QPJ36 Deep Dive
The QPJ36 is the standard line circuit for the SG-1. A single card provides facilities for two sequentially numbered lines.
The card provides battery feed, ringing and signalling, as well as signal conversion between the telephone line and the PAM-TDM bus. All of the analog functions of the card exist in duplicate, most of the logic circuity is shared by the two lines.
The QPJ36 uses the standard SG-1 bi-directional PAM interface circuity, consisting of a gate, gate drive and filter section:
Surprisingly, The primary gating element is a bridge rectifier. Because PAM-TDM requires moving all of the speech energy collected during a sample period in less than 1/25th of a sample period, high current pulses are employed. To switch these pulses, a bridge rectifier is configured as a gate. A pulse transformer provides an voltage that biases the transistors of the bridge into their conducting state.