Binary logarithmic ladder network: The network provides an attenuation between 0 dB and 78.75 dB that is presettable in 64 steps with the aid of a 6-bit code.
Six independent, relay switched attenuator sections may be connected in series in accordance with the input code. Since the sections have identical 1 kΩ impedances, they do not affect each other and can, therefore. be interchanged without any problem. The only requirement is that the network is terminated correctly. hereby R20. The fairly low characteristic resistance of 1 kf2 was chosen to keep the noise generated by the attenuator low.
Buffering of the attenuator is effected by a Type NE5532, an IC that operates effortlessly with a 1 Ic.Q input impedance. The use of relays in the various sections ensures that there are no linearity problems with switching elements.
Furthermore, relays make it possible for the control circuits and the attenuator to be electrically isolated.
The relays are driven by a transistor so that the control inputs require relatively little energy: even a simple logic circuit (TTL or CMOS) can switch the relays in this
The signal-to-noise ratio of the attenuator, with component values as shown in the diagram, is 92 dB (A-weighted even 107 dB), provided the input signal is not smaller than 1 V r.m.s. The maximum input voltage is 7 V r.m.s. Total harmonic distortion at frequencies up to 20 kHz is not greater than 0.003%. The current drawn by the circuit depends primarily on the relays: in the prototype, it was about 120 mA.