JPS6349008Y2 - - Google Patents

Description

[Detailed description of the invention] The invention relates to an SHF television receiver,
In particular, this invention relates to an improved SHF television receiver configured to visually eliminate high-frequency noise on the screen that occurs when the C/N (carrier level-to-noise ratio) of the input FM signal is significantly reduced. .

Generally, in FM transmission and demodulation methods, the input
There is a relationship between the C/N of the FM signal and the S/N (signal-to-noise ratio) of the FM demodulated output signal as shown in Figure 1, and the C/N ratio has a certain threshold level. exists, and so-called FM improvement effects cannot be expected for C/N inputs below a certain level.

Therefore, when designing a normal FM broadcast receiver, etc., the margin of the C/N ratio is usually set at several points.
Since the noise characteristics are set with enough margin to ensure that the noise level is at least dB, the receiver is not used below the C/N threshold level, and there is no need to consider the above problems at all. do not have.

However, the target of this invention is
Since the SHF television receiver uses a frequency in the 12 GHz band as the FM carrier wave, the radio waves are forced to undergo significant attenuation due to rain, and when designing the receiver, a considerable amount of C/N margin is required. (For example, even if a C/N margin of 5 dB is set due to cost and technical considerations,
If there is rainfall of more than mm/hour and there is an attenuation of more than 7 dB in C/N, the C/N level will drop below the threshold level mentioned above, resulting in significant S/N deterioration. .

The present invention has been devised in view of the various drawbacks of the conventional examples. The details will be explained below with reference to FIG. 2 which shows a block diagram of the main part and FIG. 3 which shows a circuit diagram of the main part.

FIG. 2 shows a block diagram of the main parts of the SHF television receiver of the present invention from the intermediate frequency signal input terminal 1 to the video (and audio) output terminals 10 and 11.

Diagrams 2 and 3 are intermediate frequency amplification circuits, whose amplified FM outputs are limited to a certain level by a limiter 4, and then applied as input to an FM demodulation circuit 5, which demodulates them into a composite video signal. be done. This demodulated signal passes through a low-pass filter 7 to remove residual carrier components, and is applied as an input to a video amplification circuit 8, where the video signal separated from the audio signal is amplified to a predetermined level. 10 indicates an audio signal output terminal. The amplified video signal is applied as an input to a variable frequency characteristic (or variable high-frequency attenuation) circuit 9 that is controlled by the output of a C/N level detection circuit 6, which will be described later. 11 is a video signal output terminal.

The C/N level detection circuit 6 includes an AM detection circuit 16 that performs envelope detection of the input FM signal, and a threshold level between the output of this circuit and the C/N.
Compare the reference voltage V ₀ corresponding to Vs, and
When the output V _A of _the detection circuit 16 is in the relationship that V _A > V ₀ , the DC output is sufficient to saturate the control transistor _{T 2} , which will be described later. Accordingly, the control transistor _T2 can be configured with a threshold level detection circuit 26 including a comparison circuit that generates a DC output operating in a linear operating region.

Next, details of the variable frequency characteristic (or variable high frequency attenuation) circuit 9 and its peripheral circuits will be explained with reference to FIG.

_T1 is basically an amplifying transistor that operates with a grounded emitter, and the base input of this transistor
An FM demodulated video signal is connected to derive its high frequency range from the collector in a variable form. A capacitor C and a varicap D are connected in parallel with the resistor _R1 connected between the emitter of the transistor _T1 and ground.
Connect the series circuit. A pair of series resistors R ₂ and R ₃ are connected between the ungrounded terminal of the variable cap D and the power supply terminal +B, and the collector-emitter path of the control transistor T ₂ and the resistor are connected between the connection midpoint O and the ground. The series circuit with R ₄ is connected in parallel, and the base of the control transistor T ₂ has a
As described above, it is connected to apply the output of the C/N level detection circuit 26.

