Abstract: Apparatus for measuring the noise present in a video signal is adapted to operate on a spare line of the video field. An AGC circuit is used in which a known amount of gain is applied to the input signal and compared with the reference signal. A feedback loop includes a PROM containing data indicative of the antilog of all possible difference signals and applies the antilog data as feedback data to the gain control.

Abstract: Apparatus for measuring frequency includes a first period measuring counter (14) arranged to measure a periodic signal and generate a counter enable signal an integer number of cycles long, and a second period measuring counter (15) arranged to measure frequency of an input signal over a gate period given by the counter enable duration. The apparatus may be used to remove dither in the measurement of carrier frequency of FM signals.

Abstract: In an interface arrangement for automatic test equipment a substrate, such as a circuit board, is supported between spring loaded test probes within a vacuum chamber so that application of a vacuum causes the probes to contact the board. An internal dam is mounted within the vacuum chamber and, upon the application of the vacuum, seals off an internal portion of the chamber containing the probe arrays so that cooling air may be applied to the circuit board without breaking the vacuum in the outer portion of the chamber. Access may also be provided to the circuit board in this condition.

Abstract: A six-port network for characterizing a microwave or R.F. device includes a signal source (1) capable of generating three signals of the same frequency but relative phase differences, and combines each of the signals with a signal reflected or transmitted by the device. Two three-phase sources may be used in a dual six-port network device.

Abstract: Apparatus for use in extracting clock signals from a digital data stream that periodically generates an enable window of chosen duration at periods when a data edge is expected in the stream as disclosed. If a data edge is received during a window, the apparatus of this invention resets and restarts a clock pulse generator operating at the nominal frequency of the clock to be extracted. The extracted clock remains in synchronism with that of the data and is unaffected by noise spikes occuring outside the window area. If a chosen period of time elapses without any data edges received then the apparatus of this invention increases the window period so as to allow for jitter in the clock rate.

Abstract: A test arrangement for testing high frequency electrical devices consists of two 6-port reflectometers which are both connected to a device under test. As well as being applied to the device under test, a reference sample of an excitation signal is routed to both 6-port reflectometers simultaneously. The test arrangement is capable of very wide operation and is able to fully characterize the electrical properties of a device.

Abstract: A microwave power sensor utilizing a semiconductor power absorption element which is mounted on a fin-line structure within a waveguide channel. In order to check calibration of the sensor, a coaxial line is used to feed a relatively low frequency reference signal to the power sensor. The power element is positioned between the input port of the waveguide and a coaxial line input port at which the reference signal is applied. The use of a precise and accurate reference signal enables the power sensor to be used for microwave measurements.

Abstract: An attenuator is in the form of a pad comprising a coating of resistive material which connects an input port, an output port and a pair of linked constant potential ports. The regions of the pad adjacent to the outer edges of the input port are provided with outwardly pointing cusps with curved boundary edges. These reduce current crowding at the edges of the input port, and avoid localized overheating of the adjacent regions of the pad, which could result in pad failure.

Abstract: A method and apparatus for determining the frequency of an input signal. The input signal and a sampling signal having a sampling frequency f.sub.s to a sampling mixer to produce a first intermediate frequency signal. The first intermediate frequency signal is applied to a detector having a response frequency bandwidth of at least f.sub.s /2 to determine whether the frequency value of the input exceeds a threshold value. The frequency value of the sampling signal is changed from among a plurality of frequency values and applied to the mixer until one of such frequency values results in a second intermediate frequency signal in a predetermined frequency range which is smaller than and lying within the detector bandwidth. The second intermediate frequency signal is applied to a frequency counter which produces a count corresponding to the second intermediate frequency.

Abstract: A frequency synthesizer produces a variable frequency output using a controllable oscillator and a variable frequency divider which form part of a phase locked loop. The integer division ratio of the divider can be altered during a division cycle to simulate a fractional divisor value. The phase noise or jitter which is caused by the step change in divisor value is removed by altering the divisor value in accordance with the terms of a plurality of sequences, each of which sums to zero and which represents successive rows in a Pascal's triangle. A cascaded sequence of accumulators is used to determine the starting instants of each sequence, and the length of each sequence is dependent on the number of delay lines used. Although only four accumulators are illustrated, more can be added to give any desired degree of phase noise cancellation.

Abstract: An apparatus for training a user in contamination surveillance. The contamination may be from a nuclear, bacteriological or chemical explosion or leak, for example. The apparatus is portable and can be carried by the user to various points of an exercise area. The apparatus includes a data store and an interpolation device. The data store stores data which is derived from a mathematical model of an assumed source of contamination and which defines the contamination levels at various locations within the exercise area at respective elapsed times after the assumed commencement of contamination. The interpolation device is responsive to first signals which define the location within the exercise area and to second signals which define the elapsed time when the user of the apparatus inputs the first signals. The apparatus also may include a display on which is shown the contamination level, and a keyboard from which the first and second signals may be input.

