The Battery Powered Frequency Meter is a straightforward digital frequency measuring device, comprising a frequency-to-voltage converter and an analog-to-digital display converter, all powered by a single 9-volt battery. The TC7126 ADC serves to produce the necessary voltage for the TC9400 FVC through its built-in regulators. Notably, the TC7126 is specifically engineered to directly interface with a 3-1/2 digit LCD display that doesn’t require multiplexing or additional digital conversion.
Here’s a general overview of the components and their connections for the battery-powered frequency meter circuit:
Frequency-to-Voltage Converter (FVC):
- The TC9400 is a Frequency-to-Voltage Converter (FVC) that converts the input frequency into a proportional voltage. It generates an output voltage that is linearly proportional to the input frequency.
Analog-to-Digital Converter (ADC):
- The TC7126 is an Analog-to-Digital Converter (ADC) with an integrated 3-1/2 digit LCD display driver. It converts the analog voltage from the TC9400 into a digital value that can be displayed on an LCD.
- This circuit is designed to operate from a single 9-volt battery as the power supply.
- The TC7126 includes internal voltage regulators that provide a stable supply voltage for the ADC and other components.
- Connect the 3-1/2 digit LCD display to the TC7126 according to its datasheet. The TC7126 is designed to directly drive such displays.
- You’ll need an input connector or terminals to connect the signal whose frequency you want to measure. This could be a BNC connector or other suitable connectors depending on your specific application.
Resistors and Capacitors:
- Various resistors and capacitors may be required to set the gain and filter the input signal appropriately. The values of these components will depend on the specific requirements of your circuit and the datasheets of the TC9400 and TC7126.
Ground and Power Connections:
- Ensure that all components are properly grounded, and the power supply connections are well-regulated.
The input circuit comprises several components, including a current-limiting resistor (33kΩ), a DC blocking capacitor (0.01µF), a clamping diode (1N914), and a biasing resistor (1MΩ). The diode functions as a gentle clamp, preventing negative transitions from causing the comparator input to latch, while the 33kΩ resistor restricts current during positive transitions. The TC9400’s gain (VOUT vs. FREQIN) depends on the charge-balancing capacitor and the feedback resistor in the integrator (620kΩ). These values have been chosen to yield an output of approximately +2V (with reference to ANALOG COMMON) when the frequency input is 20kHz. The bias resistor (12kΩ) establishes the input threshold for the comparator and has been selected to create an input sensitivity range spanning from 250mV to 10V peak-to-peak, accommodating sine or square wave inputs to the FVC.
The TC7126’s maximum positive input is approximately 2V since the input is referenced to ANALOG COMMON, which is only 3V below V+. However, the integrator’s internal voltage swing is not subject to the same constraint because a positive input leads to a negative swing in the integration. With a fully charged battery, the available range is about 6V. The integration components (1MΩ and 0.047µF) at pins VBUFF and VIN have been chosen to work in conjunction with the oscillator frequency, producing an integrator ramp amplitude of approximately –3V when provided with a 2V input from the TC9400.
The oscillator is configured to operate at 48kHz (using 150kΩ and 50pF components) to maximize rejection of stray power-line interference. As a result, the TC7126 will perform three conversions per second under these conditions.