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TEA6101 Arkusz danych(PDF) 5 Page - NXP Semiconductors |
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TEA6101 Arkusz danych(HTML) 5 Page - NXP Semiconductors |
5 / 13 page May 1992 5 Philips Semiconductors Objective specification Antenna diversity circuit TEA6101/T FUNCTIONAL DESCRIPTION Various forms of disturbance can affect signal reception in car radio receivers: • ignition interference produces spikes on the audio signal. Switching to another antenna will be ineffective. Strong ignition interference, however, will modulate the antenna field strength. In this instance another antenna possessing a directional pattern will suffer less disturbance and switching would be appropriate. • variation of antenna field strength due to travelling through a zone of variable signal strength will result in a variation in the signal level. Greater noise will be apparent on the audio signal whilst the IF limiter is not limiting. Switching to an alternative antenna input would increase the signal strength. • multipath reception occurs when a signal reaches the antenna from two or more directions. Often the signals will be of different phase. In certain circumstances the sum of the reflected signals results in zero and a large spike will be evident on the audio signal. It will then be necessary to switch to an alternative antenna from which the sum of the received signals will be different. The criteria for an antenna diversity system are high frequency components (spikes and noise) on the audio signal in combination with variations in signal level. Detection of spikes on the audio signal A rectifier, high pass filter, low pass filter and a comparator are used to detect spikes and noise on the audio signal (see Fig.1). The negative spikes are detected by the rectifier whilst a high pass filter removes the audio signal to leave the high frequency signal components at the negative input to the comparator. The signal at the positive input to the comparator consists of an offset together with an audio signal attenuated by the low pass filter. If the amplitude of the spikes exceed that of the attenuated audio plus offset, the output of the comparator is HIGH. When the switching rate of the comparator is HIGH, feedback increases the offset via the diode, the resistor R1, and the 100 nF capacitor. The offset is decreased by the 12 k Ω resistor and the 100 nF capacitor (pin 11 or 13). The result is an offset based upon the comparator switching rate, rapid to increase but slow to decrease, therefore permitting only the largest spikes to trigger the comparator (floating threshold). Should high noise be apparent on the audio signal, the offset is decreased by means of the rectifier and high pass filter. This will result in more frequent switching to an alternative antenna whilst the result of the switching operation will be less audible. Detection of voltage level variation A 1 µF input capacitor and 20 kΩ resistor remove the absolute level voltage to leave only variations to be detected. The level comparator output is HIGH when the variations in level voltage are greater than the offset. Similarly to the audio comparator; the feedback diode, resistor R2, the 1 µF capacitor and the 33 kΩ resistor cause the threshold level to float. During periods of high activity the comparator thus switches only on the largest variations. Switching to an alternative antenna When both the level and the audio comparator outputs are HIGH, another output of the Johnson counter will be selected. Since switching to an alternative antenna would cause a disturbance of the audio and level signals the monostable multivibrator will prohibit the counter from selecting another antenna input for 21 µs. Memory and timing Approximately similar qualities of signal originating from different antennae could result in unnecessary antenna switching. This is prevented by appointing a priority antenna. The selection of an antenna without priority results in the audio offset being decreased by 1.2 V such that the audio comparator will have a HIGH output voltage. During the period of memory timing the offset increases towards the normal offset value. Should level alterations occur during this period another antenna will be selected. If, however, the memory is timed-out without the occurrence of signal variation, priority will be appointed to the selected antenna. Thus a priority antenna will be selected for the majority of the time during reception of almost all similarly weak antenna signals. Mute A mute function should not precede the circuit. This function is therefore assumed by the TEA6101. When used in combination with the TEA6100 the 20 k Ω input of the IF IC together with the 6 k Ω output resistor of the TEA6101 cause an attenuation of 3 dB. The mute circuit therefore has 3 dB amplification of level voltages in excess of 2.75 V. |
Podobny numer części - TEA6101 |
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Podobny opis - TEA6101 |
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