JPH0611670Y2 - Channel tuning device - Google Patents

Description

The present invention relates to a channel tuning device having a local oscillation circuit controlled by the voltage of a multi-channel tuning circuit represented by a cable television (hereinafter abbreviated as CATV). is there.

2. Description of the Related Art Conventionally, a channel tuning device including this type of voltage controlled local oscillation circuit has a configuration as shown in FIG. In FIG. 2, a block A is a channel selection device, and when a TV signal as shown in FIG. 3 is input to the input terminal 1, an arbitrary TV signal is selected by the channel selection device.
For example, when the television signal in the shaded area in FIG. 3 is selected, the frequency corresponding to the television signal in the shaded area is input to the mixing circuit 2 from the voltage controlled local oscillator 3 (hereinafter abbreviated as VCO) to perform frequency conversion. , The band-pass filter 4 removes the adjacent signal and outputs the signal to the output terminal 5. At this time, the control voltage applied to the input terminal 6 of the VCO 3 is the frequency divider 7, the phase detector 8, the reference oscillator 9 and the low pass filter.
It is determined by a face-locked loop (hereinafter abbreviated as PLL) composed of 10.

Problems to be Solved by the Invention Since information of several tens of channels is transmitted as a CATV television signal, the channel selection device must also support this number of channels. At this time, the VCO has a characteristic that the relationship between the control voltage and the oscillation frequency is linearly proportional as shown in FIG. That is, Δf / ΔV is the amount of change in one channel. As is well known, several tens of low channels (eg 1 channel) and high channels (eg 1 channel)
The VCO control voltage D _IN when channel 60) is selected is different, and the VCO control voltage V _{IN (L)} of the low channel becomes a minute voltage. At this time, as shown in FIG.
The other auxiliary circuit B and the other accompanying circuit B are generally separated in relation to the frequency and space to be handled, and since the ground potentials are different from each other, when the power supply circuit 11 is present in the block B, the VCO control of the low channel is performed. The minute voltage of the voltage V _{IN (L)} becomes unstable due to the hum generated between the ground b point of the low-pass filter 10 of the block B and the ground a point of the VCO 3 of the block A from the ground plane of the power supply circuit 11, causing the voltage fluctuation. As a result, the voltage-controlled local oscillator 3 is frequency-modulated, and finally there is a problem that the operation becomes unstable with respect to channel selection.

As described above, in the conventional configuration, since a sufficient ground plane cannot be obtained, a potential difference occurs between the points a and b, and the ground potentials e ₁ and e ₂ are different as shown in FIG. The case will be described.

Assuming that e ₁ is the potential at the ground point a and e ₂ is the potential at the ground point b, the composite temporary ground potential e ₀ is as follows.

_{e 1 = αsinωt e 2 = βsinω} (t-τ 0) e 0 = e 1 + e 2 = αsinωt + βsinω (t-τ 0) However At this time, the maximum amplitude e ₀ max is when cos ω τ ₀ = 1 in the equation (1), and e ₀ max = (α + β) sin (ωt−ψ). Further, the minimum amplitude e ₀ min is cosω τ ₀ = Equation (1)
−1, which is e ₀ min (α−β) sin (ωt−ψ). Therefore, if there is a change in the potential of the ground plane at the two points, the combined virtual ground potential e ₀ between the two points will vary depending on their amplitude and phase.

The present invention solves the above-mentioned conventional problems, and an object of the present invention is to provide a channel tuning device capable of stable tuning.

Means for Solving the Problems In order to solve the above problems, the channel tuning device of the present invention is a phase lock device including a frequency divider, a phase detector, a reference oscillator, a low pass filter and a voltage controlled local oscillator. A tuning device having droop means and power supply means separated from the phase-locked loop means, wherein the ground plane of the voltage-controlled local oscillator and the ground plane of the low-pass filter are directly connected to each other to form an equipotential ground plane. In addition, the same-potential ground plane is connected to the ground point of the power supply means via a choke coil.

Operation With the above configuration, the ground potential of the voltage-controlled local oscillator and the low-pass filter are made the same, and the ground potential is grounded at a single point to the ground point of the power circuit via the choke coil for removing the AC hum of the power circuit. Therefore, the AC hum generated in the power supply circuit does not enter the voltage-controlled local oscillator or low-pass filter through the ground plane, and the control voltage of the voltage-controlled local oscillator is stabilized. Stable channel selection is possible without influence.

Embodiment An embodiment of the present invention will be described below with reference to the drawings. It should be noted that components having the same effects as those of the conventional example are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 is a block diagram of a channel selection device showing an embodiment of the present invention.

In FIG. 1, a block A of the channel selection device and a block B of other auxiliary circuits are provided with connection terminals 12 for connecting the ground points to the VCO.
3 and the low-pass filter 10 have the same ground potential, and the ground potential is grounded at a single point to the ground point of the power circuit via the choke coil 13 for removing the AC hum of the power circuit 11. Of course, at this time, it goes without saying that the choke coil 13 is selected so as to have a high impedance only for frequencies of hum noise or higher and to have a low impedance in terms of direct current.

As described above, according to the present invention, the ground plane of the voltage-controlled local oscillator and the ground plane of the low-pass filter are directly connected to form an equipotential ground plane. By connecting via a choke coil to the grounding point of the power supply means, the control voltage of the voltage controlled local oscillator becomes stable,
In particular, when a low channel is selected, the influence on the minute voltage is eliminated and stable channel selection can be performed.

[Brief description of drawings]

FIG. 1 is a block diagram of a channel tuning device showing an embodiment of the present invention, FIG. 2 is a block diagram of a conventional channel tuning device, FIG. 3 is a reception band diagram of a CATV input television signal, and FIG. Is a ground potential waveform diagram showing potential changes of two ground planes, and FIG. 5 is a control voltage characteristic diagram of the local oscillator for tuning. 3 ... VCO (Voltage Controlled Local Oscillator), 7 ... Divider,
8 ... Phase detector, 9 ... Reference oscillator, 10 ... Low-pass filter, 11 ... Power supply circuit (power supply means), 12 ... Connection terminal, 13 ... Choke coil.

Claims (1)

[Scope of utility model registration request] 1. A tuning station having phase-locked loop means composed of a frequency divider, a phase detector, a reference oscillator, a low-pass filter and a voltage-controlled local oscillator, and a power supply means separated from the phase-locked loop means. A device, wherein the ground plane of the voltage-controlled local oscillator and the ground plane of the low-pass filter are directly connected to each other to form an equipotential ground plane,
Also, a channel tuning device in which this same-potential ground plane is connected to the ground point of the power supply means via a choke coil.