Frequency deviation measuring and adjusting system

• US 4,122,391 A
• Filed: 09/19/1977
• Issued: 10/24/1978
• Est. Priority Date: 09/19/1977
• Status: Expired due to Term

• Current Assignee: AG Communication Systems Corp. (Nokia Corporation)
• Original Assignee: GTE Automatic Electric Laboratories Incorporated
• Inventors: Seaver, Terry R., Harp, Maurice C.
• Primary Examiner(s): Tokar, M.
• Application Number:
  US05/834,754
• Time in Patent Office: 400 Days
• Field of Search: 324/78 Z, 324/79 R, 324/79 D, 324/78 R, 328/141, 331/14, 331/1 A, 331/18, 331/19, 331/25
• US Class Current: 324/76.74
• CPC Class Codes: G01R 23/02 Arrangements for measuring frequency, e.g. pulse repetition rate; Arrangements for measuring period of current or voltage
• CPC Class Codes: G01R 23/12 by converting frequency into phase shift
• CPC Class Codes: H04B 14/006 Angle modulation

Abstract

A calibrated unipolar signal is used to deviate a frequency modulator by a predetermined amount Δf from a carrier f₂. An automatic frequency control circuit operates to bring the carrier back on frequency and a meter connected in the control circuit will read this departure from normal. During the test for correct deviation, a frequency shifter compensates for the predetermined frequency deviation and the meter would so indicate, e.g., would not change reading from that of carrier alone, if the deviation caused by the calibrated unipolar signal is equal to that incorporated in the frequency shifter. If the reading does change, the modulation sensitivity of the frequency modulator is adjusted until the readings before test (using carrier only) are the same as those under test.

Claims

1. Apparatus for checking the frequency deviation Δ

f of a carrier frequency f_c in a frequency modulation system comprising;

frequency modulator means, including an adjustable modulation amplifier for adjusting the modulation sensitivity and a voltage controlled frequency modulated oscillator operating at the carrier frequency f_c, said frequency modulator means having a modulation frequency input path, an oscillator frequency control input and having an output, whereby frequency deviation of the carrier f_c is obtained at said output when a modulation frequency is applied to the modulation frequency input;

means for applying to said modulation frequency input path a calibrated modulation frequency f_s having an amplitude such that a predetermined peak frequency deviation Δ

f will be obtained at the output of said frequency modulator means if the modulation sensitivity is properly adjusted;

means for changing the frequency of the frequency deviated signal said predetermined amount, but in the opposite direction so that the original diviation will be effectively cancelled if the modulation sensitivity is properly adjusted, said changing means having an input connected to the output of said frequency modulator means and having an output;

means for automatically controlling the frequency of the voltage controlled oscillator by supplying from an output a control signal to the oscillator frequency control input of the frequency modulator means, said controlling means having an input connected to the output of said changing means; and

measuring means having an input connected to the output of said automatic controlling means, for providing an indication of the frequency diviation.

2. Apparatus in accordance with claim 1 wherein said means for changing the frequency comprises:

oscillator means having an output, said oscillator means generating a frequency equal to the frequency Δ

f of the predetermined peak frequency deviation;

modulator means having as a first input the deviated signal output of said frequency modulator means, having as a second input the output of said oscillator means, said modulator means providing modulation frequencies which include a frequency f_x near the carrier frequency f_c at an output, said frequency f_x will be equal to f_c if the modulation sensitivity is properly adjusted; and

means for selecting the frequency f_x for automatic control of the voltage controlled oscillator, said selecting means having an input connected to the output of said modulator means and having an output connected to the input of said controlling means.

3. Apparatus in accordance with claim 1 wherein said means for changing the frequency comprises:

a programmable counter programmed for said carrier frequency f_c ; and

means for altering the counter program by said predetermined peak frequency deviation.

4. Apparatus for checking the frequency deviation of a carrier frequency f_c of a frequency modulated signal comprising:

frequency modulator means having a modulation frequency input, having a control input and having an output, whereby frequency deviation of the carrier f_c is obtained when a modulation frequency is applied to the modulation frequency input;

means for applying to said modulation frequency input a calibrated modulation frequency f_s hving an amplitude such that a predetermined peak frequency deviation will be obtained if the modulation sensitivity of the frequency modulator means is properly adjusted;

program gating means having an output;

digital automatic frequency control means having an output connected to the control input of said frequency modulator, having an input connected to the output of said frequency modulator means, having a programmable counter connected to said input, having a programmed input connected to the output of said program gating means, whereby said programmable counter will provide an equivalent of the carrier frequency f_c when said program gating means is in one binary state and a frequency different from f_c by Δ

f when in the other said state the latter frequency being in a direction such as to correct for the effect of the peak frequency deviation Δ

f caused by the calibrated modulation frequency f_s when the modulation sensitivity is correctly adjusted; and

measuring means having an input connected to the output of said frequency control means, said measuring means providing an indication of the frequency deviation.

