JPS6210385B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a standard signal generator with significantly reduced FM modulation noise (residual FM component). More specifically, the FM modulation noise of a standard signal generator that modulates with a composite stereo signal for FM stereo broadcasting is systematically reduced to a value that cannot be obtained with conventional standard signal generators. The purpose is to

Figure 1 shows a typical FM standard signal generator.
In the figure, a high frequency voltage oscillated by an oscillation circuit 1 is applied to a variable amplifier circuit 3 via a buffer amplifier circuit 2. This buffer amplifier circuit 2 has the function of preventing the influence of load fluctuations from affecting the oscillation circuit 1 and causing fluctuations in the oscillation frequency. The high frequency voltage that has passed through the variable amplification circuit 3 is applied to the power amplification circuit 4 and amplified to a voltage required for a standard signal generator. The output of the power amplifier circuit 4 is divided into two parts, one of which is taken out from the output terminal 9 through an attenuator 5 as the output of a standard signal generator, and the other is applied to a detector 6 to generate a DC voltage proportional to the high frequency voltage. This becomes the detection signal for the automatic output voltage control circuit. That is, the detector 6
The DC output voltage is applied to the DC amplifier circuit 7.
In addition to the DC voltage from the detector 6, a DC voltage for setting the output voltage (output voltage setting signal) is applied to the DC amplifier circuit 7 via a terminal 10, and the difference between these two signals is the DC voltage from the detector 6. The signal is amplified by circuit 7 and applied to variable amplification circuit 3. The variable amplifier circuit 3 has a characteristic that the degree of amplification of high frequencies changes depending on the DC voltage from the DC amplifier circuit 7, and from the variable amplifier circuit 3 → power amplifier circuit 4 → detector 6 → DC amplifier circuit 7 → variable amplifier circuit 3 An automatic output voltage control circuit is constructed with a negative feedback loop. Due to the function of this automatic output voltage control circuit, the output voltage of the power amplifier circuit 4 is always maintained at a constant voltage even if there are fluctuations in the oscillation voltage of the oscillation circuit 1 or fluctuations in the amplification degree of the amplifier circuit. A variable reactance circuit 8 is incorporated as a part of the resonant circuit that determines the oscillation frequency of the oscillation circuit 1, and by adding an FM modulation signal and a 19KHz pilot signal to the variable reactance circuit 8, the high frequency oscillation in the oscillation circuit 1 can be adjusted. FM modulation is applied to the voltage. FM modulation ON/OFF switch 1
1 is a switch that selects whether or not to apply the FM modulation signal applied to the terminal 12 to the variable reactance circuit 8, and is a 19KHz pilot signal ON/OFF switch.
Switch 13 is a switch for selecting whether or not to apply the 19KHz pilot signal applied to terminal 14 to the variable reactance circuit.

The following describes the procedure for measuring the S/N of an FM stereo receiver using such a standard signal generator. Using the IHF standard as an example of a measurement method, monaural S/N measurement will first be explained.
Now, switch 13 is in the OFF state, and switch 1 is in the OFF state.
1 to the ON state, and send the FM modulation signal as a 1KHz sine wave to the standard signal generator with a 1KHz modulation signal.
Apply FM modulation with 75KHz deviation (standard modulation for monaural). At this time, the FM modulated wave (output of the standard signal generator) is received by the FM receiver, and the volume of the FM receiver is adjusted so that the 1KHz signal becomes the standard output determined by the FM receiver. Next, measure the output voltage (noise) of the FM receiver when the standard signal generator's FM modulation ON/OFF switch is turned OFF so that only the carrier wave is emitted from the standard signal generator. Then, the ratio between this noise and the initial standard output is determined and taken as S/N.

Next, stereo S/N measurement will be explained.
In this case, the following two points differ from the monaural case.

(i) Always keep the 19KHz pilot signal ON/OFF switch 13 in the "ON" state both when measuring signals and when measuring noise.
Apply FM modulation.

(ii) When measuring the signal, modulation frequency: 1KHz, L = -
Apply standard modulation for R stereo.

The S/N of the FM receiver is measured as described above, but in order to make accurate measurements, the S/N of the standard signal generator must be at least 10 dB better than the FM receiver. The performance of recent FM tuners has improved significantly, and the S/N ratio when a sufficiently strong signal is input has reached a value where the FM modulation noise of the local oscillator has a large effect. The FM modulation noise of a well-designed local oscillator circuit is mainly determined by the effective noise of the oscillating active element and the effective Q of the resonant circuit. Current high-end tuners are carefully selected for their performance in terms of these factors, and use very good quality products.

