Wobble generator
A wobble generator or wobble transmitter (for short: wobbler , English: sweep generator ) is an electronic device for generating sinusoidal oscillations, with the frequency ( low frequency to high frequency ) varying cyclically between two adjustable end values.
use
By combining it with other measuring devices , the frequency-dependent behavior of a test object ( loudspeaker , amplifier , filter circuit , line) can be measured and displayed graphically.
In the telephone - and Telegrafiebereich simple wobbler be as ringing alarm at by measuring ( by ringing used) of two-wire lines.
construction
Early wobble generators achieved the cyclical change in their output frequency by mechanical means using a motor-driven frequency-determining element ( variable capacitor , variometer coil ). For the lower high-frequency range, voltage-controlled oscillator circuits , so-called VCOs ( Voltage Controlled Oscillators ) , were soon used, while the mechanical tuning in the microwave range could last longer because some components were not yet able to process the required frequencies.
The first VCO circuits implemented a variable reactance as a frequency-determining element by shifting the operating point of an electron tube . From the early 1970s, capacitance diodes took over this function, and YIG oscillators are used for very high frequencies .
The change in frequency is controlled by a separate, slow oscillator, usually a sawtooth generator ( so-called according to the form of its output signal) - the output voltage increases linearly up to a maximum value and then returns to the initial value in a short time - or by a Generator that generates a sinusoidal low-frequency voltage.
Since a representation with a logarithmic frequency axis is often desired, many wobblers contain a circuit that exponentially deforms the sawtooth signal accordingly before it is sent to the VCO.
In addition, many wobble generators have permanently adjustable markers, the frequency of which is mixed with the set frequency. This creates markings within the signal curve.
The fully digital synthesis of a wobble transmitter signal is usually carried out using Direct Digital Synthesis , or DDS for short, whereby the sweep function is achieved by continuously setting the frequency based on a desired function (e.g. sawtooth).
Spectrum analyzers equipped with a "tracking generator" are now also used for wobble measurements .
Many function generators also contain a wobble function.
application
If necessary, the output signal of the wobbler is amplified and connected to the input of the measurement object. If necessary, the signal at the output of the measurement object is amplified and processed for display (rectification, logarithmization). To measure non-electrical signals, appropriate transducers are used together with the test object (e.g. a microphone to wobble loudspeakers).
An oscilloscope is used for display or recording , the horizontal deflection of which is synchronized with the wobble generator . In the simplest case, the tuning voltage of the VCO in the wobble generator is connected to the X-channel of the oscilloscope. In order to avoid a disturbing second display of the result when the VCO voltage declines quickly, some wobble generators have an output at which a pulse is applied when the VCO voltage declines; which dark controls the beam via the Z input (brightness) of the oscilloscope.
Especially when measuring narrow-band objects (filters), the frequency must be swept slowly in order not to falsify the measurement result. As a result, however, the display on the oscilloscope starts to flicker strongly or only a slowly moving point is visible. Before the advent of digital storage oscilloscopes (DSO), photoluminescent picture tubes were used so that the entire transmission curve could be displayed, the image of which could also be photographed.
Combination devices also existed which included a wobble transmitter, demodulator / amplifier and a cathode ray tube display, e.g. B. the type "Polyskop III SWOB" from Rohde & Schwarz for 0.1 ... 1000 MHz from approx. 1970. With the device, partial ranges could be shown expanded during the frequency return (magnifying glass function for the frequency response).
For slow measurements, the use of an XY recorder came into question, which is also obsolete today due to the advent of digital technology - the results are recorded with DSO or computer and processed graphically.
Applications
- Amplitude response of the gain / attenuation of a circuit (amplifier, filter, attenuator) or a component ( transistor , resonant circuit , transformer , line ) depending on the frequency (the classic "wobbling")
- Acquisition of S-parameters (evaluation with vector analyzer )
- Loudspeaker measurement (course of sound pressure over frequency, resonance frequency )
literature
- Otger Neufang: Lexicon of Electronics. Vieweg & Sohn, Braunschweig 1983, ISBN 3-528-04099-8 .
- Wilhelm Friedrich, Franz Stejskal: Electronic paperback volume II. Verlag Dümmler, Bonn 1977, ISBN 3-427-53282-8 .
Web links
- Polyscope SWOB I on radiomuseum.org
Individual evidence
- ↑ http://www.classicbroadcast.de/downloads/rohde_SWOB3.pdf Operating instructions Polyskop III, universal four-channel frequency response display device for high-frequency technology , accessed on Jan. 22, 2019