Foster–Seeley discriminator

As a discriminator (Latin discriminare = separate, distinguish ) are referred to different devices or assemblies of equipment within the telecommunications, electronics and instrumentation. Discriminators are used to evaluate analog or digital signals, among other things, for the demodulation of frequency-modulated signals (FM ) signals.

• 3.1 distinction of pulse heights 3.1.1 threshold discriminator
• 3.1.2 single channel analyzer

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 still next to the Koinzidenzdemodulator and the PLL demodulator. PLL demodulators are sometimes also referred to as discriminators, although they have a different working principle. Also sometimes referred to as the Koinzidenzdemodulator Koinzidenzdiskriminator and on the other hand, the phase (see below) as a phase demodulator.

Phase discriminator

The modulated voltage is applied through a coupling capacitor Ck to the center tap of the coil L2 ( see diagram ). At the same time in the coil L2, a voltage is induced, which is added to the two resulting half - voltages once the same and even in opposite directions. The circuits are tuned to subtract the rectified voltages from the diodes R1 and R2 at medium frequency to zero. When frequency change these voltages are not equal. The result is a low frequency of the modulating voltage proportional to the output. The entire process before demodulation corresponds to a conversion of the frequency in a phase and finally to amplitude modulation.

Applies, the phase in older FM and TV receivers for demodulation and AFC control voltage recovery.

Ratio detector

The ratio detector provides a dependent of the relative frequency deviation with respect to its oscillator signal. It is constructed similarly to the phase discriminator, the diodes are connected in opposition, however, and the rectified voltages add up. The output voltage is taken at it between the two resistors and the two capacitors. Instead of the capacitor Ck, a third coil is connected between the center tap of L2 and the center between the two capacitors.

The ratio detector provides for the frequency demodulation of the advantage that it is less affected by amplitude variations.

Counting discriminator

The counting discriminator counts the pulses per unit time and generates a voltage that is a changing, for example, generated from a frequency-modulated alternating voltage signal pulse ratio the same. It serves inter alia, in digital radio receivers for demodulation or to view the receive frequency.

The counting discriminator delivers a dependent of the absolute frequency signal.

Pulsweitendiskriminator

The Pulsweitendiskriminator based on the conversion of frequency modulation in a pulse width modulation by a frequency to be demodulated by the triggered monoflop. From the pulse width modulated voltage can be obtained with a low pass, a frequency proportional to the signal. Application it is used for example for speed measurement and control or for flow measurement based on the signal of an incremental encoder, but there were also a few FM receiver (eg Kenwood KT -1000, KT- 1100), which with the FM signal a Pulsweitendiskriminator (pulse count detector) demodulated.

The Pulsweitendiskriminator delivers a dependent of the absolute frequency signal.

A further monostable flip-flop can be used to generate a digital signal whenever the frequency falls below a certain value higher or lower. With three one-shots can be a frequency window discriminator set up ( see below).

Window discriminator

A window comparator is used to evaluate the level of a DC voltage. Window discriminators are often composed of comparators and operational amplifiers. Comparing an input voltage with two predetermined voltage levels by the circuitry defining a "window". To the upper and lower limit of the voltage range is supplied from a reference voltage depending on a comparator. The digital outputs of the two comparators are those of the window discriminator. Report whether the input voltage is below, within or above the pane. The two output signals can be used for further evaluation linked together, for example to a signal " inside / outside " to make a decision, for example, in the manufacture of electronic components well ( within the tolerance or window area ) or bad ( out of tolerance ).

Other application examples are over-and under-voltage detection (mains voltage, batteries ) or the evaluation of process variables (eg, water flow is too large - correct - too small).

Window discriminators can also be implemented digitally for the analysis of frequencies, or as software in a microcontroller.

Radiation metrology

In the processing of analog pulses, such as pulses of particles and the radiation detectors also discriminators can be used. These discriminators are used to distinguish the useful signals from other, smaller pulses ( for example of the dark current of the detector ) or for obtaining time stamps for the occurrence of the pulses.

Distinction of pulse heights

Threshold discriminator

The threshold discriminator is a comparator substantially. It provides a standard pulse when the input pulse exceeds a selected voltage level.

Single channel analyzer

The single channel analyzer contains two identical Schwellendiskriminatoren with different set threshold voltages; it works similar to the window discriminator above. A standard output pulse appears, when the height of the input pulse is in the " window.", The voltage window is moved gradually with a constant width over the entire range of the input pulse height can be determined by registration of the output pulses of the pulse height spectrum, which is the frequency distribution of the pulse heights are measured (assuming that that distribution and the total pulse rate per unit time during the duration of the measurement does not change).

The name has been selected on the basis of the multi-channel analyzer, which is also used for analysis ( measurement) of the pulse height spectra.

Timestamp extraction

Coincidence measurements for a defined logic signal must be derived from the input pulses of different height and different shape may. In simpler cases, it takes is a threshold discriminator with a relatively low threshold. However, it gives a systematic dependence of the timing of the pulse height, because the pulses usually have a more or less constant rise time, so that a higher pulse, calculated from the start signal, exceeds the threshold earlier than a smaller one.

This dependence (English as a walk called ) is avoided or reduced by a discriminator does not determine the time from the exceeding of a fixed threshold voltage, but from the attainment of a certain fraction of the pulse height. This device is called english constant fraction discriminator. A possible German term is " Proportionaldiskriminator ".