Abstract:
A small light-weight battery operated calibrator device provides a precise sine wave output for use in calibration of test equipment, such as a RF Power Meter or a Spectrum Analyzer. The calibration device includes two power levels, one −40 dBm and one 0 dBm. The purpose of the two power levels is to obtain a slope and offset for correction of the RF power measuring device being calibrated. Operation indication LED lights are provided to indicate which of the two powers are in use, and if battery power is below acceptable levels. Miniature low power components including a crystal oscillator and a divide by 2 integrated circuit that generates a precise square wave and a low pass filter for converting the square wave into a precise sine wave allows the calibrator to be battery operated and stored as a calibration component.

Description:
CLAIM FOR PRIORITY 
       [0001]    This application is a divisional of application Ser. No. 11/856,325 filed on Sep. 17, 2007, entitled “Miniature RF Calibrator Utilizing Multiple Power Levels,” by Donald Anthony Bradley, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to components used in the calibration or verification of absolute frequency and amplitude measuring test equipment. More particularly the present invention relates to a highly accurate sine wave generation circuit used in calibrating or verifying the accuracy of test equipment such as RF power meters and spectrum analyzers. 
         [0004]    2. Related Art 
         [0005]    Existing calibration devices that can generate precise sine waves are typically provided internal to the instrument being calibrated, or as an external attachment. The internal RF calibrator in a test device typically has one power level and is accessible using the front panel space on the instrument. It may be undesirable, however, to use the front panel space which is typically in short supply on portable instrumentation. An external bench top calibrator, further, is typically bulky and requires a wall plug in line voltage for operation. The required line voltage may not be available for a field test instrument. 
         [0006]      FIG. 1  shows a block diagram of components of a typical calibrator. The system includes a precise oscillator  2 , typically operating at 50 MHz. The output of oscillator  2  is provided to a level modulator  4  that provides a stable voltage output from the oscillator  2  as controlled by a feedback signal. The output of the level modulator  4  passes through an amplifier  6 , low pass filter  8 , and attenuator  22  to a test port  24 . The attenuator  22  is shown as a variable attenuator, allowing a user to set the desired attenuation level. The amplifier  6  increases the output of oscillator  2 , while low pass filter  8  removes unwanted harmonics. The variable attenuator  22  is typically included in an external bench top calibrator that connects to a test device, allowing a user to select different output levels as desired during calibration. As an alternative to the variable attenuator  22 , a fixed attenuator can be used. A fixed attenuator is more typically included on a calibrator that is internal to a test device. 
         [0007]    The feedback signal to the level modulator  4  is provided from an amplifier  16 . The feedback signal comes to an input of amplifier  16  from the output of the low pass filter  8  through a detector diode  10  and resistor  14 . A filter capacitor  12  removes an AC component of the feedback signal. A capacitor  20  enables amplifier  16  to function as an integrator. A second input to the amplifier  16  is provided from a voltage reference  18 . The voltage reference  18  has a voltage value set to control the desired input level of attenuator  22 . 
         [0008]    It would be desirable to provide components for a calibration device that can provide a precise sine wave with two power levels that does not use up front panel space on an instrument being calibrated, is not bulky, and does not require a line voltage attachment. 
       SUMMARY 
       [0009]    According to embodiments of the present invention, a calibrator is provided that can generate precise sine waves and not suffer the drawbacks of prior art devices. 
         [0010]    The calibrator is a battery operated and provides two very precise sine wave outputs for use in calibration of amplitude or frequency measuring test equipment. With battery power, a line voltage is not required during testing. The calibrator further uses small light weight components, so it can be easily transported and used in a field test area, and will not use front panel space of an instrument being calibrated. 
         [0011]    The calibration device includes two power switches connecting the battery to a voltage regulator, one with attenuation of −40 dBm and one without at 0 dBm to provide two calibrated RF power levels. The purpose of the two power levels is to obtain a slope and offset for correction of the RF sensor in a test device being calibrated. 
