Abstract:
A multimeter with a filtered measurement mode. By pressing a single button or key, a low-pass filter is switched into the signal path to filter voltage spikes, noise, and switching transients from pulse-width modulated pulses, or lower frequency sinusoidal signals with higher frequency components, and at the same time an on-screen icon indicates to a user that the filtered mode has been selected. The user can switch back and forth between filtered and unfiltered modes. The filtered measurement mode is useful in measuring the outputs of modern adjustable-speed motor drives, uninterruptible power sources, and switch power supplies.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to digital multimeters, and in particular to a multimeter having a filtered measurement mode. 
   Digital multimeters measure a wide range of electrical parameters, such as a.c. and d.c. currents and voltages, and resistance as well as physical parameters that can be converted to electrical signals, such as temperature, capacitance, and frequency of events. One commercially available family of digital multimeters favored and used by professional electricians is the Fluke 80 Series. These multimeters have additional measurement capabilities, including true RMS (root-mean-square) voltage measurement, freezing a measurement in a so-called “hold” mode, measuring the minimum and maximum (Min/Max) voltages of a varying signal, and making diode checks. Because these multimeters are capable of accurate RMS voltage measurements over a broad band of frequencies, they are a versatile tool for investigation of power quality problems, such as transients and harmonics. 
   However, there are a number of measurement situations confronting the technician in which it is extremely difficult to make accurate measurements. For example, modern adjustable speed motor drives, uninterruptible power sources, and switching power supplies use pulse-width modulation using high frequency, high voltage pulses to generate pulse-width modulated pulses, or lower frequency sinusoidal signals, with higher frequency components in the form of switching transients and voltage spikes superimposed on the signals. The resultant RMS voltage measurement values read erroneously higher than they should because total energy of the combined outputs is being measured. 
   It would be desirable to add a measurement feature in which switching transients and voltage spikes are filtered for low-frequency measurements without compromising the broadband measurement capabilities of the multimeter. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a multimeter with a filtered measurement mode is provided. 
   A multimeter includes conventional input circuitry for signal conditioning and range selection. The output of the signal conditioning circuit is applied to both a low-pass filter and one input of a programmable switching circuit. The output of the low-pass filter is applied to a second input of the programmable switching circuit. When a filtered measurement mode is selected, a single front-panel button is pressed and a command signal from a microcontroller is sent to the programmable switching circuit to select the filter output which is then applied to an RMS converter. At the same time, an indicator alerts the user that the filtered measurement mode has been selected and is in use so that mistakes are not made when the low-pass filter is no longer needed. In the preferred embodiment, the indicator is an on-screen icon that is displayed when the filtered measurement mode is active. 
   The programmable switching circuit also allows selection between the output of the RMS converter and the output of the low-pass filter for further processing by measurement circuitry. In addition, the filtered measurement mode facilitates the ability to accurately measure the frequency of a variable-speed motor drive using the conventional frequency measurement circuit of the multimeter. 
   A multimeter with a filtered measurement mode in accordance with the present invention is extremely useful in measuring the outputs of modem adjustable speed motor drives, uninterruptible power sources, and switching power supplies which use pulse-width modulation using high frequency, high voltage pulses to generate low-frequency sinewave outputs. 
   Other objects, features, and advantages of the present invention will become obvious to those having ordinary skill in the art upon a reading of the following description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic plan view drawing of a multimeter incorporating the present invention; 
       FIG. 2  is a partial block diagram of a multimeter showing an implementation of the present invention; 
       FIG. 3  is a schematic of a low-pass filter used in the preferred embodiment; and 
       FIG. 4  is representative of a display icon indicating the filtered measurement mode. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1  of the drawings, there is shown a schematic plan view drawing of a digital multimeter  10  having a pair of test leads  12  and  14  connected to a circuit under test  16 . The details shown in the plan view are referred to as the user interface, and include a display screen  20  for viewing measurement results, a rotary switch  22  for selecting measurement functions and ranges, a plurality of input jacks  24  including at least a volts (V) jack and a common (COM) jack for receiving test leads  12  and  14 , and one or more mode-selecting push-button switches or keys  26  for selecting operating modes. For purposes of explaining this invention, key  26  selects between a filtered measurement mode and a normal operating mode. 
     FIG. 2  is a partial block diagram of the multimeter  10  of  FIG. 1 . A circuit under test  16  is connected to input jacks  24  (V) and  24 ′ (COM) via test leads  12  and  14  as described above in connection with  FIG. 1 . The input signal is applied to a signal conditioning circuit  100 , where it is processed in accordance with the operating mode, function, and ranges selected by interface switches  102  and a microcontroller  104 . Interface switches  102  includes the aforementioned push-button switch  26  for selecting between a filtered measurement mode and a normal operating mode, which in the preferred embodiment means a full-bandwidth measurement range of 20 kilohertz. Signal conditioning circuit  100  may suitably include, for example, an input ranging circuit such as that shown in U.S. Pat. No. 6,094,045 to Raymond D. Zoellick. 
   The output signal conditioning circuit  100  is applied to a low-pass filter  110  and to a BW input of a programmable switch  112 . The output of low-pass filter  110  is connected to an LP 1  input and an LP 2  input of programmable switch  112 . Programmable switch  112  includes a first independent switch  114  that selects between the filtered signal at the LP 1  input and the full bandwidth signal at the BW input upon command from microcontroller  104  in response to pressing key  26 . The selected output of switch  114  is applied to an RMS converter circuit  116 , which is a true RMS conversion circuit. That is, RMS converter  116  provides the RMS value of the applied voltage, as is well known by those having skill in the art. 
   The RMS value at the output of the RMS converter is applied to an RMS input of programmable switch  112 . Programmable switch  112  includes a second independent switch  118  that selects between the filtered signal at the LP 2  input and the RMS value at the RMS input upon command from microcontroller  104  in response to a function selected by interface switches  102 . 
   The output of switch  118  is applied to measurement circuit  124 , where the selected signals are processed and converted to measurement results for display on display device  130 , which may suitably be liquid crystal display (LCD). Measurement circuit  124  may suitably include frequency counters, Min/Max peak detection, and other signal processing as well as magnitude measurement. These functions are common in modern multimeters. 
   It can be appreciated that in the normal operating mode of the multimeter, conditioned signals may be applied directly to measurement circuit  124 , or through switches  114  and  118  and RMS converter  116  to provide an RMS value of the input voltage. For example, if the input signal is a d.c. voltage or a current, there is no need to route the conditioned signal through the RMS converter  116  and hence may be routed directly to the measurement circuit  124 . 
   The low-pass filtered signal may be either routed through switches  114  and  118  and RMS converter  116  to provide an RMS value to be processed by the measurement circuit,  124 , or via LP 2  and switch  118  directly to the measurement circuit  124 , depending on the measurement results desired. For example, if an adjustable speed motor drive is under test, the technician may be more interested in the frequency of the filtered sinewave signal from low-pass filter  110  rather than the RMS value. 
     FIG. 3  is a schematic of a low-pass filter suitable for use as low-pass filter  110  in accordance with a preferred embodiment of the invention. The input path to the + input of operational amplifier  200  is via resistors  202 ,  204  and  206 . Note that this is a d.c. path. Capacitors  208  and  210  form with resistors  202 ,  204 , and  206  an RC low-pass filter. The inverting (−) input of operational amplifier  200  is connected to its output, which is also connected to the junction of resistors  204  and  206  via a feedback capacitor  220 . With the components values shown in Table 1, this filter has a frequency range of d.c. to a cutoff frequency of 800 Hertz. That is, signals having frequency components higher than 800 Hertz are effectively removed from the signal. Thus, voltage spikes, noise, and switching transients are removed from low-frequency sine waves, allowing a pure sine wave signal to be output. 
   
