JPH0510434Y2 - - Google Patents

Description

[Detailed description of the invention] <Industrial field of application> This invention relates to head ends used in communal viewing equipment, etc., and is particularly applicable to multi-channel systems that mix and transmit multiple FM signals, each with a different carrier frequency.
Regarding FM headends.

<Prior art> A multi-channel FM head end not only receives an FM broadcast signal transmitted from a broadcast station, amplifies it, and sends it out, but also FM-modulates an independent program and sends it out. In such cases, FM
For modulation, a relatively low frequency carrier wave is FM modulated with the audio signal of an independent program, and this low frequency FM
The modulated signal is multiplied by a multiplier to obtain a predetermined frequency.
A multiplication method is used to obtain an FM modulated signal, or a relatively low frequency carrier wave is FM modulated with the audio signal of an independent program, and this low frequency FM modulated signal and a local oscillation signal are mixed in a mixer to generate a predetermined frequency. A mixed method was used to obtain an FM modulated signal.

<Problems to be solved by the invention> However, in the multiplication method, many harmonics of the FM modulation signal of a low frequency are generated as shown in Fig. 3, so in order to obtain the FM modulation signal of a predetermined frequency,
A bandpass filter must be provided at the output of the multiplier to remove unnecessary signals. In addition, in the mixed method, as shown in Figure 4, an image signal is generated above or below the local oscillation signal (lower side in the example in Figure 4), and harmonics of the local oscillation signal are also generated. Since image signals are generated above and below this harmonic, unnecessary signals cannot be removed unless a bandpass filter is provided on the output side of the mixer, similar to the multiplication method. Using a bandpass filter in this way poses a problem when the number of independent programs is large. In other words, when the number of independent programs is large, the interval between the predetermined frequencies of each independent program becomes
In this case, it is necessary to prepare a large number of high-performance bandpass filters with relatively small passbands, each with a different center frequency. Moreover, when changing the predetermined frequency of each independent program, each bandpass filter had to be readjusted.

<Means for Solving the Problems> In order to solve the above problems, this invention includes a plurality of transmitting sections and a mixing section that mixes the transmission signals of these transmitting sections. The carrier frequencies of the transmission signals of these transmitters are different from each other. These transmitting units have an FM modulating unit including an oscillating unit that directly generates a relatively high frequency carrier wave without multiplying the frequency. The output signal of the FM modulator is input to the mixer via a low-pass filter. The cutoff frequency of this low-pass filter is selected between the highest frequency of the carrier waves generated by each oscillation section and the second harmonic of the lowest frequency of each carrier wave.

<Operation> According to this invention, the FM output signals of different frequencies from each FM modulation section are supplied to the mixing section via each low-pass filter, but the cutoff frequency of each low-pass filter is determined by the frequency of each oscillation section. The harmonics of each FM output signal are selected between the highest frequency of the generated carrier waves and the second harmonic of the lowest frequency of each carrier wave, so that the harmonics of each FM output signal are filtered by the respective low-pass filters. removed,
It is not supplied to the mixing section.

<Example> This example has a mixing section 2 as shown in FIG. This mixing section 2 has a
FM output signal is supplied. Although only one transmitter 4 is shown in FIG. 1, there are actually many transmitters 4. The mixing section mixes the output signals of a large number of transmitting sections 4 and sends the mixed signals to an output terminal 6. Note that the carrier frequencies of the FM output signals of each transmitter 4 are different from each other.

Each transmitter 4 has the same configuration and includes an oscillation/modulation section 8. This oscillation/modulation section 8 performs carrier wave oscillation and FM modulation using one FET, and includes a variable capacitance diode 12 for applying FM modulation to the carrier wave by the audio signal supplied to the audio input terminal 10. and the frequency of the carrier wave will be explained later.
It includes a variable capacitance diode 14 for stabilization by PLL. The carrier wave in this oscillation/modulation section 8 is directly oscillated at, for example, 60 to 70 MHz, and is not multiplied.

For example, as shown in FIG. 5, this oscillation/modulation section 8 is based on a modified Clap oscillation circuit using one FET 36, and the fundamental frequency of the carrier wave oscillated by this oscillation/modulation section is , 60 to 70M depending on the values of capacitors 40, 42, 43, 44, 46 and variable capacitance diodes 12 and 14.
Hz, and when this fundamental frequency fluctuates, the signal of the low-pass filter 28 of the PLL, which will be described later, changes, thereby changing the value of the variable capacitance diode 14, and maintaining the fundamental frequency.

On the other hand, the value of the variable capacitance diode 12 changes depending on the audio signal supplied to the audio input terminal 10,
This causes the frequency of the carrier wave to vary from the fundamental frequency. Therefore, the carrier wave is converted to FM by the audio signal.
Modulated.

