JPH0435304A - Envelope detection circuit - Google Patents

Abstract

PURPOSE:To improve the detection efficiency by connecting the base of two transistors (TRs) to the emitter of the other TR in cascade and clamping the base voltage of the TR closer to the input at a level higher by 2Vf (Vf is a forward base-emitter voltage of TR) from a reference voltage. CONSTITUTION:A positive half cycle of an FM sound signal reproduced from an audio head 1 is detected by using a diode characteristic between a base and an emitter of a TR Q1 via a capacitor C1 to charge a capacitor C2. A negative half cycle of the FM sound signal reproduced from the audio head 1 is detected by using a diode characteristic between a base and an emitter of a TR Q2 to charge the capacitor C1. The charge is fed to the capacitor C2 with a signal of a succeeding positive half cycle signal. As a result, a peak- to-peak voltage of the input signal is obtained at an output terminal 6 with respect to a ground potential.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an envelope detection circuit, and more particularly, to a VTR detection circuit.
In (video tape recorders), the envelope detection of the FM audio signal played by the audio head is used to control the tracking control system, which is used as a tracking control signal for the head, so that it is resistant to noise and is not easily affected by fluctuations in the DC level. The present invention relates to an envelope detection circuit that can be used as an envelope detector.

[Prior art] In a high-fidelity VTR, an audio signal is FM-modulated, recorded on a tape video track, and then demodulated.
is playing the audio signal. Then, the reproduced FM signal is subjected to envelope detection, and this envelope detection signal is used to perform head tracking control.

[Problem to be solved] In this type of tracking control circuit, the detection efficiency is low because the envelope detection circuit detects only one side of the waveform.
The loss in the detector is also large. Therefore, it becomes vulnerable to noise,
Furthermore, since the circuit directly receives the FM reproduction signal reproduced by the audio head, the tracking control is easily affected by fluctuations in the DC level of the reproduced FM signal.

An object of the present invention is to solve the problems of the prior art and provide an envelope detection circuit with high detection efficiency.

[! [Means for Solving the II Problem] The features of the envelope detection circuit of the present invention for achieving such an object are that the input signal is received at the base via a first capacitor, and a diode is connected between the base and the emitter. A first transistor that generates a detected output on its emitter side due to its characteristics, a second capacitor that is inserted between this first transistor and a reference potential and receives the detected output, and a second capacitor that receives the detected output from the first transistor. The second transistor has an emitter connected to its base, and a second transistor whose base is clamped to a potential of 2 Vf from a reference potential (where Vf is a forward drop voltage between the base and emitter of the transistor).

[Operation] In this way, the bases and emitters of the two transistors are stacked and connected, and the base of the upstream transistor is clamped at a potential 2Vf higher than the reference potential, so these transistors and the first capacitor ( A circuit equivalent to a half-wave voltage doubler rectifier circuit consisting of a coupling capacitor) and a second capacitor (a capacitor for holding the detected voltage) can be constructed.

Therefore, an operation similar to voltage doubler rectification is performed on the input signal, and the capacitor for holding the detection voltage on the output side is
An envelope-detected voltage signal of a waveform detected peak-to-peak with respect to the input signal is obtained.

As a result, the envelope detection output is approximately 2 times that of the conventional one.
The signal increases by about twice as much, and noise-resistant detection can be performed, especially for VTRs.
If this is used in a tracking control system, it is resistant to noise and is not affected by fluctuations in the DC level of the input signal because a coupling capacitor is provided on the input side.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram in which the envelope detection circuit of the present invention is applied to a tracking control system of a VTR.

In the figure, reference numeral 10 indicates a tracking control signal system of the VTR, and the FM audio signal is read from the tape by the audio head 1 and reproduced, and the FM audio signal is read from the tape by the audio head 1,
It is applied to the envelope detection circuit 3 via F (bandpass filter)2.

The detection output of the envelope detection circuit 3 is outputted to the subsequent amplifier 4 as a tracking control signal. And amp 4
The output is sent via the A/D conversion circuit 7 to a circuit such as a CPU that performs tracking control.

