JPH0453095Y2 - - Google Patents

Description

[Detailed description of the invention] "Field of industrial application" This invention relates to improvements in RF modulators used in video tape recorders and the like.

"Prior Art" In a conventional RF modulator, a carrier wave is supplied from a carrier wave oscillator 1 to the primary winding of a modulation transformer T, as shown in FIG. One end of the secondary winding (point P ₁ ) is connected to the cathode of diode D ₁ ,
The other end (P ₂ points) is connected to the anode of the diode D ₂ , and the anode of the diode D ₁ and the cathode of the diode D ₂ are connected to each other at the P ₄ point.
The _P4 point is connected to the output terminal ₂ via the capacitor C2. The middle point _P3 of the secondary winding of the transformer T is grounded at high frequency via the capacitor _C1 and is also connected to the movable element of the variable resistor RV, and the power supply voltage +B is connected to one end of the variable resistor RV. The other end is grounded. The connection point _P4 between the diodes _D1 and _D2 is connected to the video signal supply circuit 4 via a high-impedance resistor R, and the DC voltage is
A superimposed signal V=v+E ₀ of E ₀ and the baseband video signal v is supplied.

The circuit made by the transformer T and the diodes D ₁ and D ₂ is an amplitude modulation circuit in which the carrier wave is modulated by the video signal v, converted to an RF signal, and sent to the load (for example, 75 ohms) via the capacitor C ₂ and the output terminal 2. Supplied. Capacitor C ₂ is connected to the RF signal (e.g.
The impedance for UHF band is small,
It is set to be large for a video signal v (for example, 0 to 6.5 MHz).

The carrier wave oscillator 1 is a well-known emitter-grounded series-tuned Colpitts oscillation circuit (also referred to as a Clapp oscillation circuit) often used in UHF tuners. One end of the oscillation coil _L1 is grounded through a variable capacitor CV and a capacitor _C3 , and the other end is grounded through a capacitor _C4 and connected to the collector of an oscillation transistor Q. The pick-up coil _L2 coupled to the oscillation coil _L1 is composed of a conductor bent into a U-shape, as shown in FIG. The oscillation circuit components are variable capacitor CV
Except for this, it is mounted on the first printed circuit board 11 and housed in the shield case 12. The RF modulator shown in FIG. 4 is mounted on both sides of the second printed circuit board 20 as shown in FIG.

Next, let's explain the operation of the RF modulator.

(1) When performing negative modulation The ground voltage V ₁ or P ₁ or P ₂ of the transformer T
V ₂ is the sum of the DC voltage E _p3 applied to the midpoint P ₃ and the carrier voltage induced between P ₁ and _{P 3} or between P ₂ and _{P 3} of the transformer T, as shown in Figure 6A. , the phases of the alternating current components differ by 180 degrees. The DC voltage E _p3 at the midpoint P ₃ is made equal to the maximum value E _p +v _nax of the superimposed signal V, and the amplitude Δ of the carrier wave voltage between P ₁ and _{P 3} and between P ₂ and _{P 3} of the transformer T is equal to the video signal. maximum value of v _nax
is set equal to or slightly smaller than v _nax .
Let the current flowing in the reverse direction through the diode D ₁ be I ₁ (therefore, I ₁ usually takes a negative value), and the current flowing in the forward direction through the diode D ₂ be I ₂ . Here, for simplification, the video signal v is expressed as 4 where v ₁ (=0) < v ₂ < v ₃ < v _nax .
It is assumed that this is a signal that takes values stepwise, and that the DC bias voltage E ₀ at _{the 4} points P is constant.

(a) The period t ₁ video signal is v=v ₁ =0, V ₁ >E ₀ , V ₂ >E ₀ ,
Further, reverse and forward DC bias voltages of magnitude E _p3 −E ₀ are applied to the diodes D ₁ and D ₂ , respectively. Its size E _p3 −E ₀ is a diode
The saturation voltage of D ₁ and D ₂ is selected to be approximately V _F (for example, 0.7 volts) or lower. Since the cathode voltage V ₁ of the diode D ₁ is always greater than the anode voltage V=E ₀ , the diode D ₁ is off and no current flows. On the other hand, since the cathode voltage V ₂ of the diode D ₂ is always larger than the cathode voltage V = E ₀ , a forward voltage is applied to the diode D ₂ , and a current I corresponding to the voltage V ₁ −E ₀ across the diode D 2 is applied. ₂ flows (Figure 6B). The alternating current component of this current is the capacitor C ₂ ,
The current flows to the load R _L via the output terminal 2, and an output voltage corresponding to the load current is generated at the output terminal 2 (FIG. 6D).

