Summary of Battery Powered Frequency Meter (0 to 20kHz)
This article describes a battery-powered digital frequency meter operating from a 9V source. It utilizes a TC9400 frequency-to-voltage converter and a TC7126 analog-to-digital converter to drive a 3-1/2 digit LCD display directly. The circuit features an input stage with specific resistors, capacitors, and diodes for signal conditioning and protection. Internal regulators power the components, while oscillator settings ensure high rejection of power-line interference, enabling three conversions per second within a 0 to 20kHz range.
Parts used in the Battery Powered Frequency Meter:
- TC7126 ADC
- TC9400 FVC
- 9-volt battery
- 33kΩ current limiting resistor
- 0.01µF DC blocking capacitor
- 1N914 clamping diode
- 1MΩ biasing resistor
- 620kΩ integrator feedback resistor
- 12kΩ bias resistor
- 1MΩ integration component resistor
- 0.047µF integration component capacitor
- 150kΩ oscillator resistor
- 50pF oscillator capacitor
The circuit is a simple digital frequency meter that is made of a frequency-to-voltage converter and an analog-to-digital display converter that can be operated from a single 9-volt battery. The TC7126 ADC generates the voltage required by the TC9400 FVC with internal regulators. The TC7126 is designed to directly drive a 3-1/2 digit, non-multiplexed LCD display so no digital conversion is required.
The input circuit is made up of a current limiting resistor (33kΩ), a DC blocking capacitor (0.01µF), a clamping diode (1N914), and a biasing resistor (1MΩ). The diode acts as a soft clamp to prevent negative going transitions from latching the comparator input and the 33kΩ resistor limits the current during the positive transitions. The gain (VOUT vs. FREQIN) of the TC9400 is determined by the charge-balance capacitor and the integrator feedback resistor (620kΩ) that has been selected for an output of approximately +2V (referenced to ANALOG COMMON) with frequency input of 20kHz. The bias resistor (12kΩ) determined the input threshold of the comparator and has been selected for an input sensitivity range of 250mV to 10V peak-to-peak of a sine or square wave on the input of the FVC.
The TC7126 will have a maximum positive input of about 2V since the input is referenced to ANALOG COMMON that is only 3V below V+. The internal voltage swing of the integrator does not have the same limitation because a positive input results in a negative swing of the integration. A fully charged battery will give a range of about 6V. The integration components (1MΩ and 0.047µF) at pins VBUFF and VIN are selected, in conjunction with the oscillator frequency to have an integrator ramp amplitude of about –3V with a 2V input from the TC9400. The oscillator is set up to run at 48kHz (150kΩ and 50pF) for maximum rejection of stray power-line pickup. This will result in the TC7126 running at three conversions per second.
For more detail: Battery Powered Frequency Meter (0 to 20kHz)
- How is the circuit powered?
The circuit operates from a single 9-volt battery. - What components generate the voltage for the FVC?
The TC7126 ADC generates the required voltage using internal regulators. - Does the system require digital conversion for the display?
No, the TC7126 is designed to directly drive a 3-1/2 digit non-multiplexed LCD display. - What is the function of the 1N914 diode?
The diode acts as a soft clamp to prevent negative going transitions from latching the comparator input. - How is the gain of the TC9400 determined?
Gain is determined by the charge-balance capacitor and the 620kΩ integrator feedback resistor. - What input sensitivity range does the bias resistor support?
The 12kΩ bias resistor sets an input sensitivity range of 250mV to 10V peak-to-peak. - Why is the oscillator set to run at 48kHz?
The 48kHz setting provides maximum rejection of stray power-line pickup. - How many conversions per second does the TC7126 perform?
The setup results in the TC7126 running at three conversions per second.