On the other hand, if we consider the oscillation circuit of a standard signal generator, (i) there is no special circuit, and it uses almost the same circuit as an FM tuner. Furthermore, since standard signal generators are generally measuring instruments and have a wider frequency range, they can be said to have worse S/N conditions than tuners. (ii) Regarding elements, in the case of transistors and FETs, while it is easy to obtain ones with characteristics of a noise figure of several dB, it is extremely difficult to obtain ones that are better than that. In addition, in principle, a transistor with a voltage of 0 dB or less
Considering the fact that there is no FET, even if a high-performance element is used, the conditions are not much better than that of a tuner. (iii) Regarding Q, unlike FM receivers, it is possible to create a high Q oscillation circuit with a standard signal generator using a cavity resonator, etc., but if you try to obtain a sufficient Q value, It has disadvantages in that it is very large and mechanically large, making it difficult to handle and very expensive. (iv)
In addition, in standard signal generators, a variable reactance circuit is connected to the resonant circuit in order to apply FM modulation, and residual FM components are generated due to noise added to this variable reactance circuit, compared to the receiver's local oscillation. This is a very poor condition. In this way, the S/N of both standard signal generators and FM tuners is suppressed by the performance of the oscillation circuit, and the circuit conditions do not change much, and the S/N of the standard signal generator is 10 compared to FM Chuna
Accurate measurements cannot be made unless it is about ~20 dB good. For this reason, there is currently no standard signal generator with truly sufficient performance regarding S/N of FM modulation.

Figure 2 also shows an FM broadcast station that uses phase modulation, such as Armstrong phase modulation.
Can be used as a standard signal generator. In the figure, a high frequency voltage oscillated by an oscillation circuit 21 is applied to a phase modulation circuit 23 after passing through a buffer amplifier circuit 22 for eliminating the influence of load fluctuations. An FM modulation signal applied to a terminal 25 is applied to this phase modulation circuit 23 through a circuit 24 having a frequency characteristic of -6 dB/octave, and the high frequency voltage is phase modulated by this signal. Since the FM deviation of the FM modulated wave obtained in this way is small, in order to obtain the required FM deviation, the FM modulated wave is passed through the multiplier 26 to have the required FM deviation, and then the power amplifier circuit It is amplified to the required power and sent to the output terminal 28.

A system using such a phase modulator is generally used for a single device or a device with a narrow frequency change range. Furthermore, in such equipment, it is possible to improve the S/N by using a crystal resonator with a very high Q in the oscillation circuit. However, even when a crystal oscillator is used, because the resonant impedance of the crystal oscillator is high, the noise is greatly improved in the close vicinity, such as within ±1KHz from the carrier frequency, but the noise at frequencies further away is reduced. The improvement is not as great as the improvement in Q. Considering this as the noise of the output of the FM receiver demodulated by the FM receiver, the noise below 1KHz is greatly improved, but the noise at higher frequencies is not improved to the extent commensurate with the improvement in Q. become. As a way to compensate for the drawback that the high-frequency component noise of the FM modulation noise of the crystal oscillator circuit is not improved much, a narrow-band filter is installed between the phase modulation circuit 23 and the buffer amplifier circuit 22 as shown in FIG. There is a method of inserting a crystal filter 29. However, such a circuit has the disadvantage that, when varying the frequency, it is necessary to vary all of the many tuning circuits included in the multiplier circuit 26 in conjunction with each other, resulting in a complicated structure.

The present invention eliminates the above-mentioned drawbacks and will be described below with examples thereof. In FIG. 4, parts corresponding to those in FIG. 1 are given the same reference numerals.
31 and 32 are interlocking modulation mode changeover switches. 33 is a crystal filter circuit, 34 is a phase modulation circuit, and 35 is a 19KHz pilot signal for the phase modulation circuit.
It is an ON/OFF switch.

Next, the operation of this embodiment will be explained. When the modulation format selector switches 31 and 32 are on the normal modulation side, the operation of the standard signal generator having this configuration is exactly the same as that described above with reference to FIG.

Next, modulation mode selector switches 31 and 32 are set to low FM.
The operation when it is on the modulation noise side will be explained.

The output of the power amplifier circuit 4 is sent to the crystal filter circuit 33
added to. This crystal filter 33 has narrow band characteristics, and when the oscillation frequency of the oscillation circuit 1 is matched to the passing frequency of the crystal filter 33, the high frequency output voltage of the power amplifier circuit is After passing, it is divided into two parts. One of them is guided to the output terminal 9 through the attenuator 5, and the other is applied to the detector 6 to become a detection signal for the automatic output voltage control circuit. The output of the power amplifier circuit 4 is filtered through a crystal filter 33 to remove sidebands due to FM modulation noise (including AM modulation noise), thereby improving the S/N ratio. However, as a matter of course, the FM modulation component by the variable reactance circuit 8 is also removed at the same time. FM as explained earlier
To measure the stereo S/N of the receiver, it is necessary to always apply FM modulation with a 19KHz signal with a deviation of 6.75KHz. Therefore, in this embodiment, the relationship between the maximum frequency deviation △m of FM modulation and the maximum phase shift θm of phase modulation is determined by θm, where P is the modulation frequency.
= △m/p When applying phase modulation of approximately ±0.355 rad with a 19KHz pilot signal, this becomes
The FM deviation is 6.75KHz, and we are focusing on the fact that this value can be achieved using only the phase modulation circuit without increasing the FM deviation by frequency multiplication as shown in Figure 2. In other words, when measuring the S/N in stereo mode, if the crystal filter circuit 33 is inserted for the purpose of improving the S/N, the 19KHz pilot signal cannot pass through, but if the phase modulation circuit 34 is inserted after the crystal filter circuit 33. If you set it up and perform phase modulation with a 19KHz signal, you can perform FM modulation for the 19KHz pilot signal.