(b) The period t ₂ video signal is v=v ₂ and E _p3 −Δ<V<E _p3 . In addition, the diodes D ₁ and D ₂ have a size E _p3 − (v ₂
+E ₀ ), respectively, are applied with a DC reverse bias voltage and a forward bias voltage. A forward voltage is applied to the diode D ₁ in the valley of the voltage V ₁ for a short time, and a slight current I ₁ flows (FIG. 6B). On the other hand, a reverse voltage is applied to the diode D ₂ during a short time width of the same length as above between the valleys of the voltage V ₂ ,
Although the current I ₂ is zero, most of the time the current I ₂ flows depending on the magnitude of the forward voltage V ₂ −(v ₂ +E ₀ ). Since the magnitude of the forward voltage is smaller than the value in the period t ₁ by the magnitude of the video signal v ₂ , the current I ₂ becomes smaller by that amount. Current I ₁ , I ₂
The sum of these results in a waveform in which both peak values of the sine wave are sliced, as shown in FIG. 6C.

(c) Period t ₃ The video signal is v=v ₃ and E _p3 −Δ<V<E _p3 . Further, a direct current reverse bias voltage and a forward bias voltage of a magnitude E _p3 -(v ₂ +E ₀ ) smaller than that in the case of b are applied to the diodes D ₁ and D ₂ , respectively. _v3 ＞
Since v ₂ , the time width and magnitude of the forward voltage applied to the diode D ₁ are both larger than in case b, and therefore the peak value of the current I ₁ is also larger. On the other hand, since the magnitude of the forward voltage of the diode D ₂ becomes smaller, the current I ₂ becomes smaller and the period during which the reverse voltage is applied (the time width is equal to the period during which the forward voltage of D ₁ is applied) increases. Both the sum current I ₁ +I ₂ and the amplitude of the output voltage are smaller than in case b.

(d) Period t _4In this period, the video signal is v=v _nax ,
V= _vnax + _E0 = _Ep3 . Therefore the diode D ₁ ,
The DC bias voltage applied to D ₂ becomes zero.
In the half cycle of the carrier wave where V ₁ < E _p3 , the largest forward voltage E _p3 −V ₁ (its peak value is equal to Δ) is applied to the diode D ₁ , and a large current |
I ₁ | flows. In this same half cycle V ₂
＞E _p3 , so a forward voltage V ₂ −E _p3 (=E _p3 −V ₁ ) of the same magnitude is similarly applied to diode D ₂ , and a current of the same magnitude as I ₁ but in the opposite direction I ₂ flows. Both the forward voltage and forward current of diode _D2 are the smallest ever. In the other half cycle of the carrier wave, the _reverse voltage _{V 1} −E _p3 =
E _p3 −V ₂ is applied and both are turned off. In the half cycle in which the forward voltage is applied, I ₁ and I ₂ are equal in magnitude and opposite in direction, so the sum current I ₁ +I ₂ becomes zero, and both the output voltage and current become zero.

As is clear from the above explanation, the video signal v
When is maximum/minimum, the modulator output voltage (RF signal output) is minimum/maximum.

(2) When performing positive modulation Adjust the variable resistor RV to adjust the voltage E _p3 at the midpoint of the transformer T to the voltage P4 supplied from the video signal supply circuit _4.
be equal to the DC bias voltage E ₀ at the point (Fig. 7A). Currents I ₁ and I ₂ shown in FIG. 7B flow through the diodes D ₁ and D ₂ , respectively. Therefore, the sum current I ₁ +I ₂ shown in FIG. 7C is obtained, and the output voltage shown in FIG. 7D is obtained. The envelope of the output voltage is zero when the video signal v is zero and is maximum when v=v _nax .