     
       
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
           
           
             
                 
               202 
               102 
               kΩ 
             
             
                 
               204 
               107 
               kΩ 
             
             
                 
               206 
               97.6 
               kΩ 
             
             
                 
               208 
               0.0027 
               μF 
             
             
                 
               210 
               390 
               pF 
             
             
                 
               220 
               0.0068 
               μF 
             
             
                 
                 
             
           
        
       
     
   
   As mentioned previously, the user of the multimeter may activate the filtered measurement mode simply by pushing key  26  on the front of the multimeter. Since a second push key  26  is required to de-activate the filtered mode, an indicator alerting the user of the selected filtered mode is useful in helping to prevent mistakes. While such an indicator could take many forms, including audible tones or lights, an on-screen icon  300  such as that shown in  FIG. 4  may be generated to provide a clear indication that the filtered measurement mode has been selected. Icon  300  in accordance with the preferred embodiment shows a low-pass filter symbol with the word “LO” positioned inside, and “˜V” to indicate a.c. voltage. Icon  300  disappears when the mode is switched back to normal. 
   It can be discerned that in a practical application of measuring the complex drive output of a variable-speed drive motor, the user can press one key or button to limit the RMS reading on the multimeter display to only the work-producing voltage applied to the motor, and at the same time, an icon appears in the display to indicate the filtered measurement mode. In addition, frequency measurements of the motor drive signal may also be made using the filtered measurement mode. 
   While we have shown and described the preferred embodiment of our invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from our invention in its broader aspects. It is therefore contemplated that the appended claims will cover all such changes and modifications as fall within the true scope of the invention.