         [0012]    The switches selecting either −40 dB or 0 dB of attenuation drive the voltage regulator that powers a crystal oscillator. The oscillator then drives a divide by two flip flop that generates a highly symmetrical square wave that has its amplitude controlled by the precision temperature corrected DC voltage regulator and its frequency stabilized by the quartz temperature corrected oscillator. The output of the divide by two frequency divider is then directed through a voltage divider to a low pass filter. With the −40 dB switch, the output of the divide by two frequency divider is connected to the low pass filter through a 10K resistor providing a 100:1 reduction. With the 0 dBm switch, the output of the divide by two frequency divider is provided to the filter through an AND gate that has a 10 Ohm resistance in series with a 90 Ohm resistor to form a total 100 Ohm resistor. An additional 100 Ohm resistor forms a two to one voltage divider with this first 100 Ohm combination to provide a two to one voltage division with a 50 Ohm output impedance to the low pass filter. With either the −40 dB or 0 dB switches, the voltage divider provides a precise square wave to the low pass filter with a matched source impedance of 50 Ohms. 
         [0013]    The low pass filter then removes all of the harmonics of the square wave to provide a precise sine wave output. The low pass filter output is provided through an attenuator and blocking capacitor to an output terminal of the calibrator. Diode protection devices are provided to divert static discharge or high power input surges applied to the output connector. The overall combination of components can be built from light weight low power components that still provide the precise sine wave output. Miniature low power components allow the calibrator to be battery operated and stored as a calibration component after use. 
         [0014]    In some embodiments, operation indication LED lights are provided to indicate the operation state of the calibrator. A green LED is connected with circuitry to provide two intensities depending on whether the −40 dBm or the 0 dBm attenuator is in use. A blinking red light is further connected with circuitry to indicate if battery power is below acceptable levels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Further details of the present invention are explained with the help of the attached drawings in which: 
           [0016]      FIG. 1  shows a block diagram of components of a conventional calibrator; 
           [0017]      FIG. 2  shows a block diagram of a miniature RF calibrator according to embodiments of the present invention; and 
           [0018]      FIG. 3  shows components providing LED lights connected to give a user a visual indication of the state of operation of the calibrator of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 2  shows a block diagram of a miniature RF calibrator according to embodiments of the present invention. The RF calibrator includes a battery  102  connected to two switches  104  and  106 . The switch  104  provides an attenuation factor of 1 or 0 dBm, while the switch  6  provides an attenuation factor of 100 or −40 dBm. The purpose of the two power levels is to obtain a slope and offset for correction of the RF sensor in a test device being calibrated. 
         [0020]    The output of the switches  104  and  106  are connected to a voltage regulator  8 . The output of the voltage regulator  8  provides power driving a quartz oscillator  10 , a divide by two flip flop  12 , and a two input AND gate. The oscillator  10  provides a highly accurate frequency at twice the output frequency to the divide by two flip flop  12 . The square wave output has its amplitude controlled by the precision temperature corrected DC voltage regulator  108  and its frequency controlled by the quartz temperature corrected oscillator  110 . An exemplary voltage regulator  108  that provides for such temperature correction is the Analog Devices ADP3336. An exemplary quartz temperature corrected oscillator  110  is the Kyocera K30-3C0-100.0000. 
         [0021]    The symmetry of the square wave is controlled by a divide by 2 frequency divider  112 . The frequency divider  112  can be constructed with complementary CMOS transistors. An exemplary frequency divider  112  is the Fairchild NC7SZ74. A slight resistance change of the output transistors in the frequency divider  112  over temperature is compensated for by the temperature dependant voltage regulator  108  to yield a constant output square wave voltage under a fixed resistive load. Output symmetry is inherent due to the frequency divider  112  changing states on only the positive going edge of the quartz oscillator. 
         [0022]    The output of the frequency divider  112  has a low 10 Ohm impedance. The 10 Ohm frequency divider  112  matches an impedance of the AND gate  114  which appears as a 10 Ohm resistor. The AND gate  114  can be constructed using complementary CMOS transistors similar to the frequency divider  112 . The AND gate  114  will provide a 10 Ohm resistance for both the 0 and 1 produced output. An exemplary circuit for the AND gate that provides a 10 Ohm resistance is the Fairchild NC7SZ02. Although an AND gate  114  is shown and described, other logic providing a Boolean AND can be used. 
         [0023]    With switch  104  used the output of the AND gate  114  is enabled. The output of the 10 Ohm AND gate  114  is then provided through a 90 Ohm resistor  116 . The total resistance of the series AND gate  114  and the 90 Ohm resistor is then 100 Ohms. This 100 Ohm total resistance is connected to a node  117  to another 100 Ohm resistance  118  that connects to ground. This forms a 50 Ohm output impedance voltage divider to drive the low pass filter  122 . 
