RF Test Equipment Series: Oscilloscopes

An Oscilloscope is an essential piece of electrical test equipment, but has a somewhat limited use with RF circuits and systems compared to digital and analog circuits. In many cases an RF engineer will default to using a spectrum analyzer for frequency domain measurements and a network analyzer to measure component behavior. An oscilloscope is solely capable of capturing voltage changes over time at the inputs of the scope. Hence, a scope is not as useful as a spectrum or signal analyzer for evaluating modulated signals.

However, this can be useful in cases where the time-domain behavior of a circuit is impacting the RF performance. As with most modern systems, many RF circuits are driven or controlled by digital and analog circuits, and an oscilloscope can be useful when measuring the amplitude and timing of multiple digital and analog signals simultaneously. An oscilloscope may also be crucial when comparing two signals over time, or determining the duration of a signal. Another use of an oscilloscope for RF circuits is the measurement of harmonic distortion of a signal.

Some modern oscilloscopes have hardware, or software that can perform Fast Fourier Transforms (FFT) signal processing calculations on the digitized time-domain signal information. With this function, an oscilloscope can perform a frequency domain transform of the time-domain information, somewhat like the functions of a spectrum analyzer. Though these functions in oscilloscopes generally aren’t a replacement for spectrum analyzers, then can be used to identify the fundamental frequency of a signal and other spectrum analysis functions that aren’t affected by the wideband oscilloscope performance and much greater noise floor compared to a spectrum analyzer.

Other uses of an Oscilloscope in RF applications include the measurement of a local oscillator, and when it’s within the bandwidth of the oscilloscope, measurement of the intermediate frequency. In the case of digital modulation and demodulation, an oscilloscope can also be used to capture and measure the digital input and output signals to/from the modulation/demodulation circuitry. Oscilloscopes may also be useful when measuring pulsed circuits, and other applications where the high impedance of the oscilloscope probe allows for minimum interference with the DUT and the capacitance of the probe doesn’t significantly impact the circuit.

Useful Oscilloscope Links

• Pasternack VNA Calibration Kits
• Pasternack Phase Stable (VNA) Coaxial Cables
• Pasternack Cable Generic Test Cable Assemblies
• Pasternack Skew Matched Pair Coaxial Cable
• Pasternack Precision Coaxial Adapters
• Pasternack In-series and Between-series Adapters
• Pasternack RF Coaxial Probe
• Pasternack Coaxial Probe Leads
• Pasternack Antennas