Settings Documentation

Access FRA4PicoScope settings by selecting Menu Tools .. Settings

FRA Execution Options

Low noise mode: Use this setting when the background noise is minimal. It samples the data at a nominal oversampling of the stimulus signal. This mode is appropriate where a filter is being characterized in isolation. If there is significant noise, it may get aliased, leading to inaccurate results. The main advantage of this mode is that it executes faster.

Noise reject mode: Use this setting when significant background noise exists that must be filtered out. In this mode, the sampling rate can be set as high as possible for the scope, or a modest oversampling above the bandwidth limiter (where available). This mode is appropriate for switched mode power supply stability measurements, where ripple noise is significant compared to the injected stimulus.

Extra Settling Time (ms): This is an additional amount of time delay to insert between changes to the stimulus frequency/amplitude and capturing samples. This can help eliminate artifacts when a circuit needs additional time to reach steady state.

Adaptive Stimulus

Adaptive stimulus was designed for SMPS stability measurements. Its purpose is to try to keep the SMPS in the linear region of operation. It seeks the minimum stimulus amplitude that still achieves the target output response (specified on the main panel) of the input or output channel with the lowest amplitude response.

Stimulus tries/step: Specifies how many times to try seeking the minimum stimulus amplitude before giving up. Default value is 10.

Response target tolerance (%): Specifies a percentage above the response target that is still considered good (target met). Setting the target (on the main panel) and tolerances may take some experimentation. Make an initial guess on the greatest gain you expect to see in the loop. For instance, if you know you’ll be measuring in a band that has 60dB loop gain, and your injection is only capable of delivering 4Vpp, a target of 100mv will never be met. The target should be less than 4 mVpp.

Autorange settings

Input/Output start range: This helps autoranging find the correct measurement ranges faster by providing a starting range. Once proper ranges are first found by autoranging, the response of adjacent/next steps is often so similar that subsequent autorange tries are less infrequent.

Autorange tries/step: Specifies how many autorange seeking tries will be attempted before giving up. The default value of 10 is more ranges than any supported scope has. Increasing beyond the default would probably only be useful if for some reason your signal had lots of some kind of noise that made autoranging continually hunt for the right ranges.

Autorange tolerance (%): Specifies a hysteresis value to avoid bouncing back and forth between ranges.

FRA Sample Settings:

This section can be a little tricky to understand because the values can mean different things depending on sampling mode, and the values are often interrelated. Also, there can be combinations of all these values that result in warnings while running FRA when all specifications can’t be met simultaneously.

Minimum cycles captured: In Low noise mode, it’s the exact number of stimulus cycles to capture for all captures. For example, with the default value of 16, at 1Hz stimulus it will try to capture 16 seconds of data. In Noise reject mode, it’s the minimum number of stimulus cycles to capture no matter what the bandwidth setting indicates.

Low noise oversampling: Only applies to Low noise mode. Specifies the scope’s minimum attempted oversampling rate (vs stimulus frequency). For example, with the default value of 64x, at a stimulus of 1Hz the scope’s sampling rate will be at least 64 Hz. At high stimulus frequencies the scope may not sample as fast as specified and will instead sample as fast as it can.

Noise reject bandwidth (Hz): Only applies to Noise reject mode. Captures enough samples so that the DFT bin is at least this narrow. Sampling always tries to target a whole number of stimulus cycles, so the actual bandwidth achieved can be narrower than specified. Smaller bandwidth values require more samples which leads to higher DFT “processing gain” and thus higher SNR.

Noise reject timebase: Only applies to Noise reject mode. Specifies the fixed timebase to be used in Noise reject mode. The corresponding sample rate is displayed immediately below this. Set this value based on the frequency content of the signal being analyzed to avoid any undesired effects of frequency folding. Since the FRA application does not use streaming and the scope’s sample buffer is limited, there is a minimum stimulus frequency that can be achieved in Noise reject mode. This value is updated and displayed based on the timebase selected.

FRA Bode Plot Options:

Phase wrapping threshold (°): Phase threshold at which to wrap results back around the unit circle. E.g. at default threshold value of 180° a result of 210° would be wrapped to -150°. If the threshold were set to 90°, a value of 100° would be wrapped to -260°. This affects the plot axis of unwrapped phase values.

Gain margin phase crossover (°): Phase crossover value to be used when computing gain margin. 0° is the default value and is appropriate for the standard SMPS measurement configuration where a resistor is inserted into the feedback loop.

Diagnostic Settings:

Time domain diagnostic plots: Generate time domain plots for each trial and store in <User Directory>\Documents\FRA4PicoScope\diag. These plots show the scope samples as well as various statistics such as channel voltage range, DFT measured amplitude and purity.

Log Verbosity:

• Instrument Access Diagnostics: Logs events related to opening and initializing the scope

• FRA Progress: Logs current FRA step number and frequency

• Step Trial Progress: Logs each try within a step, e.g. autorange and adaptive stimulus tries

• Signal Generator Diagnostics: Logs frequency of the signal generator

• Vertical Resolution Diagnostics: Logs current vertical resolution in bits

• Autorange Diagnostics: Logs information such as current range, absolute peak values, and over-range conditions

• Adaptive Stimulus Diagnostics: Logs information such as signal generator amplitude and measured input and output amplitudes

• Sample Processing Diagnostics: Logs information about the number of samples, sampling frequency, and how long it will take to sample

• DFT Diagnostics: Logs DFT computation method, actual bandwidth achieved, and DFT outputs including magnitude, amplitude, phase, purity, d.c. energy, signal energy, and total energy

• Scope Power Events: Logs events related to the status of the scope's auxiliary DC power source

• Save/Export Status: Logs status of save and export events, including location of stored files

• FRA Warnings: Logs various conditions about the FRA that may indicate issues with measurement quality

• PicoScope API Calls: Logs all calls and parameters to the PicoScope API