Abstract: An electrical interface arrangement enables a complex circuit board to be connected to an electrical test equipment to verify the correct operation of the circuit board or to identify malfunctions. The interface arrangement makes electrical contact with predetermined points of the circuit board and connects these points to predetermined terminals of the test equipment. Each electrical interface arrangement is likely to be unique to a particular type of circuit board, and it has proved very difficult and time consuming to reliably interconnect the contacts of the electrical interface arrangement--typically 200 to 300 interconnecting leads are required.

Abstract: A signal generator is arranged to produce frequency or phase modulation at frequencies which extend from a high value down to zero, i.e. to a d.c. level shift. The output oscillator forms part of a phase locked loop which also includes a variable frequency divider. A modulation signal is applied to the loop, so that high frequency modulation signals directly modify the control signal which determines the frequency of the oscillator, and so that low frequency modulation signals alter the divisor value of the variable frequency divider so as to modify the frequency characteristics of the phase locked loop. Quantization noise is reduced by applying a masking signal to an analogue-to-digital converter which utilizes the low frequency modulation signals to alter the divisor value.

Abstract: A switching circuit is suitable for pulse modulating a high frequency carrier signal. The switching circuit provides little attenuation of the carrier signal while it is in its conductive state, but provides very high electrical isolation while in its non-conductive state so that under these conditions very little signal is allowed to appear at an output terminal. The switching circuit comprises four four-arm diode bridges, two of which are in series in the signal path and the other two of which are connected in shunt. When the two serial diode bridges are conductive, and the two shunt bridges are non-conductive an output pulse is obtained, but when the conductive states are reversed no signal is present at the output. One of the shunt connected diode bridges is associated with a matched impedance which is presented to the source of high carrier frequency signals, while the switching circuit presents a high impedance and this prevents the generation of a standing or reflected wave at the input terminal.

Abstract: An electrical interface arrangement enables a circuit board to be connected to test equipment for the purpose of verifying its correct operation or enabling malfunctions to be identified. The interface arrangement carries a two dimensional array of electrical contacts which, in operation, are brought into touching contact with predetermined locations of the circuit board. It is necessary to hold the circuit board firmly in contact with the array of contacts in exactly the correct alignment. The invention provides a common housing which serves to locate the circuit board in alignment with the two dimensional array of contacts and either the circuit board can be secured to the housing by means of a suction effect generated by a suction chamber located within the housing, or alternatively the circuit board can be secured by means of an externally located pressure plate.

Abstract: A swept frequency oscillator is capable of providing an output signal whose frequency can be swept over a very wide range at a rate which is not dictated by the period of the lowest frequency in the range. The signal detector monitors the output level of the oscillator so as to generate a control signal to remove variations in the output power level. At frequencies below a particular value, the time constant of the detector is modified progressively so as to be inversely proportional to the frequency being generated. Thus at low frequencies in the swept range, the time constant of the detector is increased.

Abstract: The invention provides an automatic test system for testing and diagnosing faults in a printed circuit board carrying electronic components. The system includes an automatic test apparatus provided to receive a circuit board and produce signals indicative of faults therein which are transmitted to a repair station at which batches of faults associated with different tested boards are stored. At the repair station is also stored data enabling a display consisting of individual component parts of the circuit to be displayed and means are provided responsive to the fault indicative signals relating to a particular board for selecting an appropriate portion of the stored data to cause, under the control of a repairer a component part of the circuit wherein faults lie to be displayed in such a fashion that it usually stands out.

Abstract: An electrical probe apparatus for facilitating the testing of printed circuit boards is constructed in a gas tight manner. This allows the printed circuit board to be held by suction without losing vacuum pressure through the probe. A silicon rubber sleeve serves as the vacuum seal, and its resilience urges the probe into firm contact with the circuit under test.

Abstract: A frequency multiplier uses a pair of Schottky diodes to rectify an applied signal, with the rectified waveforms provided by both diodes being combined to produce an output signal having a predominant frequency of twice the applied frequency. In order to prevent distortion occuring at low signal levels, a d.c. bias is applied to the diodes so as to bias them at their threshold values. The effect of temperature variations on the diodes is minimized by using a further diode to determine the effective value of the bias voltage.

Abstract: A time interval measurement arrangement allows extremely accurate measurements to be performed on a complex repetitively recurring waveform. The need arises to perform such measurements in connection with television waveform calibration instruments. The time interval measurement is performed in terms of an accurate amplitude measurement which identifies the required instant in time on the waveform under test. An amplitude sampling circuit is used which repetitively samples the signal at selected points so as to progressively alter in incremental steps a sample value until it is brought into agreement with the actual value of the signal. This process removes the effect of jitter and noise from the measurement process and enables great accuracy to be achieved.