In the above explanation, in the period when the output of the modulated wave is the minimum, that is, in the case of negative modulation, in the period _t4 ,
In addition, in the case of positive modulation, during the period t ₁ , half-wave sinusoidal currents I 1 and I ₂ of equal magnitude and opposite direction flow in the diodes D ₁ and D ₂ during the half cycle of the carrier wave to _which the forward voltage is applied. flows, and in the other half cycle when the reverse voltage is applied, both diodes are off and I ₁ = I ₂
= 0. However, if we look more closely, we can see that during the half-cycle when D ₁ and D ₂ are controlled off, there is a small barrier capacitance between the terminals of the diode and stray capacitance due to the diode case or lead wire, so some currents are mutually distributed. flows in the opposite direction. If the capacitance values of both diodes are the same, the sum current I ₁ + I ₂ will be zero and there will be no problem. However, in reality, it is difficult to choose diodes with the same capacitance, so the diodes D ₁ and D ₂ The magnitudes of the currents I ₁ and I ₂ flowing through the terminals are different, so that the sum current I ₁ +I ₂ does not become zero, and a small leakage voltage is output. FIG. 8 shows the case of negative modulation. If such leakage voltage occurs, the linearity of the reproduced video signal will deteriorate, so the following precautions are taken. That is, in order to balance the capacitance values between the terminals of both diodes when off, one end of the balance adjustment lead wire 21 is connected to the connection point _P4 of the diodes _D1 and _D2 , and the other end is connected to the diode _D1 . Or bring it close to the lead wire extending to the transformer T side of D ₂ and change the stray capacitance ΔC ₁ and ΔC ₂ between it and the adjustment lead wire 21 to adjust the output voltage in the above period t ₄ or t ₁ to zero. are doing.

``The problem that the invention seeks to solve'' The spread of video tape recorders is remarkable, but behind the scenes competition between manufacturers is becoming even more intense. Good cost performance has a great impact on shear, and for manufacturers, reducing product costs can lead to ups and downs in business. This idea was made in view of this situation, and the purpose is to reduce the number of parts in the RF modulator as much as possible to make it more economical.

"Means for Solving the Problems" In the RF modulator of this invention, a carrier wave oscillation circuit having an oscillation coil is mounted on a first printed circuit board and housed in a shield case. A pickup coil is constituted by a strip-shaped printed pattern formed on the first printed circuit board in the vicinity of the oscillation coil, and first and second lead wires led out parallel to each other from both ends of the strip-shaped printed pattern. A third lead wire is led out from the middle of the printed pattern almost parallel to the first and second lead wires, and these first to third lead wires are connected to the respective leads of the second printed circuit board on which the shield case is mounted. 1,
It is connected to the second and third connection points. The third connection point is grounded at high frequency through the first capacitor and is supplied with a DC bias voltage. Further, the first and second connection points are connected to one ends of first and second diodes in opposite directions, respectively, and the other ends thereof are connected to a common fourth connection point. The fourth connection point is connected to a video signal supply circuit for modulating the carrier wave and also to one end of the second capacitor, and the RF modulation output is taken out from the other end. Connect the middle part of the third lead wire to the first lead wire.
Alternatively, the balance adjustment of the modulation section is performed by bringing it closer to one of the second lead wires.

``Embodiment'' The embodiment of this invention is shown in FIGS. 1 and 2. Parts corresponding to those in FIGS. 3 and 4 are designated by the same reference numerals, and redundant explanation will be omitted. In the RF modulator of this invention, the conventional modulation transformer T and balance adjustment lead wire 21 are omitted. The pick-up coil _L2 is constructed by a strip-shaped printed pattern 31 formed on the first printed circuit board on which the oscillation circuit is mounted, and first and second lead wires ₁ and ₂ led out in parallel from both ends of the strip-shaped printed pattern 31. configured. A third lead wire ₃ is led out from the middle of the printed pattern 31 almost parallel to the first and second lead wires ₁ and ₂ , and these first to third lead wires
₁ to ₃ are first to third connection points P 1 to _{P 3} of the second printed circuit board 20 on which the shield case 12 is mounted.
connected to. Its connection point P ₃ is the first capacitor
It is grounded at high frequency through _C1 and connected to the movable element of the variable resistor VR. the variable resistor
Power supply voltage +B is supplied to one end of VR, and the other end is grounded. Connection points P ₁ and P ₂ are connected to one end of diodes D ₁ and D ₂ connected in opposite directions, respectively, and their respective other ends are connected to a common fourth connection point.
Connected to P ₄ .