Here, the envelope detection circuit 3 receives the signal input to the input terminal 5 via the coupling capacitor CI to the base of the transistor Ql. The detected output is obtained from the emitter side of the transistor Ql via the load resistor R using the diode characteristics, and the voltage is generated in the capacitor C2 connected in parallel to the load resistor R. This voltage is applied to output terminal 6
is sent to amplifier 4. Note that the collector of the transistor Q1 is connected to the bias v1 line here.

The emitter of the transistor Q2 is connected to the base of the transistor Ql. The base of transistor Q2 is connected to power supply line VCC via current source 7, and this base is grounded via a series circuit of diodes Dt q D2.

As a result, this base is clamped at the voltage 2Vf generated by the diode Di t Dz.

In such a circuit, the base bias potential of transistor Q2 is fixed at 2Vf with respect to ground (reference potential),
The base bias potential of the transistor Q2 becomes IVf, and both transistors are in a biased state in which they operate according to the input signal. Note that the collector of the transistor Q2 is connected to the bias V2 line here.

Next, to explain its operation, for example, an FM audio signal reproduced by the audio radar 1 is transmitted to the input terminal 5 of the envelope detection circuit 3. Transistor Ql via capacitor CI
added to the base of. The positive half-cycle signal is detected by the diode characteristics between the base and emitter of the transistor Ql, and charges the capacitor C2. The voltage is thereby detected. At this time, the voltage between the base and emitter of the transistor Q2 becomes 1vf or less, so the transistor Q2 is in the "OFF" state.

On the other hand, the negative half-cycle signal is detected by the diode characteristics between the base and emitter of the transistor Q2, and charges the capacitor C1. This allows the voltage to be detected. This detected voltage, together with the next positive half-cycle signal, is again applied to capacitor C2 via the base-emitter of transistor Qt. The voltage detected at this time is the sum of the voltage due to the charge charged in the capacitor CI during the negative half cycle and the voltage during the positive half cycle. As a result, a peak-to-peak voltage of the input signal is obtained at the output terminal 6 (capacitor C2) with reference to the ground potential.

Here, the level at which the base of the transistor Q2 is clamped is determined by the diode DI+D2, so if this is thermally coupled for humidity compensation, temperature correction can be performed. As a result, the detection output of the FM signal envelope detection circuit is approximately twice the level of the peak on one side of the input signal, and temperature compensation is performed using a diode, resulting in linearity from low level to high level. A good detection output can be obtained. Moreover, since the capacitor C1 is provided on the input side, the detection output is not affected by fluctuations in the DC level of the FM audio signal read out from the audio head l.

As explained above, the diode Dt*D2 has
Of course, each includes a diode formed by diode-connecting transistors.

Further, although the tracking control system of a VTR is described in the embodiment, the present invention is not limited to application to such a control circuit.

[Effects of the Invention] As can be understood from the above explanation, in this invention, the bases and emitters of two transistors are stacked and connected, and the base of the upstream transistor is clamped at 2Vf. , an operation similar to a voltage doubler rectification operation is performed on the input signal, and the capacitor for holding the detection voltage on the output side contains an envelope detection voltage signal when a peak-to-peak waveform is detected for the human input signal. can get.

As a result, the envelope detection output has increased by about twice that of the conventional one, making it possible to perform detection that is resistant to noise.In particular, if this is used in the tracking control system of a VTR, it is resistant to noise and has a coupling capacitor on the input side. is provided, so it is not affected by fluctuations in the DC level of the input signal.

[Brief explanation of drawings]

FIG. 1 is a block diagram in which the envelope detection circuit of the present invention is applied to a tracking control system of a VTR. DESCRIPTION OF SYMBOLS 1...Audio head, 2...Amplifier, 3...Envelope line detection circuit, 4...Late stage amplifier, 5...Input terminal, 6
...Output terminal, 10...VTR tracking control system, QIT Q2...Transistor, Dt -D2.
...Diode, C! , C2... capacitor.

Claims (1)

[Claims] (1) A first transistor that receives an input signal at its base via a first capacitor and generates a detected output on its emitter side due to the diode characteristics between the base and emitter, and this first transistor and a reference potential. A second capacitor is inserted between the reference potential and receives the detected output, and its emitter is connected to the base of the first transistor, and its base is 2Vf (however, V
and a second transistor clamped to a potential of a forward drop voltage between the base and emitter of the transistor.