Many basic function generators cannot measure output frequency or amplitude. First presented to the Elektor community in 2009, this module addresses that by displaying both on a two-line LCD. It also functions as a standalone true RMS voltmeter with frequency measurement.

The Design

The voltmeter design is features two main modules: the input amplifier and the display board. The designer, Kai Riedel, explains: "The input circuitry consists of a switchable voltage divider with subsequent amplifier and a true RMS converter. Alongside this is a pulse shaper used in measuring the signal frequency. The amplifier is needed because the error in the output of the true RMS converter increases noticeably at input levels of 5 mV or less."
 
Overall wiring.

The display module includes processing circuit, consisting of a microcontroller (Renesas R8C/13 MCU), LCD panel, analog-to-digital converter and an RS-232 interface.

The design's main characteristics include:
 
  • Four measurement ranges: 0.1 V, 1 V, 10 V, 100 V
  • Voltmeter bandwidth: 20 Hz to 1 MHz (basic accuracy ±1.0% of full scale)
  • Frequency counter bandwidth: 1 Hz to 10 MHz (typ.), 25 MHz (max.) (±timebase error)
  • Minimum input voltage for counter: approximately 50 mVRMS
  • Input impedance: ≥ 1 MΩ || ≤ 50 pF
  • Counter timebase: 100 ms, 1 s
  • Adjustable trigger level: approximately ±0.15 V
  • Autoranging (may be disabled)
  • Offset correction and calibration facility
  • Automatic scaling of frequency display
  • Voltage displayed as RMS value or peak-to-peak value (for sinusoidal input)
  • Crest factor: ≤ 4
  • Two measurement inputs
 
The design and display
The circuit should be housed in a metal enclosure to minimize interference (Figure 7). DC signals are routed to the input amplifier through feedthrough capacitors, while measured signals connect via SMA or BNC connectors. The comparator’s square-wave output is also accessed through an SMA connector. If integrating the module into a function generator, use a screened cable to connect the generator’s output to the module's SMA input.

The LCD, four pushbuttons, and timebase indicator LED are mounted on the front of the display board, with other components positioned on the back. This design allows the board to function as a self-contained module within an enclosure.
 
Block diagram of the meter.

The Original Voltmeter Project

The article, “True RMS Digital Voltmeter,” appeared in Elektor June 2009. You can download the project article for free during the two weeks following the publication of this post. Enjoy the article and let us know if you start a project!
Editor's Note: The article first appeared in a 2009 edition of ElektorMag. Some of the components, products, PCBs, and/or links may no longer be available. Yet, we feel the educational content is valuable, and we think it will inspire you to start a project of your own.

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