As mentioned in the conventional example, t ₄ of negative modulation (Fig. 6)
The leakage of the carrier wave caused by the difference in capacitance between the terminals of the diode during the OFF period, which is a problem during the period _t1 of positive modulation (FIG. 7), can be suppressed as follows. That is, the middle part of the third lead wire ₃ is connected to the first lead wire ₁ so that there is no carrier wave leakage.
Or bring it closer to the second lead wire ₂ . Let us now assume that the small capacitances of the first and second diodes D ₁ and D ₂ when they are off are _CD1 and _CD2, respectively, and that _CD1 < _CD2 . At this time, as shown in Figure 3A, the third lead wire
₃ closer to the second lead wire ₂ , the stray capacitance ΔC ₂ between the second and third lead wires becomes larger than the stray capacitance ΔC ₁ between the first and third lead wires.
The carrier wave voltage V ₁₃ generated between the connection points P ₁ and P ₃ is the carrier wave voltage generated between the second and third connection points P ₂ and P ₃ .
The amplitude is larger than V ₂₃ and therefore the capacitor
The current I ₁ flowing into the load R _L through C _D1 is the capacitor
The current I ₂ flowing through C _D2 to the load R _L is equal in magnitude and opposite in direction, and the output voltage can be adjusted to zero. Both diodes D ₁ and D ₂ are on,
In the half cycle of the carrier wave in which currents I ₁ and _{I 2} of _the same magnitude flow _{respectively ,} as shown in FIG. is the operating resistance rd of the low impedance diode, the capacitor C ₂ and the load resistance R _L (e.g. 75
Since a relatively small series impedance consisting of ohms) is connected, even if there are stray capacitances ΔC ₁ and ΔC ₂ of different sizes, their values are originally small, and their respective impedances are equal to the above series impedance. Since it is extremely large and can be ignored compared to , it does not affect the magnitudes of V ₁₃ and V ₂₃ above, and |V ₁₃ |=|
You can think of it as V ₂₃ ｜.

"Effect of the invention" According to this invention, the strip-shaped printed pattern formed on the first printed circuit board of the carrier wave oscillation circuit and the first and second lead wires led out from both ends of the strip-shaped printed pattern are formed on the first printed circuit board of the carrier wave oscillation circuit.
₁ and ₂ constitute a pick-up coil _L2 , and the midpoint of the printed pattern is grounded at high frequency via the third lead wire ₃ and is supplied with a DC bias voltage E _p3 . Lead wires ₁ and ₂ are directly connected to first and second diodes D ₁ and D ₂ (for modulation) in opposite directions. In this way, the conventional modulation transformer T is omitted, and by bringing the third lead wire ₃ closer to the first or second lead wire ( ₁ or ₂ ), the balance of the modulation section is adjusted. , second diode D ₁ , D ₂
The conventional balance adjustment lead wire 21 connected to the connection point _P4 is omitted. In this way, in this invention, since the modulation transformer is omitted, the depreciation of the RF modulator can be reduced, and at the same time, it can be expected to be made smaller.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing an embodiment of this invention, Figure 2 is a circuit diagram showing an embodiment of this invention.
The figure is a perspective view of a printed circuit board on which the RF modulator of Figure 1 is mounted, and Figure 3 is a diagram for explaining the balance adjustment of the modulation section that is performed during the period when the modulated wave output of the RF modulator of Figure 1 is at its minimum. The circuit diagram of the main part, Figure 4 is the circuit diagram of a conventional RF modulator,
Fig. 5 is a perspective view of a printed circuit board on which the RF modulator shown in Fig. 4 is mounted, Fig. 6 is an ideal operating waveform diagram of the main parts when the RF modulator shown in Fig. 4 performs negative modulation, and Fig. 7 is The ideal operating waveform diagram of the main parts when the RF modulator in Figure 4 performs positive modulation,
Figure 8 shows the case where the RF modulator performs negative modulation.
FIG. 7 is an operational waveform diagram of the main part during a period _t4 during which the output of the modulated wave (RF signal) is at its minimum before the balance adjustment of the modulation section is performed.

Claims (1)

[Claims for Utility Model Registration] A carrier wave oscillation circuit having an oscillation coil is mounted on a first printed circuit board and housed in a shield case, and a strip-shaped printed pattern is formed on the first printed circuit board near the oscillation coil. and first and second lead wires drawn out parallel to each other from both ends of the pick-up coil, and a third lead wire drawn out from the middle of the printed pattern substantially parallel to the first and second lead wires. These first to third lead wires are connected to the first, second, and third connections, respectively, of the second printed circuit board on which the shield case is mounted, and the third connection point is connected to the high frequency signal through the first capacitor. The first and second connection points are connected to one ends of first and second diodes in opposite directions, and the other ends of the diodes are connected to a common diode. The fourth connection point is connected to the video signal supply circuit for modulating the carrier wave, and is also connected to one end of the second capacitor, and the RF modulation output is taken out from the other end. Connect the middle part of the lead wire to the first or second
An RF modulator that adjusts the balance of the modulation section by bringing it close to one side of the lead wire.