JPH0115216Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a frequency converter, and particularly to a frequency converter using a PLL (phase locked loop) circuit.

Conventionally, there is a device shown in FIG. 1 as this main device. In the figure, the input is, for example, the horizontal synchronization frequency of a television video signal, which is one input of the phase comparator 1. The output of this comparison is
It becomes a control signal for a voltage controlled oscillator (hereinafter simply referred to as VCO) 3 via an LPF (low pass filter) 2. The output of this VCO 3 becomes one input of the mixer 4 and is mixed with the output of the local oscillator 5, which is the other input. This mixed output becomes another input to the phase comparator 1 via the frequency divider 6, and a frequency-converted signal is derived from the VCO 3.

Here, the frequency _H of the input signal, the oscillation frequency _L of the local oscillator 5, and the division ratio of the frequency divider 6 are N (positive integer).
Then, the output frequency of VCO3 will be _L −N _H or _L +N _H.

Since the conventional device is configured as described above,
Even if the input is restored after the input is interrupted,
The drawback is that it is difficult for the PLL to lock, and the output frequency cannot lock to the normal frequency unless the power to the circuit is turned off. The reason for this will be explained below using FIGS. 2 to 5.

FIG. 2 is a diagram showing the frequency arrangement when the two input frequencies to the mixer 4 are not close to each other, and in this case, when the input signal is interrupted,
The so-called free-run frequency range of the VCO 3 is indicated by diagonal lines for each of the VCO oscillation frequencies _L ± _NH . Therefore, the signal (N _H ) frequency-converted by the mixer 4 also free-runs in the frequency region shown by diagonal lines. FIG. 3 shows the relationship between the output voltage level of the phase comparator 1 and the oscillation frequency of the VCO 3 at that time.
In this figure, when the frequency of the VCO 3 is _L - _NH , the frequency region above _L + _NH becomes an undefined region, and when the frequency is _L + _NH , the frequency region below _L - _NH becomes an undefined region.

Therefore, if the input signal is interrupted and the VCO 3 free-runs and the oscillation frequency shifts to this undefined region, the lock will not be applied even if the input signal returns again, but the two frequencies input to the mixer 4 will be the second frequency.
In the case where they are not close to each other as shown in the figure, the low level range of the comparison voltage of phase comparator 1 is wide as shown in Figure 3, so the VCO oscillation does not reach the above-mentioned undefined area. There is no possibility of the frequency going off.

However, as shown in FIG.
If the two input frequencies are close to each other, the free running frequency range of the VCO 3 will overlap for each of the input frequencies _L ±N _H of the mixer 4. FIG. 5 shows an enlarged view of the relationship between the comparison voltage of the phase comparator and the input frequency of the mixer 4 in this case.

As can be seen from FIG. 5, since the low level range of the comparison voltage is extremely narrow, a phenomenon occurs in which the oscillation frequency of the VCO 3 deviates from the above-mentioned undefined region. As a result, even if the input signal returns again, the lock will not be applied.

The purpose of this invention is to create a PLL that can always lock to the normal output frequency even if the input signal is interrupted and restored.
An object of the present invention is to provide a frequency converter using a frequency converter.

The frequency converter of the present invention includes a local oscillator,
It has a VCO, a mixer that mixes the output of this VCO and the output of a local oscillator, and phase comparison means that generates a control voltage for the VCO according to the phase difference between this mixed output and an input signal. This is a frequency converter that derives a frequency-converted signal from the output, and its characteristics are that the input signal is supplied to a separately added PLL circuit, and the oscillation output of the VCO in this PLL circuit is phase-compared. The purpose is to serve as an input signal of the means.

The present invention will be explained below using the drawings.

FIG. 6 is a diagram showing an embodiment of the present invention, and the first
Parts equivalent to those in the figures are indicated by the same reference numerals. That is, another PLL circuit consisting of a phase comparator 7, an LPF 8, and a VCO 9 is provided, and an input signal to be input to the phase comparator 1 is applied as one input of the phase comparator 7 in this PLL circuit. this
1 of phase comparator 1
It is used as input. The other circuit configuration is the same as that in FIG.

In such a configuration, if the free run frequency of the VCO 9 of the newly added PLL circuit is made approximately equal to the frequency of this input signal, even if the input signal is interrupted, the input to the phase comparator 1 will be the oscillation output of the VCO 9. Since the voltage is supplied to the VCO 3, the VCO 3 will not go out of the above-mentioned undefined area. Therefore, when the input signal is restored again, the VCO 3 is stably locked to the normal frequency.

As described above, according to the present invention, even if the input signal is interrupted, the circuit immediately locks against the return of the input signal, which is extremely convenient.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional frequency converter, Figure 2 is a diagram showing the frequency arrangement of each part when the two input frequencies of the mixer are not close to each other, and Figure 3 is a diagram showing the frequency arrangement of each part when the two input frequencies of the mixer are not close to each other. Figure 4 explains the relationship between the free run area and the undefined area of the block in Figure 1.
The figure shows the frequency arrangement of each part when the two input frequencies of the mixer are close to each other, and Figure 5 shows the relationship between the free run region and the undefined region of the block in Figure 1 under the conditions of Figure 4. The explanatory figure, FIG. 6, is a block diagram of an embodiment of the present invention. Explanation of the symbols of the main parts, 1, 7... Phase comparator, 3, 9... VCO, 4... Mixer, 5... Local oscillator.

Claims (1)

[Scope of utility model registration request] a local oscillator, a voltage controlled oscillator, a mixer that mixes the output of the voltage controlled oscillator and the output of the local oscillator, and a control voltage of the voltage controlled oscillator according to a phase difference between the mixed output and the input signal. a frequency converter for deriving a frequency conversion signal from the output of the voltage controlled oscillator; A frequency converter characterized in that an oscillation output of a voltage controlled oscillator in a droop circuit is used as an input signal of the phase comparison means.