFC-controlled by the voltage VAFC and held at a specified value. That is,
The FM signal S is changed by the temperature change of the FM modulation circuit 6.
If the carrier frequency of TX is going to increase, for example, FM
The carrier frequency of the signal SIF also increases, for example, and as a result, the level of the voltage VAFC decreases, and the carrier frequency of the FM signal STX from the FM modulation circuit 6 decreases.

On the contrary, if the carrier frequency of the FM signal STX from the FM modulation circuit 6 becomes low, the carrier frequency of the FM signal SIF also becomes low, and as a result, the voltage VAF
The level of C becomes high, and the carrier frequency of the FM signal STX from the FM modulation circuit 6 becomes high.

Therefore, the carrier frequency of the FM signal STX from the FM modulation circuit 6 is feedback-controlled and held at a specified value.

In this way, according to this FM transmission circuit, the carrier frequency of the FM signal STX is held at the specified value, so that the FM modulation circuit 6 uses the resonance circuit, the coil, the capacitor and the variable circuit as described above. It can be configured by a capacitance diode, and thus a sufficient modulation degree can be obtained.

Further, since a circuit such as a PLL is not required and the circuits 11 to 14 can use the IC or the like used in the FM receiver, the cost increase can be minimized. . In addition, it is advantageous for miniaturization and lower voltage.

Further, the FM signal S from the FM modulation circuit 6
Since TX can be transmitted without frequency conversion,
Generation of spurious components can be suppressed. Also, F
Since the M signal STX is frequency-converted into the FM signal SIF having a low frequency, and the FM signal SIF is frequency-discriminated to perform the AFC control, the signal is compared to the case where the FM signal STX is directly frequency-discriminated to perform the AFC control. The processing is easy and the AFC control can be stably performed.

In the example shown in FIG. 2, the audio signal is a stereo signal. Further, in this figure, the left and right channel circuits corresponding to the circuits 1 to 8 in FIG. Also, FM modulation circuits 6L and 6
The carrier frequency of R is, for example, 238.48 MHz and 238 MHz.
z

The circuits 11 to 14 are common to the left and right channels, and the FM modulation circuit 6
The FM signal STX from R is supplied to the mixer circuit 12, and the voltage VAFC from the low pass filter 14 is supplied to the adding circuits 5L and 5R.

Therefore, as described above, the AFC control is performed by the voltage VAFC, and the FM from the FM modulation circuit 6R is controlled.
The carrier frequency of the signal STX is kept at the correct frequency.

At this time, the FM modulation circuits 6L and 6R
The carrier frequencies of the FM signals STX and STX are substantially equal to each other.
The FM modulation circuits 6L and 6R can have the same configuration, and as a result, the FM modulation circuits 6L and 6R
When the carrier frequencies of the FM signals STX and STX from 6R fluctuate, they can be set to change similarly.

Therefore, the FM from the FM modulation circuit 6R
When the carrier frequency of the signal STX is AFC controlled to the correct frequency, the carrier frequency of the FM signal STX from the FM modulation circuit 6L is also controlled to the correct frequency.

Thus, also in this example, the carrier frequencies of the two-channel FM signals STX and STX can be held at the correct frequencies, and a sufficient degree of modulation can be obtained. Moreover, since the AFC circuit is commonly used for the left and right channels, the overall configuration is simple and the cost is reduced.

In the above description, the present invention is applied to a wireless headphone stereo, but it can also be applied to a wireless microphone or the like. Further, the FM signal STX from the FM modulation circuit 6 may be multiplied before transmission.

[0031]

According to the present invention, since the carrier frequency of the FM signal STX can be held at the specified value, the FM signal
The resonance circuit of the modulation circuit 6 can be composed of a coil, a capacitor, and a variable capacitance diode, so that a sufficient modulation degree can be obtained.

Further, since a circuit such as a PLL is not required and an IC or the like used in the FM receiver can be used, cost increase can be suppressed to a minimum. In addition, it is advantageous for miniaturization and lower voltage.

Further, the FM signal S from the FM modulation circuit 6
Since TX can be transmitted without frequency conversion,
Generation of spurious components can be suppressed. Also, F
Since the M signal STX is frequency-converted into the FM signal SIF having a low frequency, and the FM signal SIF is frequency-discriminated to perform the AFC control, the signal is compared to the case where the FM signal STX is directly frequency-discriminated to perform the AFC control. The processing is easy and the AFC control can be stably performed.

Even when the audio signal is a stereo signal, the carrier frequency of both channels can be stabilized by the circuit for one channel.

[Brief description of drawings]

FIG. 1 is a system diagram showing an example of the present invention.

FIG. 2 is a system diagram showing another example of the present invention.

[Explanation of symbols]

 1 cassette tape 2 playback head 3 playback equalizer circuit 4 pre-emphasis circuit 6 FM modulation circuit 8 band pass filter 11 crystal oscillator circuit 12 mixer circuit 13 FM demodulation circuit 14 low pass filter

Claims (4)

[Claims] 1. An FM modulation circuit in which a resonance circuit is composed of a coil, a capacitor and a variable reactance element, an oscillation circuit forming an oscillation signal of a stable frequency for frequency conversion, and an output signal of the FM modulation circuit. , By the above oscillation signal,
A mixer circuit for frequency-converting into an FM signal having a carrier frequency lower than that of the output signal, an FM demodulation circuit for discriminating the frequency of the FM signal of low frequency output from the mixer circuit, and an output signal of the FM demodulation circuit, A low-pass filter for extracting a DC voltage whose level changes in accordance with the frequency fluctuation of the output signal of the FM modulation circuit, and an audio signal is supplied to the variable reactance element of the FM modulation circuit as its control voltage. The FM modulation is performed, and the DC voltage extracted by the low-pass filter is supplied to the variable reactance element of the FM modulation circuit as its control voltage so that the carrier frequency of the output signal of the FM modulation circuit has a specified value. FM transmitter circuit that is AFC controlled. 2. The FM transmission circuit according to claim 1, wherein the output signal of the FM modulation circuit is supplied to a transmission antenna through an amplifier. 3. The FM transmission circuit according to claim 1, wherein the output signal of the FM modulation circuit is multiplied before being supplied to a transmission antenna. 4. The FM transmission circuit according to claim 1, 2 or 3, further comprising a second FM modulation circuit configured similarly to the FM modulation circuit, wherein the second audio signal is , Is supplied as a control voltage to the variable reactance element of the second FM modulation circuit, and F
An FM transmission circuit in which M modulation is performed and the DC voltage extracted by the low-pass filter is also supplied to the variable reactance element of the second FM modulation circuit as its control voltage.