Patent GB-788565-A (GB)
Title: Improvements in and relating to frequency response determining apparatus
IPC: G01R31/26
Filed: 1956-08-10
Abstract: 788,565. Frequency response measurement. GENERAL ELECTRIC CO. Aug. 10, 1955 [Aug. 11, 1954], No. 23082/55. Class 37. Apparatus for comparing the frequency response of an electric signal translating device at two frequencies comprises means for simultaneously applying a pair of voltage signals of the first and second distinct frequencies having a predetermined amplitude relationship to the input of the device, means for applying the output thereof simultaneously to a pair of selective amplifier channels adapted respectively to pass the first and the second frequency, and means for comparing the rectified amplitudes of the signal outputs of each channel in a null device by means of an attenuator network in one selective channel; the response over the working range being determinable with regard to the response to a reference frequency by holding the latter constant and varying the other frequency over the range. In Fig. 1 a low-frequency (e.g. 60 c.p.s.) sine wave oscillator 1 and a high-frequency sine wave oscillator 2 are connected through series resistances to the emitter 22 of a grounded base transistor amplifying device 21 together with a direct voltage applied over high resistance 7, and an amplified signal is derived from collector 23, which is energized through low resistance 10 from a direct voltage source 11 shunted by capacitance 12. The output is applied over lowfrequency amplifier 15, variable attenuator 16 (Fig. 2, not shown) and rectifier 17; and over high-frequency amplifier 18 and rectifier 19, to the differential measuring circuit 20 wherein (Fig. 3, not shown) the respective inputs are balance amplified by resistance coupled triodes and compared in opposition in a single measuring instrument. Oscillator 2 is adjusted to provide a pre-set high-frequency signal at the minimum value allowable for the frequency at which the short-circuit current gain for the transistor is 3 db. below the low-frequency short-circuit current gain, and since the frequency distortion values of amplifiers 15, 18 are respectively zero at, e.g. 60 c.p.s. and the pre-set frequency, the rectified output signals are equal as indicated by the differential measuring circuit when the insertion loss of attenuator 16 represents the amplifier loss at the frequency of oscillator 2. The device is applicable to grounded emitter semi-conductor amplifiers, and with the insertion of, e.g., 3 db. preset attenuation at 18 may be applied to the testing of semi-conductor amplifiers for acceptability; the differential meter 20 being deflected in one direction when the gain at a predetermined highfrequency exceeds, and in the other direction when it is less than the present attenuation. For measurement of overall frequency response characteristics of an amplifier inserted between the output of oscillators 1, 2 and the inputs of amplifiers 15, 18, the frequency of the oscillator 2 is varied over the measurement range while attenuator 16 is adjusted to hold differential meter 20 at zero; the frequency response may also be measured with regard to a high-frequency value as standard by inserting attenuator 16 between amplifier 18 and rectifier 19 and adjusting the frequency of oscillator 1.