The operation of each block has been explained above, and next, the case where it is used as a standard signal generator will be explained. When the standard signal generator is used in the same normal state as the conventional one, the modulation mode changeover switches 31 and 32 are set to the normal modulation side. Next, when a measurement with a particularly high S/N ratio is desired, first the oscillation frequency of the oscillation circuit is set to the passing frequency of the crystal filter circuit 33. The modulation mode selector switches 31 and 32 are used for normal modulation, and in the case of monaural, the modulation signal is 75K with a 1KHz modulation signal.
Mono standard modulation with Hz deviation, FM modulation for pilot signal with 6.75KHz deviation and modulation frequency 1KHz with modulation frequency of 19KHz for stereo, L = -R
Standard modulation for stereo is applied, this FM modulated wave is received by the FM receiver, and the volume of the FM receiver is adjusted so that the standard output is determined by the receiver.

Next, FM modulation ON/OFF switch 11, and
Both the 19KHz pilot signal ON/OFF switches 13 are turned "OFF" so that no signal is applied to the variable reactance circuit 8, and the modulation mode selection switches 31 and 32 are set to the low FM modulation noise side. The 19KHz pilot signal applied to the phase modulation circuit 34 is a 19KHz pilot signal for the phase modulation circuit.
Turn ON/OFF switch 35 to "OFF" for monaural and "ON" for stereo. Measure the noise level of the FM receiver in this state, and calculate the ratio to the output during standard modulation and use it as the S/N.

As is clear from the above explanation, high S/N measurements can be performed only at the frequencies of the points of the crystal filter circuit 33. In general, the standard measurement points of a stereo tuner are five frequencies for both Japanese and foreign bands, and although it is not shown in Figure 4, the crystal filter circuit 33 allows these necessary frequencies to be switched with a switch. Covers the familiar and necessary points.

In the above explanation, the oscillation circuit is explained using the same oscillation circuit as in the conventional example, but the oscillation circuit 1 is operated by a crystal oscillation circuit that oscillates a signal at the frequency passed by the crystal filter circuit 33 in conjunction with the modulation mode changeover switches 31 and 32. It goes without saying that it is possible to switch between different circuits, and in this case, a higher S/N value can be obtained. Also, almost the same effect can be obtained by locking a regular oscillator to a reference crystal oscillator using PLL technology. The reason why the crystal filter circuit 33 is used is that at present, the only element that can be used to remove noise components that exist as sidebands of such high-frequency signals is a filter using a crystal resonator. It goes without saying that if a narrow band pass filter element that can be used for such a purpose is developed, that element can be used.

As is clear from the above embodiment, the present invention includes a crystal filter circuit of a specific frequency and a phase modulation circuit that performs phase modulation using a 19KHz pilot signal in the loop of a variable amplifier circuit of a conventional FM modulation signal generator. This configuration has the effect that high S/N characteristics can be obtained at the specific frequency.

[Brief explanation of the drawing]

1, 2, and 3 are block diagrams of conventional standard signal generators, and FIG. 4 is a block diagram of a standard signal generator according to an embodiment of the present invention. 31, 32...Modulation mode selection switch, 33...
...Crystal filter circuit, 34...Phase modulation circuit.

Claims (1)

[Claims] 1. An oscillation circuit that inputs an FM modulation signal to a variable reactance element and outputs an FM signal, a variable amplifier circuit that inputs the output signal of this oscillation circuit, changes the amplification degree according to a control signal, and outputs it. Input the output signal of the variable amplification circuit, normally on the modulation side or low
A first modulation format selector switch that switches to either the FM modulation noise side and outputs the output signal, a crystal filter that inputs the output signal of the low FM modulation noise side of this switch and has narrow band characteristics, and this crystal filter. a phase modulation circuit that inputs the output signal of the phase modulation circuit, modulates the phase according to the phase modulation signal, and outputs the output signal; a second modulation format changeover switch whose side is connected to the normal modulation side and which switches and outputs one of the output signals in conjunction with the first modulation changeover switch; and a DC amplifier circuit that outputs the difference signal between the output signal of
A standard signal generator characterized in that it outputs a 19KHz pilot signal via an OFF switch, and outputs this 19KHz pilot signal as the above-mentioned phase modulation signal via an ON/OFF switch.