JPS6314533B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present description relates to a carrier-to-noise power ratio (hereinafter abbreviated as C/N) detector provided on the receiving side of a satellite communication system.

Conventionally, this C/N detector consists of an automatic gain control (abbreviated as AGC) amplifier that receives a received input signal in the intermediate frequency (hereinafter abbreviated as IF) band that includes a carrier wave and noise, and an output signal in the IF band of this AGC amplifier. It has a local oscillator and mixer for converting the output into the baseband band, and a baseband band pass filter for extracting noise from the output of the baseband band.
The passband width of this baseband bandpass filter is wide in order to facilitate C/N detection. However, because the passband width of the baseband bandpass filter is wide, conventional C/N detectors cannot extract only the noise power, and the A drawback is that the C/N cannot be detected accurately when the modulation product (hereinafter abbreviated as IM) noise is large.

The purpose of the present invention is to solve the above-mentioned conventional drawbacks,
The object of the present invention is to provide a C/N detector that can accurately detect C/N without being affected by IM noise.

According to the present invention, there is provided an AGC amplifier that receives an IF band reception input signal including a carrier wave and noise, and
In a C/N detector having means for converting an output of an IF band of an amplifier into a baseband band, and a baseband band bandpass filter for extracting noise from the output of the baseband band, the baseband band bandpass filter A C/N detector is obtained in which the passband width of the C/N detector is set to a narrow band that is not affected by IM noise caused by nonlinearity of a transmission path.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an example of the configuration of a C/N detector applied to a single channel per carrier (hereinafter abbreviated as SCPC) system. In the figure, 1 is an IF band reception bandpass filter, and 2 is an IF band receiving bandpass filter.
AGC amplifier, 3 is a distributor, 4 is an IF band narrow band pass filter, 5 is a detector, 6 is a mixer, 7 is a local oscillator, 8 is a baseband band band pass filter, 9 is a baseband band amplifier, 10 indicates a detector.

Next, the operation of the circuit shown in FIG. 1 will be explained. The received input signal Sin in the IF band including the carrier wave and noise is the IF
Passes through band receiving band pass filter 1 and AGC amplifier 2,
It is divided into two by the distributor 3. Here, in the SCPC method,
Generally, an unmodulated IF band pilot signal _SP is used, and the AGC amplifier 2 controls the level of the IF band pilot signal _SP among its outputs to be always constant. Therefore, one of the IF band outputs divided into two by the divider 3 is input to the IF band narrow band pass filter 4 to extract only the IF band pilot signal _SP .
By passing the IF band pilot signal through the detector 5 and controlling the AGC amplifier 2 by the AGC voltage output from the detector 5, the signal output from the AGC amplifier 2 is determined by the IF in the output signal. The level of the band pilot signal S _P is controlled to be always constant.

On the other hand, the other of the IF band outputs divided into two by the divider 3 is input to the mixer 6 and converted to the baseband band by the local oscillator 7. Then, noise power is extracted from this baseband output S _B by a baseband band pass filter 8, and this extracted noise power passes through a baseband band amplifier 9 and is input to a detector 10. and output as voltage Sout.

Here, since the pilot signal level is always kept constant by AGC amplifier 2, the C/N
When C/N changes, the output noise of AGC amplifier 2 changes
Since the output voltage Sout of the detector 10 also changes in response to the change in the output noise, the amount of change in the output voltage Sout corresponds one to one to the amount of change in C/N. Therefore, the output voltage Sout
The C/N can be detected by reading.

In FIG. 1, in the conventional C/N detector, the passband width of the baseband bandpass filter 8 is
The wider the passband width, the greater the noise power that can be taken out as an output, making it easier to detect C/N, so it was set to a wide band. Figure 2 shows this situation.

Referring to Figure 2, in general, in the SCPC method,
Baseband carrier waves C are arranged for each Δf, and a guard band GB is provided near the baseband pilot signal P in order to perform automatic frequency control of reception by this pilot signal P. Therefore, in the conventional C/N detector, the passband width of the baseband bandpass filter 8 in FIG. 1 is changed to the guard band width as shown by the dotted line in FIG.
It was set for the entire GB, and the noise power of this guard band GB was extracted.

However, in an actual satellite loop, as shown in FIG. 3, IM noise exists due to the nonlinearity of the transmission path of the satellite loop, so the IM falls even within the guard band GB. Here, the IM positions are at intervals of Δf, but each level is constantly changing depending on the frequency of the baseband carrier wave C, voice activation, etc. Therefore, with conventional C/N detectors, it is not possible to extract only the noise power excluding IM noise, and when the influence of IM is large,
Accurate C/N cannot be detected.

In the present invention, in order to prevent the IM from being removed,
The passband width of the baseband bandpass filter 8 in FIG. 1 is set within Δf, and the narrowband Set to . An example of this situation is shown in FIG.

Referring to FIG. 4, in the present invention, either the area indicated by the dotted line [ ] or [ ] is
This is used as the passband width of the baseband bandpass filter 8 in FIG. Since the noise power excluding the IM noise extracted in this way is very small,
Output from the baseband bandpass filter 8
It is necessary to considerably increase the gain of the baseband amplifier 9 to which the noise power excluding IM noise is input. In this case, the stability of the baseband amplifier 9 becomes a problem, but it is recommended to use an extremely stable amplifier as the baseband amplifier 9, or to add an automatic level control function to the baseband amplifier 9. Therefore, the above stability problem can be solved.

As is clear from the above explanation, according to the present invention, by setting the passband width of the baseband bandpass filter to a narrow band that does not extract IM, the modulation caused by the nonlinearity of the satellite loop can be reduced. This has the effect that it is possible to remove the influence of the IM caused by the IM, and it is possible to detect an accurate C/N.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing an example of the configuration of a C/N detector applied to the SCPC method, and Figure 2 shows the passband width of a baseband bandpass filter in a conventional C/N detector. 3 is a diagram showing the positional relationship of IM caused by nonlinearity of the satellite loop, and FIG. 4 is a diagram showing the passband width of the baseband bandpass filter in the C/N detector according to the present invention. It is a figure showing one example of this. 1...IF band reception bandpass filter, 2...AGC
Amplifier, 3...Distributor, 4...IF band narrow band pass filter, 5...Detector, 6...Mixer, 7...Local oscillator, 8...Baseband band pass filter, 9... Baseband amplifier, 10...detector.

Claims (1)

[Claims] 1. An automatic gain control amplifier that receives a received input signal in an intermediate frequency band containing a carrier wave and noise, means for converting the output of the intermediate frequency band of the automatic gain control amplifier into a baseband band, and a means for converting the output of the intermediate frequency band of the automatic gain control amplifier into a baseband band, and converting the output of the baseband band to noise. In a carrier-to-noise power ratio detector having a baseband bandpass filter that extracts A carrier-to-noise power ratio detector characterized in that it is set to a narrow band that is not affected by carrier waves.