With this kind of circuit connection, if the C/N of the input FM signal is above a predetermined level (V _A > V ₀ ), the above C/N
The output of the N level detection circuit 6 causes the control transistor _T2 to operate at a fully saturated level. While the control transistor _T2 is saturated and a low voltage is applied between the connection midpoint O and ground, the capacitance value of the varicap diode D takes into account the capacitance value of the capacitor C. The capacitance values of the series circuits C and D are set to be sufficiently large to bypass the video signal over its entire band.

Next, the C/N level of the input FM signal decreases,
When V _A ≦V ₀ , the control transistor T ₂ becomes
It is controlled according to the DC output of the C/N level detection circuit 6 to change its internal impedance. Correspondingly, since a variable voltage determined by the collector-emitter path impedance of the control transistor _T2 and the resistors _R2 and _R4 is applied to the variable cap D, its capacitance value decreases proportionally.
When the control transistor _T2 is operating in its linear region, the series capacitance of the capacitor C and the variable cap D is at the threshold level region of the graph showing the relationship between C/N and S/N in Figure 1. The NF of the amplifier transistor _T1 changes according to the following changes and exhibits a considerable impedance for the high-frequency components of the video signal where noise on the screen is noticeable.
The amount is increased in inverse proportion to the C/N level of the input FM signal, and the high frequency range of the signal is attenuated in accordance with the amount of decrease in the C/N level. Therefore, in such a case, although the image becomes somewhat soft in tone, the noise is not noticeable.

The above C/N threshold level detection circuit 6
In place of the comparison circuit of , the above control transistor T ₂
The bias point may be set such that the control transistor _T2 operates linearly with a base input below the threshold level Vs of the C/N level.

Since the present invention has the above-described configuration, even if the C/N level of the input FM signal decreases due to an increase in rainfall and falls below the threshold level, the high Change the area characteristics and improve the S/N
Because it cuts out noise components caused by deterioration of
No influence of C/N level is visible on the screen. Furthermore, when designing the FM demodulation circuit of the SHF television receiver, there is no need to provide an unnecessary extra C/N level margin, and costs can be reduced.

[Brief explanation of the drawing]

Figure 1 shows the input CN ratio of the FM transmission/demodulation system,
A diagram showing the relationship between the SN ratio, FIG. 2 is a circuit block diagram of the main part of the present invention, and FIG. 3 is a circuit diagram of the main part of the present invention. 4... Limiter, 5... FM demodulation circuit, 8...
Video amplification circuit, 6...C/N level detection circuit,
9...Variable frequency characteristic circuit.

Claims (1)

[Claims for Utility Model Registration] An FM demodulation circuit that demodulates an input FM signal, a variable frequency characteristic circuit that varies the high frequency characteristics of the output of this FM demodulation circuit, and an AM detection circuit that performs envelope detection of the input FM signal. And, the output of this AM detection circuit is the output of the FM demodulation circuit.
In an SHF television receiver comprising a threshold level detection circuit that detects that C/N is below a threshold level based on CN/SN characteristics and controls the variable frequency characteristic circuit with this detection output. , the variable frequency characteristic circuit is based on the variable frequency characteristic circuit.
The output of the FM demodulation circuit is applied to an amplification transistor whose emitter is connected in parallel with a series connection of a capacitor and a varicap, and whose collector outputs an output, and the connection point between the capacitor and the varicap and a DC current. It is composed of a pair of series resistors connected between power supplies, and a control transistor to which the output of the detection circuit is applied to its base, whose collector is connected to the connection point of the pair of series resistors, and whose emitter is grounded. When the C/N of the FM signal level is above a threshold level, the control transistor operates in a saturated state, and when the C/N is below the threshold level, the control transistor operates in a linear region. According to the SHF television receiver, the series capacitance between the capacitor and the variable cap changes as the C/N decreases, thereby attenuating the high frequency components of the demodulated video signal.