JPS5922402B2 - Carrier leakage wave suppression method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scheme for suppressing carrier leakage of a carrier frequency still present in residual sidebands modulated through a balanced or double balanced modulator.

The magnitude of the carrier leakage still present in the residual sidebands modulated through a balanced or double balanced modulator depends primarily on the degree of balance of the circuit elements constituting the modulator.

However, it is extremely difficult and uneconomical to carefully select circuit elements and maintain their balance.

It is well known that when the carrier leakage wave becomes a signal, a normal carrier wave is not reproduced on the receiving side, resulting in extremely poor sound quality or image quality.

Generally, when the input signal is an audio signal such as a telephone transmission, the carrier frequency exists outside the band of the modulated output signal, so carrier leakage waves can be sufficiently suppressed by simply providing a filter following the modulator. .

However, when data or video is used as an input signal, the input signal spans a wide band from DC to several MHz and has a high maximum frequency. exist nearby,
Difficult to suppress by subsequent filters.

Therefore, regardless of the filter, the amount of carrier leakage wave suppression in the modulator alone becomes a problem.

As a means to solve this problem, the
No. 5 or Special Publication No. 33-8882 has been proposed.

In principle, these are adjusted so that a part of the carrier wave is shunted at the output of the modulator, and the carrier wave at the output of the modulator is canceled by changing the phase and magnitude of the carrier wave. Carrier leakage waves from the modulator itself are suppressed.

By using such a carrier leakage wave suppression adjustment circuit, carrier leakage waves can be removed extremely effectively.

However, on the other hand, for reasons explained in detail later, it has become clear that the modulator itself has the disadvantage of deteriorating its distortion rate.Especially when video or data is used as an input signal, the distortion rate is required due to the wide band nature of the signal. Since the value was strict, it became an essential issue to eliminate this drawback.

Therefore, an object of the present invention is to provide a carrier leakage wave suppression method that can suppress carrier leakage waves without deteriorating the distortion factor of the modulator itself.

In accordance with the above object, the present invention provides a carrier wave distribution which acts to input a carrier wave on the one hand into a modulator and on the other hand into a compensation channel added to send a compensation wave for said carrier suppression to the modulator output. The feature is that the circuit has directionality.

The present invention will be explained below with reference to the drawings.

First, a conventionally used carrier leakage wave suppression adjustment circuit will be described with reference to FIGS. 1 and 2.

However, Figures 1 and 2 are based on the already mentioned
No. 75 and Japanese Patent Publication No. 33-8882, respectively.

In the figure, 1 and 2 are input terminals of a modulator M to which input signals such as video or data to be modulated are input, and 3 and 4 are input terminals of a modulator M that have passed through the modulator M to be transmitted. an output terminal of the modulator M, at which the signal is output;
5 and 6 are carrier wave input terminals.

Furthermore, a compensation circuit C is inserted between terminals 3 and 4 and terminals 5 and 6.

This compensation circuit C outputs a compensation wave for suppressing the carrier leakage wave mentioned above, and this compensation wave is created from two waves having different phases and sizes including polarity, and is a carrier leakage wave of the modulator itself. It has completely opposite phase to the output of the modulator M, and is combined with the output of the modulator M.

In this case, the output impedance ZOUT of the compensation circuit C is designed to be sufficiently high in correspondence with the output impedance of the modulator so as not to harm the characteristics of the modulator, and the input impedance ZIN of the compensation circuit C is also specially limited. Although not very high, it is designed to be sufficiently high in order to keep the phase sufficiently stable when creating the above-mentioned compensation wave, to minimize the influence on the modulator side, and to minimize the branching loss of the carrier wave power.

Carrier leakage waves can be suppressed very effectively by the compensation waves output from the above-mentioned compensation circuits shown in FIGS. 1 and 2.

However, this also has the undesirable effect of deteriorating the distortion factor of the modulator M itself, and the elimination of this phenomenon has become an essential issue.

It has already been mentioned that this phenomenon is remarkable when a signal covering a wide band and a high frequency, such as a video or data signal, is modulated.

Therefore, the inventor investigated the cause of the deterioration of the distortion factor of the modulator due to the insertion of the compensation circuit C, and found the following.

As already mentioned, the compensation circuit C is designed so as not to affect the characteristics of the modulator M as much as possible, but the fact that the compensation circuit C itself is not unidirectional with respect to the signal flow but bidirectional. This causes the following phenomena.

That is, among the frequency components of the modulated output signal appearing at the output terminal 3-4 of the modulator M in FIGS.
If f is the frequency of the input signal to the modulator M, then Cf,
Cf±Pf, 30f soil Pf...=C2n+1)Cf
±Pf (where n = o, i.

2...).

Due to the bidirectional nature of the compensation circuit C, these powerful frequency components pass through the compensation circuit C and reach the carrier wave input terminals 5 and 6.

The carrier wave input terminals 5 and 6 are originally connected to the carrier wave (Cf).
However, the above-mentioned strong frequency component that has entered through the compensation circuit C is input again to terminals 5 and 6 as if it were a carrier wave, resulting in a different modulation than originally intended. .

This is an undesirable feedback effect, and the distortion components of the modulated output signal caused by this feedback effect are 2Pf above Cf, 2Cf±2
Pf, 3C1±2Pf.

2Pf, 30f±4pf, etc.

Furthermore, among these distortion components, the one with relatively strong force is Cf
Upper 2Pf and BCf±2Pf.

In this case, C±
Suppression of 2Pf is particularly problematic.

