Analysis of several unusual causes of satellite receivers
The satellite TV receiver is the part with the most functions, the most complex circuit, and the largest power consumption in the satellite TV receiving equipment. It not only needs to complete a series of signal processing from the first IF to the image and the accompanying sound, but also realizes various functions such as selection, control, conversion, remote control, and display.
Once the satellite TV receiving device fails, the first thing is to determine where the fault is. That is the most important part of the maintenance process. Sometimes the abnormality of the satellite TV receiver is not a problem with the internal components of the satellite TV receiver, but the wrong installation due to the lack of understanding of its characteristics. Today let us learn some knowledge of why the satellite receiver works abnormally. The following is an analysis of the causes of several abnormalities in this regard.
1. Tuner medium frequency amplitude frequency characteristics are not flat
There is a mosaic phenomenon in one or two channels. The main reason for this failure is that the tuner used by the receiver does not have flat amplitude-frequency characteristics, and the receiving intermediate frequency range may also exist in some frequency points (segments) amplitude-frequency attenuation. The gain of the tuner is near the frequency point of 1068-1099MHz, which is in the trough region, resulting in a weak signal after degradation, which is between the receiver thresholds, resulting in the generation of the mosaic phenomenon.
2. Tuner IF frequency range is narrow
Some receivers have a narrow IF frequency range, which cannot reach the standard bandwidth, resulting in some transponders cannot receive. Therefore, It will lead to the receiving channel having no signal or poor signal quality. Generally speaking, when the satellite receiver is working, the received signal first passes through the high-frequency head of the outdoor unit, amplifies and down-converts the received satellite signal, and converts it into an intermediate frequency signal of a unified frequency band that the receiver can process. In the early satellite receivers, the IF bandwidth was 500MHz and 800MHz, and the corresponding IF frequency range was 950-1450MHz and 950-1750MHz. Now the standard bandwidth of a satellite digital receiver is 1200MHz, the intermediate frequency range is between 950 and 2150MHz, and the C and Ku-band HF heads with various local oscillator values can receive satellite transponder signals of any frequency band and frequency.
3. Narrow range of symbol rate
Another cause of satellite receiver anomalies is the narrow symbol rate range. Symbol rate refers to the rate of transmitted data. Restricted by past reception conditions, the symbol rate range of early satellite receivers was 2~35MS/s, and now it is 2MS/s~45MS/s. At that time, the satellite transponder bandwidth was 36MHz and based on the ratio of the transponder bandwidth to the signal symbol rate of 1.2:1, the maximum symbol rate was only 30MS/s. At present, the bandwidth of the transponder is 54MHz or 72MHz. For example, for a bandwidth of 54MHz, the highest symbol rate can reach 45MS/s. When the FEC is higher than 1/2, the CPU is required to provide a higher input code rate, otherwise, the signal with a high symbol code rate will not be received at the upper end.
4. Slow transient response of 22V power supply.
There is no problem to switch when connecting to the same star, but when changing stars, you must switch from the vertical polarization (V) program of the first star to the horizontal polarization (H) signal of the other star. In addition, the channel occasionally shows no signal when the power is turned on. Since the receiver not only provides 13/18V power supply to the LNB, but also supplies power to the DiSEqC four-in-one, 0/22kHz, 13/18V toggle switch, so the load is relatively heavy. The power supply with large internal resistance and slow transient response has a large voltage drop at the moment of switching, resulting in a decrease in the amplitude of the output 22kHz pulse signal, and a lower voltage at the switching point, which affects the identification of the 22kHz pulse.
5. 13/18V polarization voltage is high
If the receiver output polarization voltage of 13/18V is high, using a long feed line can play a role in voltage reduction, which can make the receiver send the polarization voltage to the LNB back to the normal range; while the feed line is too short, the polarization voltage sent to the LNB is still high. For example, the normal polarization voltage is 13/18V, and the polarization switching voltage action point inside the LNB is 15V. But if the received polarization voltage is very high, such as 15/20V, then the feeder is short, and the voltage input to the switch is still 15/20V, which is higher than the operating point voltage, so of course it cannot be switched to the 13V corresponding to the vertical polarization port.
6. 22kHz switching pulses are disturbed
Switching is not sensitive, sometimes difficult to switch properly. This is mainly due to interference with the 22kHz switching pulse output of the micro-DC satellite digital receiver. Miniature machine using DC-12V DC-DC switching power supply, when the 13/18V power supply filtering performance is poor, it will affect the 22kHz switching pulse signal carried. As the operating frequency of the DC-DC converter circuit of the receiver is 33kHz, which is close to the switching pulse frequency, the capacity of the filter capacitor (100μF/25V) of the 13/18V booster circuit is obviously small, resulting in the unstable and distorted 22kHz waveform output from the F22/DiSEqC port of the QPSK demodulator chip. In the 0/22k switch, there is a 22kHz frequency selection network circuit, which can detect the presence of 22kHz pulses in the feeder connected to the receiver, and thus control the on/off of the two input interfaces. When the 22kHz switching signal with unstable waveform and excessive distortion enters the 0/22k switch, the internal circuitry is mis-sampled and not correctly identified, resulting in unresponsive switching. the DiSEqC switching switch has more stringent requirements for 22kHz pulses, and when its waveform distortion is too large, the circuitry inside the switching switch refuses to process it, making it difficult to switch properly.
Satellite TV reception has become widely popular in recent years. Through the above analysis of abnormal causes, it will provide accurate judgment of some abnormal causes in time for installation and commissioning work. Further improve the quality of satellite TV reception work, to avoid the impact of some wrong judgment on satellite TV reception work.