5. Apparatus in accordance with claim 4 wherein said frequency modulator comprises:

a modulation amplifier having a modulation frequency input, having an output, said modulation amplifier having controllable gain for adjustment of the modulation sensitivity of the frequency modulator; and

a voltage controlled frequency modulation oscillator having one input connected to the output of said modulation amplifier having a control input connected to the output of said digital automatic frequency control means, and having an output.

6. Apparatus in accordance with claim 5 wherein said digital automatic frequency control means further comprises:

a reference oscillator having an output;

a reference counting means having an input connected to the output of said reference oscillator, said reference counting means providing a reference pulse repetition rate at an output;

compare and integrate means having an output connected to the control input of said controlled voltage oscillator, having a first input connected to the output of said reference counting means, and having a second input connected to the output of said programmable counting means, said compare and integrate means providing a control signal for automatically controlling the frequency of said controlled oscillator, and for providing an input to said measuring means.

7. Apparatus in accordance with claim 6 wherein said means for applying comprises:

a first gating means having a first input connected to the output of said reference counting means, having a second input connected to said programmed gating means, said gating means controlling the phase of the reference counting means appearing at its output; and

amplitude adjustment means having an input connected to the output of said gating means, said adjustment providing a calibrated modulation frequency to an output path.

8. Apparatus in accordance with claim 7 wherein said program gating means comprises:

a second gating means having one input connected to a reference voltage source, having a second input and having an output connected to an input of said programmable counter;

switching means for disconnecting the input of the frequency modulator from the normal signal modulation frequency path and connecting the calibrated modulation frequency to the modulation frequency input of said frequency modulator; and

means for connecting a second voltage reference source to said second input of said second gating means whereby the output changes state.

9. Apparatus for adjusting the frequency deviation of a carrier frequency f_c of a frequency modulated signal comprising:

a frequency modulator having a modulation frequency input path, having a control input path and having an output path whereby frequency deviation of the carrier f_c is obtained when a modulation frequency is applied to the modulation frequency input path, said frequency modulator including means for adjusting the modulation sensitivity;

digital automatic frequency control means having an input path connected to the output path of said frequency modulator, having a first output path connected to the control input of said frequency modulator and providing a reference frequency at a second output, said digital automatic frequency control including a programmable counter which is programmed for normal operation with the carrier frequency f_c and having a selected program input path, whereby for a binary input signal of one state on said input path normal operation is obtained and for a binary input signal of the other state the programmable counter is set to operate with an effective carrier which is different from f_c by predetermined deviation frequency Δ

f;

modulation frequency means having an input connected to the reference frequency output of said digital automatic frequency control means and having an output, said modulation frequency means providing at said output a calibrated unipolar signal having an amplitude such that a predetermined peak frequency deviation Δ

f will be obtained from said frequency modulator if the modulation sensitivity is properly adjusted;

a first switching means having a first terminal connected to the modulation frequency input of said frequency modulator, having a second terminal connected to the through modulation path and having a third terminal connected to the output of said modulation frequency means, whereby said modulation frequency input is connected to the through modulation path when the first switching means is in one state and is connected to said output of the modulation frequency means when the first switching means is in an other state;

program gating means having a first input connected to a first voltage source, having a second input and having an output path connected to the selected program input path of said programmable counter, whereby the binary signal on said output path shifts the counter by the equivalent of Δ

f when in one binary state;

second switching means having one terminal thereof connected to a second reference voltage and having a second terminal thereof connected to the second input of said program gating means, whereby operation of said switching means from one state to the other state connects said second reference voltage to the second input and changes the state of the binary signal on the output path; and

measuring means having a first input connected to the first output of said digital automatic frequency control, having a second input connected to said first voltage source, said measuring means providing a first indication when said switching means is in said other state, said measuring means providing a second indication when said first and second switching means are in said other state, said modulation sensitivity being adjusted so that the first and second indications are substantially the same.

10. A method of checking the modulation sensitivity of a frequency modulator consisting of the steps of:

measuring the amplitude of the automatic frequency control signal during normal operation of the frequency modulator;

switching from normal to a test mode to apply a calibrated modulation frequency to the modulation input of the modulator;

shifting the frequency of the modulated carrier by a fixed amount and in a direction opposite to that effected by the calibrated modulation frequency; and

again measuring the amplitude of the automatic frequency control signal during the test operation, whereby the difference between the normal amplitude and the test amplitude is determined.

11. A method of adjusting the frequency deviation of a frequency modulator consisting of the steps of:

first, measuring the amplitude of the automatic frequency control signal during normal operation of the frequency modulator;

second, switching from normal to the test mode to apply a calibrated modulation frequency to the modulation input of the modulator;

third, shifting the frequency of the modulated carrier a fixed amount and in a direction opposite to that effected by the calibrated modulation frequency;

fourth, measuring the amplitude of the automatic frequency control signal during the test operation;

fifth, adjusting the modulation sensitivity; and

repeating the first through fifth steps until the measurements during the first and fourth steps are substantially the same.