For example, if the carrier leakage wave attenuation of the modulator M alone is 45 dB and the Cf -2Pf distortion attenuation (distortion rate) of the modulator M alone is 70 dB, in this case, the carrier wave input terminals 5 and 6 (the 1) or 5' and 6' (
The amount of carrier leakage wave attenuation from FIG. be equivalent.

Conversely, the amount of carrier leakage wave attenuation due to the compensation circuit C from the output terminals 3 and 4 of the modulator M to the carrier wave input terminals 5 and 6 of the modulator M is approximately 45 dB in the circuit shown in FIG. 1, and approximately 45 dB in the circuit shown in FIG. So, roughly 5 things that vary depending on the turns ratio of the transformer PT.
0 to 60 dB.

Here, the frequency component c1-2P already described, the amount of distortion attenuation is Cf-P between the frequency component C1-Pf, which is re-input to the modulator M due to the feedback effect, and the frequency Pf of the input signal,
It is equal to the modulation loss of the modulator M itself because it occurs in the modulation product of the M type.

When the compensation circuit C is added and the carrier leakage wave is optimally adjusted and suppressed, the distortion factor of the modulator M becomes 45 to 60 dB.

From the above, the distortion rate degradation due to the addition of compensation circuit C is 10 (70-60') to 25 (70-45) dB.
It is calculated that the

The present invention is based on the clarification that the deterioration of the distortion factor of the modulator M, which ranges from about 10 to 25 dB, is caused by the bidirectionality of the compensation circuit C. This is a proposed circuit.

FIG. 3 is a block diagram showing an embodiment based on the principle of the carrier leakage wave suppression method proposed by the present invention.

In this figure, 1-2.3-4°5-6. M and C are completely equivalent to those explained in FIGS. 1 and 2, and the distribution circuit to the unidirectional circuit U and the compensation circuit C are newly added circuits according to the present invention.

Therefore, the portion surrounded by a dashed line 11 in the figure constitutes a carrier wave distribution section.

Note that 7 and 8 are input terminals for this carrier wave.

The unidirectional circuit U in the distribution section 11 prevents the carrier leakage wave mentioned above from returning to the input terminals 5 and 6, and the distortion factor deterioration of the modulator M is improved.

In the principle diagram of the present invention shown in FIG. 3, the unidirectional circuit U is concretely realized, for example, by a buffer amplifier, but in the embodiment of the present invention, this is most An example realized using an inexpensive and stable hybrid coil will be shown.

It should be noted that the hybrid coil has not only the function of the unidirectional circuit U but also the function of a distribution circuit.

FIG. 4 is a block diagram showing an example of a carrier leakage wave suppression circuit using this hybrid coil.

In FIG. 4, N. C, 1-2, 3-4, 5-6 and 7-8 are completely equivalent to those shown in FIG. 3, and the reference symbol He indicates the aforementioned hybrid coil.

Note that the input impedance of the compensation circuit C is terminated by a characteristic impedance R for the purpose of achieving sufficient matching with the hybrid coil HC.

According to the circuit shown in FIG. 4, carrier waves input from input terminals 7 and 8 are distributed as shown by arrow C8IC,
Although the transmission from the carrier wave input terminal 7-8 side to the carrier wave input terminal 6-9 side of the compensation circuit C is carried out without any trouble,
Carrier wave input terminal 5-6 of modulator M from the terminal 6-9 side
The amount of transmission of the carrier leakage wave (illustrated by arrow C2 in the figure) toward the side has the effect of attenuating by the amount of rotation into the hybrid coil He.

Since this wrap-around attenuation i can be secured by 25 to 30 dB in a normal hybrid coil, the above-mentioned distortion of the modulator M caused by the carrier leakage wave fed back to the modulator M due to the bidirectionality of the compensation circuit C is This value is sufficient to improve the rate deterioration.

The table below shows the attenuation of Cl-2Pf distortion (
distortion factor) without adding any compensation circuit (a in the table),
When the compensation circuit C is added as shown in Figs. 1 and 2 (b in the table), the compensation circuit C as shown in Fig. 3 is added based on the present invention.
A case in which a distribution section 11 is further added to (C in the table) is shown in comparison.

However, the carrier frequency is approximately 7MHz. Cf −2Pf distortion frequency is 4.2 MHz.

As shown in the above table, according to the present invention (Case C), the distortion rate is almost the same as that in the case where no compensation circuit is added (Case a), and is good.

Furthermore, carrier leakage wave suppression is almost the same as when only a compensation circuit is added, and is good.

In this case, the distortion factor of the present invention (case C) is at most 20 d compared to that of the one with only the compensation circuit added (case b).
It has been improved by B level.

As explained above, according to the present invention, carrier leakage waves can be sufficiently suppressed without deteriorating the distortion factor of the modulator.

[Brief explanation of drawings]

1 and 2 are block diagrams showing a conventional carrier leakage wave suppression circuit, FIG. 3 is a block diagram showing the principle configuration of a carrier leakage wave suppression method based on the present invention, and FIG. 4 is a block diagram showing a carrier leakage wave suppression system based on the present invention. FIG. 2 is a block diagram showing an example of implementing the principle configuration of a leakage wave suppression method. In the figure, M is a modulator, C is a compensation circuit, D is a distribution circuit,
U is a unidirectional circuit.

Claims (1)

[Claims] 1. The carrier wave input side of a modulator that performs balanced or double balanced modulation of the carrier wave, and the carrier wave input side of a compensation circuit that is connected to the output terminal side of the modulator and sends out a compensation wave for suppressing carrier leakage waves. the carrier wave is transmitted in the distribution unit in only two directions toward the carrier wave input side of the modulator and the carrier wave input side of the compensation circuit; A carrier leakage wave suppression method characterized by having the following characteristics.