The FM output signal of the oscillation/modulation section 8 is the buffer 1
6 to the prescaler 18, the frequency is divided to 1/64, the frequency is further divided to 1/N by the programmable counter 20, and the frequency is supplied to the comparator 22. The comparison section 22 is also supplied with a reference oscillation signal from the reference oscillation/frequency division section 24 . As the reference oscillation signal, a signal with a frequency of 3.2MHz is oscillated using the crystal 26, and this is divided into 1/4096 to 781.25Hz.
I'm using one. Since the frequency of the reference oscillation signal is selected as described above, if the carrier frequency in the oscillation/modulation section 8 is in the range of 60 to 70 MHz, the frequency division number N of the programmable counter 20 is 1200 to 70 MHz.
The value will be in the range 1400. The comparator 22 generates an output signal according to the phase difference between the frequency-divided signal and the reference oscillation signal, and this output signal is supplied to the variable capacitance diode 14 via the low-pass filter 28. A prescaler 18, a programmable counter 20, a comparator 22, a reference oscillation/frequency divider 24, and a low-pass filter 28 constitute a PLL.

The FM output signal of the oscillation/modulation section 8 is the buffer 1
6 to a low pass filter 30.
This low-pass filter 30 has a cutting frequency of
It is chosen to be lower than the second harmonic of the FM output signal. The output signal of this low-pass filter 30 is
The signal is amplified by a broadband amplifying section 32 and supplied to the mixing section 2. This wideband amplification section 32 is a wideband class A amplifier that covers the entire frequency band used or covers a divisional band.

In this embodiment, since the FM output signal of the oscillation/modulation section 8 is supplied to the PLL, it is possible to use a known PLL.
Following operation, the carrier frequency is stabilized. Note that since the oscillation/modulation section 8 is composed of one FET, the signal supplied to the PLL is an FM modulated signal, that is, the frequency is almost constantly changing. Therefore, when the frequency of the reference oscillation signal is increased, even if the frequency of the FM modulated signal is divided, the frequency difference between this reference oscillation signal and the frequency-divided signal of the FM modulated signal increases, and this The comparator generates an output signal to correct the difference, resulting in increased distortion of the FM modulated signal. Therefore, in this embodiment, the frequency of the reference oscillation signal is selected as low as 781.25 Hz. For example, the carrier wave of the oscillation/modulation section 8 is 70MHz, and as a result of FM modulation with an audio signal, it shifts by 15KHz,
If the frequency becomes 70.015MHz, the frequency-divided signal supplied to the comparator 22 will be approximately 781.42Hz, and the frequency difference from the reference oscillation signal will be approximately 0.17Hz, and the correction signal generated by the comparator 22 will be too large. Not FM
The distortion of the output signal is a value within practical use.

When the oscillation/modulation section 8 generates the FM output signal, harmonics are also generated at the same time. For example, if the FM output signal is 70MHz, the second harmonic is 140MHz,
The third harmonic is 210MHz. Therefore, as shown in Figure 2, these harmonics have a shearing frequency.
It is cut off by a low pass filter 30 whose frequency is lower than the second harmonic of the FM output signal.
Only the FM output signal is amplified by the broadband amplifier section 32 and supplied to the mixing section 2.

Note that although the carrier frequencies of each transmitter 4 are different, if a cutting frequency is set between the highest and lowest second harmonic frequency of each of these carrier frequencies, Each transmitter 4
The same low-pass filter 30 can be used.

In the above embodiment, the oscillation and modulation section 8 simultaneously performs the oscillation and modulation of the carrier wave, but the oscillation section and the modulation section for the carrier wave may be provided separately. In that case,
The output signal of the oscillation section may be supplied to the PLL.

<Effects> As mentioned above, according to this invention, the oscillation section directly oscillates the relatively high frequency carrier wave without multiplying it, unlike the conventional multiplication method or mixing method. Only harmonics are generated as unnecessary signals. This harmonic is 2, 3... times the carrier frequency. Therefore, unnecessary signals can be suppressed simply by providing a low-pass filter having a suppression frequency lower than this harmonic. Therefore, since this low-pass filter has a simple circuit configuration, the circuit configuration of the multi-channel FM head end can be simplified even when transmitting a large number of independent programs. Also, even if the frequency of each carrier wave is changed, the cutoff frequency is selected between the second harmonic of the lowest frequency of each carrier wave and the highest frequency of each carrier wave, so it is not necessary to change the low-pass filter. There is no. Further, even if a wideband amplifier is used, unnecessary signals are not amplified, so no interference occurs.

[Brief explanation of the drawing]

Figure 1 is a block diagram of one embodiment of the multi-channel FM head end according to this invention, Figure 2 is a diagram showing the relationship between the FM output signal of the same embodiment and its harmonics, and Figures 3 and 4 are Traditional multi-channel
FIG. 5 is a diagram showing the relationship between the FM output signal of the FM head end and unnecessary signals generated together with it, and is a circuit diagram of the oscillation/modulation section 8 of this embodiment. 2... Mixing section, 4... Transmission section, 8... Oscillation/modulation section, 30... Low pass filter.

Claims (1)

[Scope of utility model registration request] It has a plurality of transmitting units that FM modulate carrier waves of different frequencies and transmitting the same, and a mixing unit that mixes the respective output signals of these transmitting units, and each of the transmitting units has:
The FM modulation section includes an oscillation section that directly oscillates carrier waves with relatively high and different frequencies;
a low-pass filter to which the output signal of the FM modulator is supplied and whose cutoff frequency is selected between the highest frequency of the carrier waves and the second harmonic of the lowest frequency of the carrier waves; Multi-channel FM headend with