         [0024]    During use of switch  106 , the output of AND gate  114  is disabled. The disabled AND gate  114  provides a 10 Ohm resistance to ground. With switch  104  disabled, the output of the frequency divider  112  is provided through a 10,000 Ohm resistor  120  to node  117  to connect to the low pass filter  122 . The attenuation factor of the voltage divider formed by the 10 Ohm AND gate  114  in series with the 90 Ohm resistor  116  and the 10,000 Ohm resistor  120  presents a 100:1 reduction of the precision square wave available to the low pass filter  122  compared with the signal available when switch  104  is enabled. 
         [0025]    With either switch  104  or  106  used, the precision square wave from node  117  now enters the low pass filter  122 . Filter  122  removes all harmonics of the fundamental frequency. The filter  122  is designed to present a 50 Ohm output impedance at the desired output frequency. It is also designed to accept slight variations on its input impedance without affecting its output impedance. This can be accomplished at a single frequency of interest. With switch  106  enabled, the filter  122  output frequency is now a pure sine wave with an amplitude of −36.5 dBm. The filter  122  is followed by a fixed 3.5 dB attenuator  124 . The final output at terminal  128  is, then, a −40.0 dBm pure sine wave. A source match is tightly controlled to provide the greater than 40 dB return loss and a SWR&lt;1.02 by precision design of the attenuator  124  and low pass filter  122 . Although specific attenuation values for the switches  104  and  106 , resistance values of resistors  116 ,  117 ,  118  and  120 , AND gate  114 , and attenuation of attenuator  124  are given, these exemplary values may be changed depending on desired design requirements. 
         [0026]    A DC blocking capacitor  126  follows the attenuator  124 . The DC blocking capacitor  126  is used to reference the output to 0 volts DC. The blocking capacitor  126  is further used to block any unintended DC from being applied to the calibrator output. Back to back diodes  130  and  132  at the input to filter  122  also prevent unintended RF energy as well as static discharge from destroying CMOS device components. The CMOS components that could be damaged include those in the AND gate  114  or the frequency divider  112 . A first diode  130  in the back to back diodes connects node  117  at the input of filter  122  to ground, while the diode  132  connects node  117  to the battery  102 . Neither diode conducts current during normal operation. 
         [0027]    Operation of the calibrator of  FIG. 2  is described as follows. Depressing the −40 dBm push button enables power to the circuit and disables AND gate  114 . The quartz oscillator  110  produces a very stable frequency at 2 times the output frequency. This signal has no amplitude control or duty cycle control, but is suitable to drive the divide by 2 divider  112 . The output of divider  112  has a low 10 Ohm impedance. The slight resistance change of the output transistors in the divider  112  over temperature is compensated for by the temperature dependant voltage regulator  108  to yield a constant output square wave voltage into a fixed resistive load. Output symmetry is inherent due to the frequency divider  112  changing states on only the positive edge of the quartz oscillator  110 . 
         [0028]    With AND gate  114  disabled when using switch  106 , the square wave is then presented to the approximately 10,000 Ohm resistor  120  and the disabled AND gate  114  and 90 Ohm resistor  116 . Disabled AND gate  114  appears as a 10 Ohm resistor to ground. The attenuation factor of this voltage divider represents a 100:1 reduction of the precision square wave available at the output of divider  117  compared with the signal available when switch  104  is enabled. The precision square wave now enters low pass filter  122  which filters all harmonics of the fundamental frequency. The filter  122  presents a 50 Ohm output impedance at the desired output frequency. Filter  122  also accepts slight variations on its input impedance without affecting its output impedance. This can be accomplished at a single frequency of interest. 
         [0029]    With switch  106  enabled, the output of filter  122  is now a pure sine wave with an amplitude of −36.5 dBm. The filter  122  is followed by a fixed 3.5 dB attenuator  124  and has DC blocked by capacitor  126 . The final output is a −40.0 dBm pure sine wave. The blocking capacitor  126  references the output to 0 VDC. It also blocks any unintended DC from being applied to the calibrator output. Back to back diodes  130  and  132  at the input to filter  122  prevent unintended RF energy as well as static discharge from destroying its CMOS components. 
