GB673141A - Improvements in tuning indicator systems for frequency-modulation receivers - Google Patents

Abstract

673,141. Valve circuits for radio reception; limiters. HAZELTINE CORPORATION. March 10, 1950 [May 28, 1949], No. 6077/50. Classes 40 (v) and 40 (vi). In a frequency-modulation receiver, in which the presence of a limiter renders accurate tuning by ear difficult, tuning is facilitated by the use as a limiter of a valve in which the transconductance is controllable subsequent to the point at which limiting occurs, being controlled by a voltage dependent upon the amplitude of the signal before limiting. The limiter 15 is of the grid-bias.type in which the grid leak 20 is returned to the cathode; the suppressor 25 is earthed, while the cathode circuit comprises resistance 24 and a decoupling condenser 30. In the absence of a signal the grid bias is small and the cathode current high, so that a negative potential is developed across resistance 24, i.e. between cathode and suppressor which renders the transconductance to the anode low. Grid current rectification, which occurs when a signal is applied, produces a negative grid-bias causing reduction in cathode current to a value which is a minimum when the set is accurately tuned; the corresponding decrease in potential difference between cathode and suppressor produces an increase in the transconductance to the anode despite the fall in cathode current, so that the output increases to a maximum at the proper tuning point. The variation in output may be made greater by applying to the suppressor grid a positive bias derived from the signal before limiting, by means of a diode rectifier fed either from the same I.F. circuit as the limiter, Fig. 2 (not shown), or alternatively from a sharply-tuned circuit which is loosely coupled to the primary of the I.F. transformer feeding the limiter, Fig. 3 (not shown). Specification 673,142 is referred to.