The Discriminator

Author: J.B. Hoag

A typical detector or discriminator circuit is shown in Fig. 33 E.

Fig. 33 E. A typical discriminator circuit

The secondary of the input transformer is center-tapped and is adjusted to resonance by condenser C₄ to the same frequency as the primary, i.e., the center of the i.f. band. The voltages on the plates of the diodes are those induced in the secondary of the input transformer, together with those developed across condenser C₂ and fed back through the radio-frequency-choke, r.f.c. When the input signal is at its center (unmodulated) frequency, equal and opposite voltages are set up across R₁ and R₂ and there is no output. This is the case when there is no modulation at the transmitter. When, however, the signal's frequency changes, one of the diode's voltages lags behind the other. Thus, at a given moment in a cycle, the voltage on one of the diode plates is greater than that on the other. The output voltage is then the difference between the voltages across R₁ and R₂. The faithfulness with which the output voltage changes in strength with change of input frequency is called the "linearity" of the detector. It is greater if the Q's of the input transformer's primary and secondary are small and if they are closely coupled.