         [0030]    Depressing the 0 dBm switch  104  enables the AND gate  114 . The output of the AND gate  114  is a precision square wave switching between ground and the regulated voltage. It has a 10 Ohm output resistance, which in series with the approximately 90 Ohm resistor  116  appears at 100 Ohms. The slight resistance change of the output transistors in the AND gate  114  over temperature is compensated for by the temperature dependant voltage regulator to yield a constant output square wave voltage into a fixed resistive load. This 100 Ohms is provided in series with the 100 Ohm resistor  118  to ground and creates a divide by two voltage divider at node  117 . The Thevinin equivalent impedance of the input of filter  122  then appears as a fixed 50 Ohms for both 0 and −40 dBm selections, and further operation of the calibrator is similar to that described with the −40 dBm switch depressed. 
         [0031]      FIG. 3  shows components providing dual colored LED lights  206  and  228  connected to give a user a visual indication of the state of operation of the calibrator of  FIG. 2 . Depressing the −40 dBm switch button  106  causes the green LED  206  to illuminate at a visible brightness. Depressing the 0 dBm switch button  104  causes the green LED  206  to illuminate twice as bright. Battery voltage below a usable range needed to keep the regulator  108  in regulation causes the red LED to flash  228 , indicating a low battery condition for battery  102 . In one embodiment, the LED lights  206  and  228  can be provided by a single red/green LED. An example of such a red/green LED is the Lumex SSL-LX30591 GW. 
         [0032]    The state indication circuit includes a comparator amplifier  201  having a first input connected to the output of voltage regulator  108 , and a second input connected through a voltage divider formed by resistors  220  and  222  to the input of voltage regulator  108 . Power is supplied to the comparator  201  from the input to the voltage regulator. The output of comparator  201  drives a resistor  204  that connects to the green LED  206 . An exemplary circuit for the comparator is the National Semiconductors LMV7239. Under normal conditions the comparator  201  provides an output of logic one or the voltage of battery  102 . To increase the intensity of the green LED  206  when switch  104  is depressed, a PMOS FET transistor  208  is provided with a gate connected to the ground connection of the switch  104 . An exemplary PMOS FET transistor  208  is the Zetex ZXM61P02F. With the switch  104  depressed, the source-drain path of transistor  208  connects the output of comparator  201  through a resistor  210  to the green LED  206 , thus reducing the overall resistance from the output of comparator  201  and LED  206  and increasing intensity of LED  206 . With switch  104  open, the transistor  208  will remain off and the intensity of LED  206  will be reduced when switch  106  is connected. 
         [0033]    An oscillator  224  is connected by a resistor  226  to the red LED  228 . The input of the oscillator  224  receives a disable signal from the output of comparator  201 . Thus, when the oscillator  124  is not receiving a disable signal from comparator  201 , it will enable the oscillator  224  and the red LED  228  will blink on and off at the oscillator  224  frequency of approximately 10 Hertz. For convenience, components in  FIG. 3  that are carried over from  FIG. 2  are similarly labeled. 
         [0034]    Operation of the circuitry of  FIG. 3  used in driving the green LED  206  is described as follows. First, selection of the −40 dBm switch  106  and sufficient voltage from battery  102  for proper operation will illuminate the green LED  206  at moderate brightness. The selection of 0 dBm switch  104  and sufficient battery voltage enables the boost transistor  208  that applies approximately twice the current to the green LED  206  so that it provides twice the illumination. 
         [0035]    Operation of the circuitry used in driving the red LED  228  is described as follows. First, the voltage regulator  108  provides a reference voltage used to compare to the voltage of the battery  102 . If the voltage of battery  102  drops below approximately 0.2V above the voltage of regulator  108  output the comparator  201  will change state from a 1 to a 0. This will enable the 10 Hz flashing oscillator which drives the red LED  228 . The green LED  206  will be disabled. 
         [0036]    Although specific voltages for battery  102 , oscillation frequencies for the LEDs, and LED colors are described, these are exemplary and may be changed based on design requirements. 
         [0037]    Although the present invention has been described above with particularity, this was merely to teach one of ordinary skill in the art how to make and use the invention. Many additional modifications will fall within the scope of the invention, as that scope is defined by the following claims.