&. Frequency Response Analysis Probe

Symbol Name: FRAPROBE

Syntax:

&xxx o+ o- i+ i-

The fraprobe device is used in conjunction with a @. Frequency Response Analyzer during frequency response analysis simulation (.fra). The fraprobe device analyzes gain between any two differential points. The fraprobe does not accept control parameters — the stimulus is controlled by the fra device.

The result of the fraprobe analysis is the complex quantity V(o+,o-)/V(i+,i-) versus frequency, which appears as the signal named probe_<fraprobe_instance_name> in the fra complex raw output file, <circuit>.fra_<fra_instance_name>.raw. The fraprobe result can be plotted on the FRA Bode plot.

The fraprobe is useful for applications such as:

SMPS micromodules with integrated top feedback resistors
Analyzing gain of intermediate portions of a control loop (such as from the compensation point of a SMPS to the output, also known as the modulator gain)
Differential feedback such as current-feedback circuits
Inverting (negative output) SMPS circuits

For circuits with more than one feedback loop, the loops can be analyzed simultaneously by configuring a separate fra device for each independent loop. fraprobe devices may also be used in simulations with multiple fra devices, but each fraprobe must be associated with a specific fra device. If there are multiple fra devices, LTspice will associate fraprobe devices with fra devices based on the instance name. For example, LTspice will associate fraprobe &1 with fra @1, fraprobe &2 will be associated with @2, etc.

See fraprobe examples by choosing File > Open Examples... and navigating to subdirectory Educational\FRA\. Example schematics with the word "probe" in the filename demonstrate use of the fraprobe device.