Discriminator
As a discriminator (lat. Discriminare = separate part) may be different devices or assemblies within designated devices in communications engineering, electronics, and instrumentation. Discriminators are used to evaluate analog or digital signals, including demodulating frequency-modulated signals (FM signals).
Evaluation of frequency-modulated signals
An FM discriminator converts the frequency modulation (FM) into an amplitude modulation (AM) or pulse width modulation (PWM), which is then demodulated. The FM discriminators described here are not the only ways to demodulate FM; there are also the coincidence demodulator and the PLL demodulator. PLL demodulators are sometimes also called discriminators, although they work on a different principle. The coincidence demodulator is also sometimes referred to as a coincidence discriminator and, on the other hand, the phase discriminator (see below) as a phase demodulator .
Phase discriminator
The modulated voltage is sent to the center tap of coil L2 via a coupling capacitor Ck (see circuit diagram). At the same time, a voltage is induced in the coil L2, which is added once in the same direction and once in opposite directions with the two resulting half-voltages. The circles are tuned in such a way that the voltages rectified by the diodes at R1 and R2 subtract to zero at medium frequency. When the frequency changes, these voltages are no longer the same. A voltage proportional to the modulating low frequency is generated at the output. The entire process before demodulation corresponds to a conversion of the frequency into a phase and finally into an amplitude modulation.
The phase discriminator is used in older VHF and television receivers for demodulation and AFC control voltage generation.
Ratio detector
The ratio detector delivers a signal that is dependent on the relative frequency deviation compared to its resonant circuit. It is constructed similarly to the phase discriminator, but the diodes are switched in opposite directions and the rectified voltages add up. The output voltage is picked up between the two resistors and between the two capacitors. Instead of the capacitor Ck, there is a third coil between the center tap of L2 and the center between the two capacitors.
With frequency demodulation, the ratio detector has the advantage that it is less influenced by amplitude fluctuations.
Counting discriminator
The counting discriminator counts the pulses per unit of time and generates a voltage that is proportional to a changing pulse frequency, for example generated from a frequency-modulated AC voltage signal.
He serves u. a. in digital radio receivers for demodulation or to display the reception frequency.
The counting discriminator supplies a signal that is dependent on the absolute frequency.
Pulse width discriminator
The pulse width discriminator is based on the conversion of a frequency modulation into a pulse width modulation by means of a monoflop triggered by the frequency to be demodulated . A signal proportional to the frequency can be obtained from the pulse width modulated voltage with a low pass .
It is used, for example, for speed measurement and control or also for flow measurement based on the signal from an incremental encoder , but there were also a few VHF receivers (e.g. Kenwood KT-1000, KT-1100) that transmit the FM signal demodulated a pulse width discriminator (pulse count detector).
The pulse width discriminator supplies a signal that is dependent on the absolute frequency.
Another monoflop can be used to generate a digital signal whenever the frequency exceeds or falls below a certain value. A frequency window discriminator (see below) can be set up with three monoflops.
Window discriminator
A window discriminator is used to evaluate the level of a DC voltage . Window discriminators are often made up of operational amplifiers connected as comparators . They compare an input voltage with two voltage levels determined by the circuit, which define a "window". For this purpose, the upper and lower limit of the voltage range is provided from a reference voltage to one comparator. The digital outputs of the two comparators are those of the window discriminator. They report whether the input voltage is below, within or above the window area. The two output signals can be linked to one another for further evaluation, for example to form an "inside / outside" signal, for example in order to make a decision good (within the tolerance or window range) or bad (outside the tolerance range) in the production of electronic components .
Further application examples are overvoltage and undervoltage detection (mains voltage, accumulators) or the evaluation of process variables (for example water flow too large - correct - too small).
Window discriminators can also be implemented digitally to evaluate frequencies or as software in a microcontroller .
Radiation measurement technology
Discriminators are also used when processing analog pulses, such as those from particle and radiation detectors . These discriminators are used to differentiate the useful signals from other, smaller pulses (for example from the dark current of the detector) or to obtain time stamps for the occurrence of the pulses.
Differentiation of pulse heights
Threshold discriminator
The threshold discriminator is essentially a comparator . It delivers a standard pulse when the input pulse exceeds a selected voltage level.
Single channel analyzer
The single-channel analyzer contains two identical threshold discriminators with different threshold voltages; it works similarly to the window discriminator mentioned above. A standard output pulse appears when the height of the input pulse is in the "window". If the voltage window is gradually shifted with a constant width over the entire range of the input pulse height, the pulse height spectrum , i.e. the frequency distribution of the pulse heights, can be measured by registering the output pulses (provided that this distribution and the total pulse rate per time unit vary during the duration of the measurement).
The name was chosen based on the multichannel analyzer , which is also used to analyze (measure) pulse height spectra.
Time stamp acquisition
For coincidence measurements , the input pulses of different height and u. U. different forms a defined logic signal can be obtained. In simpler cases, a threshold discriminator with a relatively low threshold is sufficient. However, it results in a systematic dependence of the point in time on the pulse height, because the pulses usually have a more or less constant rise time , so that a higher pulse, calculated from the start of the signal, exceeds the threshold earlier than a smaller one.
This dependency (called walk ) is avoided or reduced by a discriminator which does not determine the point in time from exceeding a fixed threshold voltage, but from reaching a certain fraction of the pulse height. This device is called English Constant Fraction Discriminator . A possible German term is "Proportionaldiskriminator".
literature
- Modern radio reception technology , Martin Gerhard Wegener, Franzis-Verlag, Munich 1985, ISBN 3-7723-7911-7 & Yüce-Group, Istanbul 1989, ISBN 975-411-058-1
- Expertise in radio, television and information electronics , Europa-Verlag 2000, ISBN 3-8085-3284-X
- Signals - Processes - Systems , Ulrich Karrenberg, Springer-Verlag 2002, ISBN 3-540-41769-9
- Radiation Detection and Measurement , 2nd edition, GF Knoll, Wiley, New York 1989