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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Adaptive receiver of pulses of slowly varying amplitude. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer An impulse signal transmitted through a communication channel or coming from a moving object is always unstable in amplitude and is often affected by impulse noise. If the rate of change of the communication channel parameters is relatively small and they do not have time to change significantly from pulse to pulse, the amplitude of each subsequent received pulse differs only slightly from the amplitude of the previous one, although it can change several times over a long period of time. Such a signal can be called inertial. Using its features can improve reception reliability and noise immunity. In order to increase the probability of suppressing false and useful pulses, it was proposed to distinguish them by the criterion of amplitude change inertia [1–5]. To do this, the amplitude selector should provide for forced recharging of the capacitor storing the detection threshold with each selected useful signal pulse to a voltage equal to a certain fraction of its amplitude. The threshold voltage must remain unchanged until the next pulse is emitted. In the absence of such recharging, the threshold in the pauses between useful signal pulses is usually gradually reduced so that those whose amplitude is less than the previously set threshold are not suppressed. However, the use of a gradually decreasing threshold leads to the fact that the maximum amplitude of the suppressed false pulse depends on its delay relative to the useful signal pulse. The later the interference comes, the smaller the amplitude it must have in order to be suppressed. With a long absence of useful pulses, the threshold will spontaneously decrease to almost zero and interference of even the smallest amplitude will not be suppressed. To suppress larger amplitude noise, the threshold decay should be as slow as possible. At the same time, the rate of decline must be greater than the maximum possible rate of decrease in the amplitude of the useful signal so that it cannot fall below the threshold.
The figure shows a diagram of a selector that selects the useful signal pulses according to another criterion that takes into account the inertia of changing their amplitude. The pulses arriving at the input with a duration of about 0,5 μs (the element ratings are calculated for this pulse duration) have a negative polarity at a zero level of +10 V. Further, speaking about the amplitude of the pulses and the threshold, we will compare them by absolute value relative to this level. The pulse repetition rate for selected types of transistors and capacitor ratings can be in the range from approximately 50 Hz to 1 MHz. The selector remembers the amplitude of each pulse that exceeds the threshold and sets a new threshold at 80% of this amplitude. Pulses that do not exceed it are considered interference and do not pass to the output. With each new pulse passing the selector, the threshold adjustment is repeated. The storage capacitor C3 is always charged to a voltage equal to the amplitude of the last detected useful pulse. Approximately 20% less voltage on the capacitor C3, the threshold voltage is formed on the resistor R6. It is applied to the emitter of the transistor VT4, which is opened by a pulse entering its base through the VD4 diode only if the peak value of this pulse exceeds the voltage at the emitter. The threshold voltage is also supplied through the resistor R2 to the anode of the diode VD2. Part of the useful pulse that has exceeded the threshold passes through the diode VD2 to the differentiating capacitor C1, which forms a short pulse from it that opens the transistor VT1. Capacitor C3Z is partially discharged through an open transistor, and then charged through diode VD3 and resistor R1 to the amplitude of the input pulse. Thus, each useful pulse corrects the threshold voltage in proportion to its amplitude. Interference that does not exceed the threshold does not pass through the VD2 diode and does not change the voltage across the capacitor C3. The time constant for discharging capacitor C3 by the gate current of transistor VT2, the reverse current of the collector of transistor VT1 and diode VD3 exceeds 0,02 s. Therefore, until the arrival of the next pulse of the useful signal, the voltage across the capacitor remains practically unchanged. This provides increased noise immunity in the pauses between useful pulses. The amplitude of these pulses can vary from 2 to 10 V, respectively, its threshold varies from 1,6 to 8 V. The pulses that have exceeded the threshold from the collector of the transistor VT4, having passed the emitter follower on the transistor VT5, start the single vibrator on the transistors VT6 and VT7. It generates selector output pulses with TTL levels and a duration of 0,5 µs. Literature
Author: V. Solonin
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