Bridged T delay equaliser
Linear analog electronic filters |
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The bridged-T delay equaliser is an electrical
Applications
The network is used when it is required that two or more signals are matched to each other on some form of timing criterion. Delay is added to all other signals so that the total delay is matched to the signal which already has the longest delay. In television broadcasting, for instance, it is desirable that the timing of the television waveform synchronisation pulses from different sources are aligned as they reach studio control rooms or network switching centres. This ensures that cuts between sources do not result in disruption at the receivers. Another application occurs when
Operation
The operation is best explained in terms of the
The relationship between phase shift (φ) and time delay (TD) with angular frequency (ω) is given by the simple relation,
It is required that TD is constant at all frequencies over the band of operation. φ must, therefore, be kept linearly proportional to ω. With a suitable choice of parameters, the network phase shift can be made linear up to about 180° phase shift.
The network is terminated in a characteristic impedance (not shown in the circuit diagram), ideally a resistance R, which is the input impedance to the successive circuit or transmission line.
Design
The four component values of the network provide four degrees of freedom in the design. It is required from image theory (see Zobel network) that the L/C branch and the L'/C' branch are the dual of each other (ignoring the transformer action) which provides two parameters for calculating component values. These are
- and
Equivalently, every transmission
There is one remaining degree of freedom that the designer can use to maximally linearise the phase/frequency response. This parameter is usually stated as the L/C ratio. As stated above, it is not practical to linearise the phase response above 180°, i.e. half a cycle, so once a maximum frequency of operation, fm is chosen, this sets the maximum delay that can be designed in to the circuit and is given by,
For broadcast sound purposes, 15 kHz is often chosen as the maximum usable frequency on landlines. A delay equaliser designed to this specification can, therefore, insert a delay of 33μs. In reality, the differential delay that might be required to equalise may be many hundreds of microseconds. A chain of many sections in tandem will be required. For television purposes, a maximum frequency of 6 MHz might be chosen, which corresponds to a delay of 83ns. Again, many sections may be required to fully equalise. In general, much greater attention is paid to the routing and exact length of television cables because many more equaliser sections are required to remove the same delay difference as compared to audio.
Superconductor planar implementation
Losses in the circuit cause the maximum delay to be reduced, a problem that can be ameliorated with the use of
See also
References
Cited references
- H. J. Chaloupka, S. Kolesov, "Design of lumped-element 2D RF devices", H. Weinstock, Martin Nisenoff (eds), Microwave Superconductivity, Springer, 2012 ISBN 9401004501.
General references
- Jay C. Adrick, "Analog television transmitters", in, Edmund A. Williams (editor-in-chief), National Association of Broadcasters Engineering Handbook, 10th edition, pp. 1483-1484, Taylor & Francis, 2013 ISBN 1136034102.
- Phillip R. Geffe, "LC filter design", in, John Taylor, Qiuting Huang (eds), CRC Handbook of Electrical Filters, pp. 76-77, CRC Press, 1997 ISBN 0849389518.