Voltage Controlled Oscillator With Correction of Tuning Curve Non-Linearities

This invention relates to voltage controlled oscillators, and particularly to a voltage controlled oscillator which is useful for ESM (Electronic Support Measures) receivers where both linearization and temperature compensation of the tune voltage input/frequency output transfer function are important considerations.

Voltage controlled oscillators (VCO) generally have a non-linear tuning characteristic, which means that the transfer function between the tune voltage input and frequency output is non-linear. This tuning characteristic is commonly linearized and temperature compensated using PROMs (Programmable Read Only Memories).

An ESM receiver is often a scanning superheterodyne type receiver which is used to intercept and derive information from RF and microwave signals.

Most ESM Receivers require one or more VCOs to provide the function of the local oscillator. These systems require the voltage input/frequency output transfer function to be linearized and temperature compensated. This function has been traditionally implemented with PROMS or breakpoint generators. This is expensive in terms of hardware cost, testing time, and does not work if some component characteristic drifts with age. In addition, if certain components within the VCO fail, the calibration data in the PROM becomes invalid. New linearization and temperature compensation data must be retaken and input to the PROM.

The use of programmed control of a VCO with a varactor tuned filter has been illustrated in a number of U.S. Patents. U.S. Pat. No. 4,641,101, issued Feb. 3, 1987, to Harold N. Selim provided a VCO with a programmable tuner which generated voltages for two voltage dividers. A tuning curve was programmed in a RAM (Random Access Memory) with tuning voltage data values stored in memory to cause the dividers to follow the input. There the VCO tuning voltage was quantitized into sixteen steps.

This may be contrasted with the approach of shaping networks and PROMS incorporated into microprocessor controlled signal generators mentioned, along with thermistors for temperature correction, in U.S. Pat. No. 4,728,906, issued Mar. 1, 1988 to Turl et al. That patent controlled a varactor-tuned oscillator by using look-up tables for fine tuning with a program for a microprocessor to perform the various frequency establishing steps and calibration steps of the apparatus.

Microcomputers have been used to measure the frequency of an output from a VCO, as illustrated by U.S. Pat. No. 4,823,399 issued Apr. 18, 1989 to Ashok K. George.

Programmed tuning has been used in tuning television oscillators, as illustrated by the U.S. Patents to Rast et al., U.S. Pat. Nos. 4,078,212 issued Mar. 7, 1978 and 4,077,008 issued Feb. 28, 1978.

The aforesaid programmatical and hardware approaches are an expensive solution to linearizing a VCO throughout its frequency range and requires additional elements to handle temperature compensation.

Linearization and temperature compensation of a voltage controlled oscillator (VCO) may be accomplished using a stable combine or harmonic oscillator for generating known frequencies throughout the VCO tuning range, reducing not only the component cost but the time required for tuning. This solution provides a more suitable approach where an ESM system is involved, since a combline oscillator used in accordance with the preferred embodiment of the inventions may be already used for calibration and built-in-test (BIT) and does not constitute additional hardware.

The VCO tune voltage is adjusted under microprocessor control such that each combline frequency is sequentially down-converted to the center of the intermediate frequency (IF) passband of the receiver. Temporary data is collected on the VCO frequency and tune voltage.

The temporary data is utilized as input to the microprocessor and in accordance with our preferred embodiment the non-linear turning function is characterized with a third order polynomial, and this is repeated often enough that temperature compensation is not required. A Cubic Spline program is used to calibrate a specific desired frequency and the coefficients for the particular frequency range are used to generate a polynomial equation which is solved to determine the corresponding turning voltage. This permits implementation of the temperature compensation by rapid repetition of the complete calibration process when it is determined that the VCO tuning curve has drifted significantly from a previous calculation.

In accordance with the preferred embodiment of our inventions the drift is detected by periodically comparing the tune voltages corresponding to the combline frequencies to the tune voltages for the same comblines frequencies calculated during the previous calibration. The combline oscillator generates a series of stable harmonic frequency components, which the local oscillator sequentially converts to the same intermediate frequency and the calculated local oscillator frequency and the tune voltage may be stored in local memory.

The approach significantly reduces the recurring costs of VCO's with minimal impact on processor memory and processing time.

For a further description of the inventions, reference should be had to the following description and drawings.

FIG. 1 illustrates schematically an ESM receiver incorporating the preferred embodiment of the inventions.

FIG. 2 is flow chart of the corrective program for the receiver.

FIG. 3 consisting of FIGS. 3A and 3B is an illustration in Psuedo Code of the use of a Cubic Spline best fit polynomial function for linearization and temperature compensation of a VCO tuning characteristic.

As previously stated, in an ESM receiver combline oscillators are used routinely. It does not constitute additional hardware to use this oscillator to generate known frequencies that may be used to calibrate the VCO, as may be seen by referring to our preferred embodiment illustrated by FIG. 1. In accordance with this embodiment a combline or harmonic oscillator 101 generates a series of stable frequency components which are harmonically related to a base frequency (i.e. Fbase, 2*Fbase, 3*Fbase . . . N*Fbase, where N is a positive integer and Fbase is a fixed frequency).

In accordance with out invention the VCO input voltage is adjusted until a combline signal is down-converted to the center of the IF passband. The input voltage and the computed VCO frequency are recorded in temporary memory. In accordance with the illustrated embodiment, the VCO frequency is computed knowing the combline frequency and the IF center frequency. This procedure is repeated in each repeated calibration for all combline frequencies and a table of voltage/frequency data points is generated.

We have improved the time of calibration by providing that the table of data points is provided as an input to a Cubic Spline program which generates the best fit, third order polynomial curve between each consecutive pair of data points. The output of the Cubic Spline program is a set of four polynomial coefficients for each pair of consecutive data points which can be used to describe the curve between the data points.

