EE-465 Instrumentation and
Measurements
Engr. Zagham Muhio Din
Lecture 7,8,10 (Week 11)
18-05-2026
Today, Topic
• Digital Voltmeters and Frequency Meters
• Time and Ration Measurements
• Counter Input Stage
• Counter/Timer/Frequency Meter
Electronic Ohmmeter (Already discuss in Lecture 4)
Low, High and Precise Resistance Measurements
Counter Input Stage
• counter input stage:The counter input stage of a digital counter or digital
frequency meter consists of an amplifier/attenuator circuit followed by a waveshaping circuit. Its purpose is to condition the input signal into a form suitable for
accurate digital counting.
• Amplifier/Attenuator Stage: The amplifier increases low-level input signals, while
the attenuator reduces high-level signals to safe operating levels. This ensures
that the signal amplitude is suitable for processing by the counter circuitry.
• Wave-Shaping Circuit: The wave-shaping circuit converts the analog input
waveform into clean digital pulses. A Schmitt trigger is commonly used for this
purpose because it provides good noise immunity and sharp switching
characteristics.
• Schmitt Trigger: A Schmitt trigger operates with two threshold voltages:
• Upper Trigger Point (UTP)
• Lower Trigger Point (LTP)
When the input voltage rises above the UTP, the output switches to a high state.
When the input voltage falls below the LTP, the output returns to the low state.
Counter Input Stage
• Hysteresis Band: The difference between the UTP and
LTP is called the hysteresis band. The hysteresis band
prevents false triggering caused by small noise variations
around the threshold voltage.
• Effect of Clean Input Signal: For a clean sine-wave input,
the waveform crosses the hysteresis band only twice
during each cycle. Therefore, the Schmitt trigger
produces one output pulse per cycle, resulting in
accurate frequency measurement.
• Effect of Noisy Input Signal: If the input signal contains
noise, the waveform may cross the hysteresis band
several times during one cycle. This produces extra
output pulses and causes counting errors in the digital
frequency meter.
Counter Input Stage
• Reducing Noise Errors
• Noise errors can be minimized by:
• increasing the hysteresis band
• attenuating the noisy signal
• using low-pass filters
• adjusting the input attenuator .
• Pulse Width Measurement Errors
• When measuring pulse width, long rise and fall times can
produce timing errors. The signal crosses the hysteresis band
slowly, causing the measured pulse width to become larger
than the actual pulse width.
• Minimizing Pulse Width Errors
• Pulse-width measurement errors can be reduced by:
• decreasing hysteresis
• amplifying the input pulse
• producing steeper pulse edges
• Amplification improves switching speed and measurement
accuracy.
Counter/Timer/Frequency Meter
Counter/Timer/Frequency Meter
Quiz type Questions