Marconi Instruments Limited

Pickering Interfaces Limited

Aeroflex International Limited

David P Owen

Keith Thomas Moore

John Norman Wells

Adrian Jones

Donald Pearce

Adam Charles Mitchell

An array of reed switches () is assembled on a circuit board () having a plurality of through-apertures () with one reed switch () disposed in each aperture with the axis of the switch envelope () extending perpendicularly to the plane of the board. A plurality of coils () is provided on the board so that each switch () is surrounded by a respective coil, the coils being connected to conductors on the board for operation of the switches. Also disposed is a control circuit wherein the reed switch drive coils () are connected in series across a constant-current voltage source and a respective solid-state switch TR, TR. . . connected across each coil, with divers for each solid-state switch to permit operation of a selected reed switch.

Reed switch arrays

A variable resistor network has a coarse resistance network and a fine resistance network connected in series with the coarse network. The coarse resistance network comprises a chain of series-connected resistors any one or more of which may be switched out of the series by individual parallel-connected relays. The fine network comprises a shunt resistor together with a fixed resistor and a series-connected adjustable resistor together connected in parallel with the shunt resistor. A control circuit is arranged to control the relays for the switching of the resistors of the coarse network and also to control adjustment of the adjustable resistor. A chosen resistance value can be set by appropriate switching of the coarse network and adjustment of the adjustable resistor.

Resistor network and variable resistor simulator

A low noise sinusoidal signal at a desired output frequency is synchronised using a first variable frequency oscillator () and providing a feedback control loop around the first oscillator () to generate a feedback control signal in successive frequ comparison steps using second and third frequency reference signals (). Each of the second and third frequency reference signals are derived from a first frequency reference signal, the second frequency reference signal () being compared with the output frequency of the first oscillator () to generate a frequency difference signal (), and said frequency difference signal ( being compared in frequency with the third frequency reference signal () to generate said feedback control signal for the first oscillator (), the frequency of the third frequency reference signal () being equal to the difference of the frequency of the se frequency reference signal () and the desired output frequency. The second and third frequency reference signals () are generated by respective second and third frequency reference sources, the second frequency reference source comprising a second variable frequency oscillator () the frequency of which is maintained in a limited range around its optimum natural frequency to give optimum performance in producing a low noise output from the first oscillator ().

Low noise synthesizer and method employing first tunable source and first and second reference sources

A signal generator comprises a number of independent signal sources each of which can be controlled to generate predetermined frequencies and modulation. The outputs of these signal sources can be made available separately at output ports or can be combined within the signal generator to provide a combined output signal. The output signals are sensed at the output ports, and are calibrated against a stored correction tabulation to ensure that they have the correct signal level at the output ports. Such a signal generator is useful for the measurement and testing of complex electrical and electronic products.

Signal generator for testing radio frequency components

A waveform generator suitable for use in the testing of instrument landing systems comprising: a first generator (1) for generating digitally a first waveform comprising the superposition of a second waveform (19) of a first frequency and a third waveform (21) of a second frequency; a second generator (3) for generating digitally a fourth waveform comprising the superposition of a fifth waveform (25 or 27) comprising a phase reversed version of one (19 or 21) of said second (19) and third (21) waveforms of the frequency of said one waveform (19 or 21) and a sixth waveform (23 or 29) in phase with and of the frequency of the other one (19 or 21) of said second (19) and third (21) waveforms; and a circuit (7) for combining said first and fourth waveforms thereby to produce a waveform suitable for said use.

Waveform generators using digitally generator superimposed waveforms

A variable frequency signal generator includes a phase locked loop having a variable frequency oscillator which has a control port to which a frequency determining signal is applied, the output of the oscillator being fed via a frequency divider to a phase sensitive detector where it is compared with a reference frequency signal, the result of said comparison being arranged to generate a comparison signal which is fed to a loop filter which is coupled to said control port; and the signal generator having a frequency control signal applied thereto and comprising a first path being arranged to adjust the division ratio of the frequency divider, and a second path including a combiner for combining said frequency control signal and said comparison signal to produce said frequency determining signal; and a calibrator for adjusting the relative characteristics of said first and second paths to compensate for effects arising from tuning sensitivity dependence on frequency of said variable frequency generator.

Variable frequency signal generator

A signal source from a transmitter is down-converted by being mixed with a locally generated signal so that a desired adjacent channel band is converted to base band and the adjacent channel power measured after filtering by a low pass filter. To eliminate the notch in the filter characteristics that occurs when the power is measured by a.c coupled means, the locally generated signal is modulated or swept around the center of the adjacent channel band.

Measuring adjacent channel power

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.

Electrical apparatus

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. Additional diodes are associated with the two series connected diode bridges to further attenuate any high frequency signal which partially breaks through.

Diode bridge switching circuit with high attenuation

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.

Phase locked loop frequency and phase modulators

A frequency synthesizer which is capable of providing very high output frequencies of the order of 500 MHz is designed to produce a very pure signal having very low noise. An overtone crystal oscillator which is constrained to oscillate at a harmonic frequency of the fundamental frequency determined by the physical dimensions of the crystal is mixed with a variable low frequency signal which is varied to provide the necessary output frequency steps. It is not possible to adjust the frequency of the overtone crystal oscillator without degrading its stability and introducing noise, and any departure of the frequency to the overtone crystal oscillator from its nominal value is compensated by adjusting the low frequency oscillator.

Frequency synthesizer having overtone crystal oscillator

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.

Frequency multipliers

In a frequency modulation system, the frequency modulation sensitivity of an oscillator is itself a function of the carrier frequency of the oscillator. The frequency modulation sensitivity is measured and stored for particular carrier frequencies, and subsequently used to control the amplitude of the modulation signal, so that when a frequency modulation signal is fed to the oscillator, a frequency deviation is obtained which is accurately related to the amplitude of the modulation signal.

Frequency modulators with compensation for variations in modulation

An R.F. signal generator is arranged to provide an output signal having a controlled level, by means of a feedback loop into which an additional frequency dependent control signal is injected. This additional control signal is derived from a resistor network in which particular resistors, or combinations thereof, are selected in dependence on the output frequency of the signal generator.

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Dunstable, United Kingdom

David P Owen