Patent GB-817821-A (GB)
Title: Improvements in or relating to electronic frequency calibrating devices
IPC: H03B23/00
Filed: 1955-10-17
Abstract: 817,821. Frequency response measurement and calibration. SANGAMO WESTON Ltd. Oct. 17, 1955 [Oct. 18, 1954], No. 29596/55. Class 37. An electronic frequencycalibrating device comprises a variable R.F. oscillator having a calibrated frequency control, an input terminal for connection to a source of R.F. (which may be a sweep oscillator), a mixer receiving the input signals from the terminal and from the oscillator, a gas discharge tube energized by signals derived from the output of the mixer having a predetermined firing voltage, and circuit means for adjusting the relation between the amplitude of the signals applied to the gas discharge tube and the firing voltage thereof so that it flashes to indicate coincidence between the input and local oscillator signals. Fig. 4 shows an electronic frequency calibrating device and marker signal generator for use in the apparatus described in Specification 817,633 for displaying on a cathode-ray oscillograph the frequency response curve of a television receiver energized from a sweep oscillator, in combination with intensity modulation of the display at selected calibration points (marker signals) determined by a variable-frequency oscillator 32 whose operating frequency is indicated by a pointer on a lineal scale which is controllably displaceable for calibration setting (Fig. 2, not shown). The output of oscillator 32 is connected to the cathode of heptode V1, to whose first control grid is applied a signal of unknown frequency from the sweep oscillator over terminal 33, and a calibrating signal of 1.5 or 4.5 mc/s. controlled by crystals 35, 36 is generated, by the left-hand section of double triode V2; the calibration frequency being selected by two gang rotary switch S1 shown in the " off " position, for application to the second control grid of the heptode. Operative functions of the apparatus are controlled by six-gang rotary switch S2 shown in the " calibrate and heterodyne " position, wherein the calibration signal is coupled to the second control grid of heptode VI, whose modulation products developed across aperiodic anode load 50<SP>1</SP> are amplified by the remaining section of double triode V2 and by pentode V3, the signal across load inductance 37 being connected to one plate of gas discharge indicator tube 21, whose remaining plate is biased so that it strikes at a predetermined level of amplitude. With the crystal oscillator disabled by switch 1 at " off," and an unknown frequency signal (e.g. from the sweep oscillator) applied to terminal 33, the variable frequency oscillator may be adjusted so that the gas tube strikes when the difference frequency is small but not zero on either side of zero beat; all other spurious signals being of amplitude insufficient to fire the tube, and the precise zero beat point being determined by extinction of the tube between two adjacent flashes thereof (Fig. 3, not shown). Calibration of the variable frequency oscillator 32 is effected by removing the unknown signal from terminal 33 and operating switch S1 to select one or other of the crystals 35, 36 to generate oscillating at 1.5 or 4.5 mc/s. with plural higher harmonics; the gas tube then striking at equality between the variable oscillator frequency and the successive harmonics, so that the oscillator scale may be adjusted for correct calibration with reference to either control crystal frequency. For beam intensity modulation of the oscillograph display at predetermined calibration points, switch S2 is set to position Z+, switch S1 is set to its off position, and the sweep signal is applied over terminal 33 to the mixer V1, whose anode load 50<SP>1</SP> is exchanged for tuned circuit 50 of, e.g., 75 kc/s. to extract signals of frequency Œ75 kc/s. with respect to the variable oscillator frequency from the numerous intermodulation products at the mixer anode, which are rectified by diode 51 after amplification by the remaining section of double triode V2, and further amplified by pentode V3 to positivegoing pulses which are clamped to earth potential by the left-hand section of double diode V4 and applied to terminal 53 for modulation connection to the oscillograph cathode. In the position Z - of switch S2, the oppositely-poled diode 54 is used for rectification to produce negative-going modulating pulses which are clamped by the right-hand section of the double diode. Introduction of either crystal frequency by operation of switch S1 produces further modulation pulses corresponding to Œ 1.5 or Œ 4.5 Mc/s. with respect to the variable frequency oscillator signal.