Pulse-step modulation
The pulse step modulation ( PSM , English pulse step modulation ) is a technical modulation method from the field of electrical energy technology . Applications are in electronic AC power supply systems and in radio broadcasting systems with higher transmission power in the short wave and medium wave range . The modulation method was invented in 1994 by Hilmer Swanson at Harris Corporation .
Procedure
The method uses a so-called pulse step modulator . It consists of several individual stages which are connected in series and each consist of a direct voltage source U , a switch S and a freewheeling diode D , as shown in the simplified circuit diagram with four stages. Real pulse-step modulators consist of 32 or more stages; the number depends on the specific application.
The voltage source U of each stage supplies a direct voltage which can be switched by the switch S to the output of a stage. The switch is usually implemented in the form of a bipolar transistor with an insulated gate electrode (IGBT) or a power MOSFET which allows switching frequencies of a few 10 kHz . In addition, there is a freewheeling diode D at the output which ensures the current flow of the active stages in series. In this way, an instantaneous voltage can be set at the load resistor R L , which corresponds to the sum of the active switched stages. With four levels there are four possible voltage levels (1U, 2U, 3U and 4U) at the load resistor which can be activated one after the other in order to simulate a certain signal curve. This is shown in the figure in the timing diagrams on the right with colored bars representing the individual active levels. The course of a sine half-wave is approximated in a rough approximation.
The control of the individual switches can be optimized, as shown in the second time diagram below. The stage that has been on the longest is switched off first, similar to the first-in-first-out principle. This ensures that the load is evenly distributed over time and each stage supplies an approximately equal amount of energy. In order to reduce the step formation inherent in the process, as many steps as possible are used. In addition, a pulse width modulation (PWM) can also take place during the transition from one stage to the next . The subsequent filter effort can thus be minimized.
The individual stages are controlled via a control unit, not shown in the sketch, which defines the individual switch-on and switch-off times. The communication between the control unit and the individual PSM stages takes place with fiber optic cables , which also ensure galvanic isolation .
application
The pulse-step modulator is used, among other things, in large-scale transmitter systems for amplitude modulation at powers around 100 kW to 1 MW in frequency ranges such as medium wave and short wave. Typically 32 to 36 PSM stages are used, with a voltage in the range from 500 V to 750 V and a maximum pulse current of a few amperes per stage. Each individual stage is fed via a galvanically isolated connection; connections for three-phase alternating current are common . The transformer required for this has a primary side, which, depending on the power, is for example directly connected to the medium-voltage network, and a large number of galvanically isolated secondary connections. A separate secondary winding on the transformer is required for each PSM stage.
The total average DC voltage that can be achieved with the PSM is approx. 10 kV to 14 kV, depending on the configuration and system; this corresponds to the case when 50% of the stages are active and serves as the anode DC voltage for the transmitter tube . The transmission tube is usually designed as a single transmission stage with a water-cooled triode . The anode voltage is controlled upwards and downwards by the pulse step modulator depending on the low-frequency useful signal to be modulated, in extreme values between 1 kV and just over 20 kV. In the case of analogue transmissions, only the unmodulated carrier frequency (transmission frequency) is fed to the control grid of the triode . The amplitude-modulated transmission signal can then be capacitively decoupled via the anode circuit and fed to the transmission antenna. This means that compact transmission systems, which are also suitable for digital radio broadcasting according to the Digital Radio Mondiale (DRM) standard , can be implemented with high transmission power.
literature
- Graham A. Jones, David H. Layer, Thomas G. Osenkowsky: National Association of Broadcasters Engineering Handbook . Taylor & Francis, 2013, ISBN 978-1-136-03410-7 , pp. 687 .
Individual evidence
- ^ RJ Adler, B. Ashcraft, RJ Richter-Sand: Pulse step modulation to produce very fast solid state pulses . In: Pulsed Power Plasma Science, 2001. IEEE Conference Record - Abstracts . 2001, doi : 10.1109 / PPPS.2001.961029 .
- ^ PJ Patel et al .: A Regulated Power Supply for Accelerator Driven System. Retrieved June 6, 2015 .
- ↑ Patent US5309114 : Pulse step modulator. Applied June 30, 1993 , published May 3, 1994 , applicant: Harris Corporation, inventor: Hilmer I. Swanson.
- ^ W. Tron: Modern and Crowbarless HVPS. (No longer available online.) Archived from the original on September 23, 2015 ; Retrieved June 7, 2015 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
