Patent GB-997079-A (GB)
Title: Electric circuits for time-interval measurement
IPC: G04F10/00
Filed: 1961-10-27
Abstract: 997, 079. Time interval measurement; transistor testing. WESTERN ELECTRIC CO. Oct. 27, 1961 [Nov. 1, 1960], No. 38486/61. Heading G1U. [Also in Divisions H1 and H3] Apparatus for determining the time elapsed between application of an input pulse and the attainment of a predetermined output voltage in an electric circuit device whose output voltage magnitude is a function of an input pulse duration comprises a pulse generator having means for adjustment of pulse duration to a known amount, means for application of the pulses to the circuit device and means connected to the output of the device for indicating the attainment of a predetermined output voltage. For measuring the delay time for a transistor output to increase to 10% of the output voltage swing after application of a turn on signal and for the rise time for the output to increase from 10% to 90% of the output swing by pulsing the transistor tor from a generator of pulses of adjustable duration and measuring the magnitude of the output voltage, whereby the time from application of an input signal to the attainment of a predetermined output voltage from the transistor may be measured; there is provided a free running transistor multivibrator 10 (Fig. 2) with back-up diodes 20, 21 producing a pulse train which is capacitance coupled through a ferrite cared inverting isolating transformer 26 to D.C. restoring diode 30 and resistor 31 at the input of a switching circuit comprising normally nonconducting transistor 28 whose base receives the input pulses, whose collector is connected to D.C. source 33 over resistor 32, and whose emitter is grounded over resistor 29. The collector is connected over line 36 to an open circuit coaxial cable pulse width changer circuit (Fig. 3) wherein switches 37, 38 select critical lengths of cable introducing delay times graduated from 1 to 10 and from 10 to 100 milliseconds respectively; with a characteristic impedance of Z 01 , while switch 42 is operable to connect in other cable lengths of characteristic impedance Z 02  introducing delay times graduated from 1 to 10 and from 10 to 100 milliseconds respectively selected by switches 43, 44 with a variable resistor 46 in series therewith and calibrated dials operating the switches. Variable resistor 47 and diode 48 across the cable input restrict the charge voltage across the cable to protect transistor 28, and increase the maximum charge/discharge frequency of the cable, and on pulsing transistor 28, collector/emitter conduction drives line 36 negatively as a step voltage charge to discharge the cable at a constant rate until the charge is propagated along the cable and reflected back to the input at full discharge to reduce the voltage across resistor 29 to a low value. The latter voltage is coupled through clipping diodes 49, 50, 51 and line 52 to test circuit 12 (Fig. 4) and the base of transistor 53 under test in a common emitter circuit is normally cut-off by bias 56 and is then pulsed into conduction after a delay time, and a positive voltage at the collector is capacitance coupled to a peak responsive circuit comprising biased diode 61 and storage capacitance 62 developing a direct voltage in bridge with resistors 63, 64 and switchable bias voltage sources 65, 66 representing respectively 10% and 90% of the available output voltage swing of the test transistor. A voltage detector 67 is connected across the bridge diagonal. Since if the test transistor is not driven to full conduction the time from application of a drive pulse to peak output voltage equals the duration of the drive pulse, the transistor may be operated through any proportion of the operating range by adjustment of switches 37, 38, 43, 44 to vary the drive pulse duration and the calibrated dials then indicate the time to attain a desired transient peak. If battery 65 is switched in and switches 37, 38 are adjusted for null of detector 67, the switch dials read the transistor delay time, and if battery 66 is switched in switches 42, 43, 44 enables additional delay sections to be connected to obtain null; when the dials of switches 43, 44 read the transistor rise time. Resistor 46 and the differing characteristic impedance of the cable sections introduced by switches 43, 44 introduce an adjustable step in the pulse waveform applied to transistor 53 corresponding to the point at which its output represents 10% of the available output voltage swing, which is effective to compensate the transistor rise time variation due to changes in the transistor input voltage during its turn-on transient interval. Clipping diodes 49, 50, 51 also remove reflected voltages from the differing cable characteristic impedance which would otherwise prevent the test transistor from cutting off at the termination of the drive pulse. Transistor turn off times (i.e. storage and fall times) are measurable using a test circuit (Fig. 4A not shown) in which bias source 56, diode 61, bias source 59, and the polarities of batteries 65, 66 are reversed, with the transistor normally conducting and driven to cut off by the input pulses; the polarity of the input pulses being reversed by modification to the circuit of transistor 28 (Fig. 2A not shown).