Frequency Counter
Understanding Frequency Counters
Frequency counters are fundamental test instruments for verifying oscillator frequency, synthesizer accuracy, and signal source performance. They provide the most accurate frequency measurement available in the lab.
Counter Types
- Direct count: Counts input cycles during gate time. Resolution = 1/gate_time Hz.
- Reciprocal: Measures period of input and gate clock simultaneously. High resolution at all gate times.
- Continuous timestamp: Records every zero-crossing with a high-resolution timebase. Provides frequency, period, phase, and time interval simultaneously.
Key Specifications
- Frequency range: DC to 20 GHz (standard), to 100+ GHz (with prescaler or downconverter).
- Resolution: 12-14 digits in 1 second gate time.
- Timebase accuracy: TCXO (+/- 0.5 ppm), OCXO (+/- 0.01 ppm), or GPS-disciplined (~0 error).
Frequently Asked Questions
What is a frequency counter?
A frequency counter measures signal frequency by counting cycles in a gate time. Modern counters: DC to 100+ GHz, 12+ digits resolution. Reciprocal counting provides high resolution even with short gate times.
How accurate is a frequency counter?
Accuracy = timebase accuracy + trigger error. With TCXO: +/- 0.5 ppm. With OCXO: +/- 0.01 ppm. With GPS-disciplined oscillator: essentially perfect (10^-12 accuracy). Plus trigger error from noise, typically 1 count +/- 1.
What about frequency counter vs spectrum analyzer?
Counter: highest frequency accuracy (12+ digits) but no spectral information. Spectrum analyzer: shows all signals and their relative levels but lower frequency accuracy (kHz level). Use counter for precise frequency, SA for spectral analysis.