Bode Equaliser
A Bode network, as with a Zobel network, is a symmetrical bridge T network which meets the constant k condition. It does not however meet the constant resistance condition, that is, the bridge is not in balance. Any impedance network, Z, can be used in a Bode network, just as with a Zobel network, but the high pass section shown for correcting high-end frequencies is the most common. A Bode network terminated in a variable resistor can be used to produce a variable impedance at the input terminals of the network. A useful property of this network is that the input impedance can be made to vary from a capacitive impedance through a purely resistive impedance to an inductive impedance all by adjusting the single load potentiometer, RL. The bridging resistor, R0, is chosen to equal the nominal impedance so that in the special case when RL is set to R0 the network behaves as a Zobel network and Zin is also equal to R0.
The Bode network is used in an equaliser by connecting the whole network such that the input impedance of the Bode network, Zin, is in series with the load. Since the impedance of the Bode network can be either capacitive or inductive depending on the position of the adjustment potentiometer, the response may be a boost or a cut to the band of frequencies it is acting on. The transfer function of this arrangement is:
The Bode equaliser can be converted into a constant resistance filter by using the entire Bode network as the Z branch of a Zobel network, resulting in a rather complex network of bridge T networks embedded in a larger bridge T. It can be seen that this results in the same transfer function by noting that the transfer function of the Bode equaliser is identical to the transfer function of the general form of Zobel equaliser. Note that the dual of a constant resistance bridge T network is the identical network. The dual of a Bode network is therefore the same network except for the load resistance RL, which must be the inverse, RL', in the dual circuit. To adjust the equaliser RL and RL' must be ganged, or otherwise kept in step such that as RL increases RL' will decrease and vice versa.
Read more about this topic: Zobel Network, Video Equalisers