1. Field of the Invention
The present invention relates to a receiver for receiving multi-channel broadcast and more particularly to a frequency converter which enables a VHF or UHF television receiver set to receive television broadcast waves using as their carriers superhigh frequency signals.
2. Description of the Prior Art
Today, the television broadcast uses electromagnetic waves belonging to VHF or UHF band, but the future trend in this field is toward the use of much higher frequencies, e.g. signal waves of SHF band.
In the practice of such a broadcasting service as using SHF band, it is of great economy if the currently used receiver set can be utilized in the reception of the broadcast. For this purpose, there is a need for a frequency converter which, if it is only attached to a conventional television receiver, makes the receiver adaptable for the new broadcast band. Above all, it is necessary to employ in the SHF broadcast the same television signals as the conventional ones and the same modulation system, i.e. vestigial-sideband amplitude modulation, as the one now in practice.
The frequency converter to enable the conventional TV receiver to receive the SHF broadcast, comprises, for example, an antenna for catching the broadcast waves in the air, a local oscillator for generating at a time a single oscillation frequency, a mixer for mixing the signals induced in the antenna and the output of the local oscillator, and an amplifier for amplifying the output, that is, frequency-converted signal, from the mixer. If the antenna catches three kinds of broadcast waves having their respective carrier frequencies f.sub.1, f.sub.2 and f.sub.3, the amplifier mentioned above will deliver the corresponding three kinds of signals having frequencies f'.sub.1, f'.sub.2 and f'.sub.3. Therefore, if the oscillation frequency of the local oscillator is so determined as to make the frequencies f'.sub.1, f'.sub.2 and f'.sub.3 belong to the conventional VHF or UHF band for TV broadcast, the SHF signal can be effectively received by simply connecting the converter with the conventional television receiver set. According to the convention of broadcast, the frequencies f'.sub.1, f'.sub.2 and f'.sub.3 are so determined as to correspond to the channels to which no broadcast waves are alloted in the district in consideration. The most important thing with such a frequency converter is the stability of the oscillation frequency of the local oscillator. The problem concerning the stability of the oscillation frequency will be described by the use of concrete numerical example.
Let a case be considered in which an SHF broadcast channel having a video carrier frequency f.sub.1 of 12,000 MHz is received by converting the SHF channel to the fiftieth channel (video carrier frequency 693.25 MHz) of the present Japanese Standard TV Broadcast System. In this case, the local oscillation frequency f.sub.l, assumed to be set lower than the carrier frequency of the SHF channel, is such that EQU f.sub.l = 12,000 - 693.25 = 11,306.75 (MHz).
The video carrier frequency f.sub.1 of the broadcast wave can be considered almost fixed since the broadcast station is always monitoring and correcting the fluctuation in the frequency, so that the fluctuation in the frequency f'.sub.1 of the output of the converter depends mainly on the deviation of the local oscillation frequency f.sub.l, the variation +.DELTA.f of the frequency f.sub.l causing the variation -.DELTA.f of the frequency of the output of the converter.
In order to effectively receive television broadcast with conventional receiver sets, the video carrier frequency of the broadcast signal induced in the antenna must be confined within a range of at least the predetermined value .+-.0.1 MHz. Accordingly, the local oscillator of the frequency converter must have a frequency stability as high as 0.1/11,306.75, i.e. about 1 .times. 10.sup.-.sup.5. Such a high frequency stability can be attained by (1) using an oscillator having a high frequency stability, such as quartz or crystal controlled oscillator and (2) automatically controlling the local oscillation frequency in such a manner that the detected deviation of the output frequency is always rendered to zero.
The first method cannot be employed in the television sets for domestic use since the associated device is very complicated and expensive. Moreover, in the present level of technique, even a quartz controlled oscillator having the highest frequency stability attainable will not be able to maintain the above said stability in the circumstances varying in ambient temperature, power source voltage etc.
The second artifice, which has been employed in the domestic TV sets for the reception of VHF and UHF bands, cannot be applied, as it is, to the reception of the SHF television broadcast without encountering technical difficulties and above all without causing too much expense. This is described below.
In order to automatically control the local oscillation frequency, it is necessary to design the local oscillator of the frequency converter so that it may generate in a change-over fashion different frequencies in accordance with the number of the broadcast waves, and to incorporate in the frequency converter mentioned above a frequency discriminator which detects the frequency of the output of the mixer or the output of the amplifier and whose output is fed to the local oscillator to automatically control the oscillation frequency. In such a case, the carrier frequency in the output of the mixer remains to be a fixed value f.sub.o for any channel received since the local oscillation frequency is changed over in accordance with the frequencies of the received signals. Accordingly, the channels to be received are selected by changing over the oscillation frequencies of the local oscillator and the channel selecting device or channel selector of the conventional TV receiver set to which the frequency-converted signals are applied, is adjusted to the frequency f.sub.o.
The frequency converter described above has the following drawbacks.
1. Since each of the plural TV broadcast channels will be separated from one another by about 12 MHz, as in the UHF band, the different frequencies generated by the local oscillator must differ from one another by about 12 MHz. As described before, however, the local frequency is to be around 11 GHz and it is technically very difficult to make a difference of 12 MHz with respect to such a high frequency.
2. The condition necessary for the frequency stability proper to the local oscillator can be relaxed due to the use of the automatic frequency control, but the frequency excursion of the oscillator must be limited within a certain range so as to prevent the erroneous operations of the control system. Of all the erroneous operations, the one due to the influence of sound carrier takes place most usually. Namely, the frequency discriminator in its normal operation compares the video carrier frequency with the associated fixed value f.sub.o, but when the excursion of the local oscillation frequency exceeds a certain limit, the frequency discriminator will compare the value f.sub.o with the sound carrier frequency so that the control system operates erroneously. It is known from this that the allowable breadth of the excursion of the local oscillation frequency is about half (i.e. 2.3 MHz) the difference (4.5 MHz) between the video and the sound carrier frequencies. The breadth of 2.3 MHz is about 20 times the value 0.1 MHz obtained above in the absence of frequency control. The frequency stability in this case is 2 .times. 10.sup.-.sup.4 and this is a much relaxed condition in comparison with the previously obtained value of 1 .times. 10.sup.-.sup.5. In order to maintain the improved value under various conditions, however, the local oscillator must be provided with some auxiliary stabilizing means. Thus, under these circumstances, the local oscillator for generating not a single frequency but plural ones in change-over manner will add considerably to technical difficulties.
3. As described above, the conventional TV receiver sets are utilized in the reception system using such a frequency converter as enlarged upon above. Since the receiver set has a channel selector, the separate provision of a channel selecting function in the converter will cause an operative complexity and above all be uneconomical.