A CIRCUIT FOR DETERMINING FREQUENCY RESPONSE

Abstract: Prior art circuits for determining the frequency response of a unit under test comprise a VCO and e.g. a cathode ray tube on which the respective spectrum from the unit under test is displayed. For accurate allocation between wobble frequency and spectral amplitude, the VCO had to be periodically calibrated. For simplification purposes, the invention provides a counter which either counts quartz-stabilized pulses during a VCO period or which counts, during a quartz-stablized counting interval, the VCO output pulses themselves. The counts represent with quartz accuracy the test frequency, which corresponds to the mean value within the counting interval.

Description: This invention relates to a circuit for determining the frequency response of a unit under test, and is more particularly concerned with such a circuit of the kind comprising a variable frequency oscillator for producing a variable frequency test signal for application to the unit under test, a control signal generator for generating a control signal to vary the frequency of the oscillator, and a processing circuit for determining a parameter of the unit under test from measured values of the output signal produced by the unit under test in response to the test signal. Such circuits are used, for example, to measure the amplitude-frequency characteristic of a network or to determine the frequency spectrum of a received signal. Typically, the oscillator comprises a voltage-controlled oscillator which generates a test signal whose frequency is wobbled (ie. varied) by a control voltage generator such as a saw-tooth voltage generator. Usually, the variation with frequency of the parameter of the unit under test determinded by the processing circuit is displayed on display means, typically a cathode ray tube. This requires signals representative of the actual frequency corresponding to each value determined for the parameter. This has been achieved in the past either by assuming that the control voltage applied to the voltage-controlled oscillator is representive of the frequency produced by the oscillator, or by periodically calibrating the oscillator, for example during the line return interval of the cathode ray tube, and for example by temporarily connecting the oscillator in a phase locked loop. Circuits operating generally in the latter way are disclosed for example in United Kingdom Patent Specification No. 1 546 667 and European Patent Application No. 0 095 732 However, these prior circuits suffer from the drawback that the frequency response of the oscillator to its control voltage is not necessarily linear, and subject to drift, both of which clearly adversely affect the accuracy of the parameter vs. frequency determination. It is therefore an object of the present invention to provide a circuit of the kind specified above, in which simple variable-frequency oscillators and control signal generators can be used without the adverse effects encountered in the prior circuits. A circuit having the features as set forth in the pre-characterizing part of patent claim 1 is known from HEWLETT PACKARD SPECTRUM ANALYZER 8553B, 8443 Technical Data 1 Nov 74 and Wobbelmeßtechnik ISBN 3-922120-12-1 by Markt und Technik 1981, pages 92-94, pages 111, 112. In this known apparatus, the counter is used to produce marks upon stopping of the wobbling process. The object of the present invention is addressed by the combination of features defined by patent claim 1. The invention will now be described, by way of example only, with reference to the accompanying drawing, which is a simplified block circuit diagram of a circuit in accordance with the present invention. It is assumed that the unit 10 under test is a network whose frequency response (attenuation in response to frequency) is to be determined. Its input is fed with a AC voltage having a predetermined fixed amplitude but variable frequency, generated by voltage controlled oscillator (VCO) 12. A level detector 14 is connected to the output of the unit 10. The control voltage for VCO 12 is supplied by a sweep generator 16, which may be a simple saw tooth generator variable with respect to speed and direction. Test frequencies generated by VCO 12 are fed to a counter 18. If the latter is unable to operate with extremely high clock frequencies, a mixer 20 may be used to convert the VCO output into a band of lower frequencies. The counter has a quartz-stabilized clock generator 21, which either determines the counting interval if the test frequencies are applied at the counting input of the counter, or which supplies pulses to be counted during a counting interval depending upon the test frequency period. So that the apparatus may operate in a free running mode at the end of each counting interval the count is transferred to a comparator 22, which compares it with a reference value representative of the wobble frequencies and supplied by a control unit 30. If the count exceeds or falls below the reference value (depending upon the actual wobble direction of wobble generator 16), i.e. whenever the sign of the difference changes, the wobble direction is reversed via line 38, so that the sweep or wobble generator 16 is always within the range defined by control unit 30 by means of reference 24. Further, and more significantly in relation to the present invention, the counts are applied to the display processing and storing unit 28. Each such count is allocated by the unit 28 to the respective level value from level detector 14 which is present simultaneously with the count. Each level value and its corresponding count are then converted into a form suitable for display by means of display device 26. It will therefore be appreciated that during each upward or downward sweep of the output frequency of the VCO 12, the respective actual output frequency (or period) is accurately determined by the counter 18 and the generator 21 for each value of the signal at the output of level detector 14. Consequently, problems due to non-linearities in sweep rate and frequency drift in the VCO 12 and its control unit 30 are avoided. The embodiment described above serves to generate a triangular periodic control voltage for VCO 12 via sweep generator 16. Alternatively by reseting or holding the sweep when the reference value is exceeded (for example) saw tooth or single shot sweep may be obtained. Where the sweep generator is operating in a free running mode, since the counting interval is not synchronised with the sweep, the instantaneous amplitude and frequency co-ordinates obtained by the display processing and memory unit will be different between individual sweeps. Thus a display of accurately recorded values may be built up as sweeping progresses, resolution increasing with each sweep, there being no theoretical limit on the resolution which can be achieved.

Claims: A circuit for determining the frequency response of a unit under test, the circuit comprising: a variable frequency oscillator (12) for producing a test signal to be applied to said unit under test (10), a control signal generator (16) producing a control signal to vary the frequency of said oscillator, a processing circuit (14, 28) for measuring the output value of a selected parameter of the unit under test from measured values of its output signal produced in response to said test signal, and a counter (18) having a count input and a start/stop input, one of said inputs being fed with a signal derived from said test signal while the other of said counter inputs is coupled to the output of a quartz-stabilized reference frequency generator (21), characterized in that a control unit (30) pairs said output value of said selected parameter with counts of said counter occuring simultaneously therewith the accuracy of said pairing being determined by the stability of said reference frequency generator. A circuit as claimed in claim 1, wherein the control unit (30) is operatively connected to said control signal generator (16) so as to define a predetermined frequency range for the oscillator (12) in response to respective detected counts A circuit as claimed in claim 1 or claim 2, further comprising a mixer (20) circuit connected between the oscillator (12) and the counter (18) for converting the test frequencies into a range in which the counter (18) is able to operate. A circuit as claimed in any preceding claim, further comprising a comparator (22) circuit for comparing detected counts with predetermined reference values and for generating a signal causing the control signal generator (16) to reverse the direction in which the frequency of the oscillator (12) is being varied upon sign change of the difference between count and reference value. A circuit as in claim 4, comprising a memory (28) for holding said reference values, a new reference value being read out from said memory (28) upon each sign change. A circuit as claimed in any preceding claim, comprising display means (26) having orthogonal axes, said counts appearing along one axis and said measured values appearing along the other axis.

Inventors: SCHUTZ, JOHANN
Applicants: SCHLUMBERGER TECHNOLOGIES GMBH
Application year: 1987
Main classification: G01R23
IPC codes: G01R23, G01R31