The advantage of the described preferred Cubic Spline program over an N-th order polynomial function which might have been used (such as the Lagrange Integration Polynomial) is in the ease of computation and the numerical accuracy. The Lagrange Integration Polynomial requires a polynomial with the same number of elements as the number of data points. Thus, if the combline oscillator has 38 discrete frequencies, a polynomial of degree 38 would need to be solved. This would be too time consuming for the microprocessor and could result in significant error if the coefficients are not carried out to many significant digits, so the Cubic Spline calibration offers a substantial improvement over this other possibility.

Once the Cubic Spline calibration is complete and a particular VCO frequency is desired, the coefficients for the particular frequency range which contains the desired frequency are used to generate a polynomial equation. The equation is solved to determine the corresponding tune voltage. Temperature compensation is achieved by periodically repeating the complete calibration process. The process is repeated when it is determined that the VCO tuning curve has drifted significantly from the previous calibration. The drift is detected by periodically comparing the tune voltages corresponding to the combline frequences to the tune voltages for the same comblines calculated during the previous calibration.

FIG. 1 illustrates schematically a scanning-superhetrodyne ESM receiver incorporating the preferred embodiment of the inventions. The preferred receiver will comprise a stable combline oscillator (101), a frequency converting device (103), a VCO (104) and a frequency measurement device (107). A VCO (104) will consist of a bipolar or field effect transistor used as a negative resistance element, with a voltage input and frequency output transfer characteristic.

In accordance with our preferred embodiment, we utilize the computational capabilities of the frequency measurement device (107) to compensate for temperature fluctuations. The method of linearizing and compensating for temperature fluctuations which we employ uses the voltage/frequency transfer characteristic of a VCO using a stable combline frequency source to generate known voltage/frequency points. The set of points is input to a Cubic Spline program which creates the coefficients of a set of polynomial curves that together characterize the transfer curve of the VCO as computed by a microprocessor (108). With this method of temperature compensating the VCO transfer curve is determined by repeating the entire calibration procedure periodically whenever the known voltage/frequency points have drifted by an unacceptable amount.

A preferred scanning superhetrodyne ESM receiver is provided, with the circuit schematically coupled as illustrated in FIG. 1. A single-pole, double throw switch (102) is used to select either the receiver input (usually an antenna) or the combline oscillator (101). During VCO linearization, the switch (102) is connected to the combline oscillator position. The mixer (103), IF amplifier (105), and the bandpass filter (106) are used to convert the combline frequencies to an IF frequency that is measurable by the frequency discriminator (107). The mixer mixes or converts each stable frequency component (N*FBASE) of the combline oscillator sequentially to an intermediate frequency (IF) using the VCO (104) as a local oscillator. The frequency discriminator (107) measures the intermediate frequency after amplification. A VCO (104) is provided having a transistor coupled as a negative resistance element and having a voltage input, frequency output transfer characteristic. The VCO may have either a bi-polar or field effect transistor. The VCO (104) is used as the local oscillator input to the mixer (103) and receives its tune voltage command from a microprocessor (108). This microprocessor (108) adjusts the tune voltage input to the VCO such that each combline frequency is sequentially converted to the same intermediate frequency and the calculated local oscillator frequency and tune voltage stored in temporary memory. The local oscillator frequency (Flo) is calculated using the known combline frequency (Fcomb) and the known intermediate frequency (Fif) as Flo=Fcomb+/-Fif where the + sign is used if the combline frequency is below the local oscillator frequency and the - sign is used if the combline frequency is above the local oscillator frequency.

FIG. 2 is a flow chart of the corrective program for the receiver. The process starts with power up of the receiver whereupon the microprocessor (108) resets the combline counter to the first combline frequency N=1 (110). The first calibration following receiver power up is different from all subsequent calibrations. During the first calibration, the system must find all the combline frequencies knowing only the tune voltage of the first combline. Once the first calibration is complete and all tune voltages corresponding to the combline frequencies are known, all subsequent calibrations use the known combline tune voltages as a starting point for centering the signals in the IF passband. The process (111) decides if the calibration is the first calibration following application of power. If it is, the tune voltage is set to either the predetermined voltage for the first combline (114) if the first combline is being calibrated; or, the modulation sensitivity is computed in process (113) if it is not the first combline frequency. Computing the modulation sensitivity (or slope of the frequency out/voltage in curve) is performed in both process (115) and (118). This process involves recording the tune voltage/frequency output point (V1,F1) then adjusting the tune voltage a fixed amount and recording the second voltage/frequency point (V2,F2). The modulation sensitivity is then computed as (F2-F1)/(V2-V1). Process (116) computes the tune voltage required to center the Nth combline in the IF bandpass. At this point in the program, the microprocessor (108) has attempted to center the Nth combline in the IF passband. Processes (117), (118), and (116) from a repetitive loop wherein the combline signal is adjusted to the center of the IF passband. At this point, the tune voltage/frequency data point may be recorded in process (119). If all combline data points have been taken, the Cubic Spline coefficients are computed and stored in process (122). If the combline frequencies have not been completed, the combline counter is incremented in process (121) and the process is repeated starting at (111).

FIG. 3 composed of FIGS. 3A and 3B is an illustration in pseudo code showing the calculation of the Cubic Spline coefficients (122) using the voltage/frequency points as inputs. As the illustrated code is self explanatory, no further discussion will be needed at this point.

It will be appreciated by those skilled in the art, that having described our preferred embodiment of our inventions, that various improvements and modifications may be made, both now and in the future, and that such modifications will be within the scope of our claimed inventions.