Originally posted May 8, 2016
With a benefit or two that should not remain invisible
Though we don’t always think of them in quite this way, signal measurements such as low-level spurious involve the collection of a great deal of information, and thus can be frustratingly slow. I’ve described how the laws of physics sometimes help us, but this bit of good fortune confers only a modest benefit.
Some years ago, the advent of digital RBW filters in signal analyzers brought gains in speed and performance. The improved shape factor and consistent bandwidth yielded better accuracy, and the predictable dynamic response allowed sweep speeds to be increased by a factor of two to four. The effects of a faster sweep were correctable in real time as long as the speed wasn’t increased too much.
The idea of correcting for even faster sweep speeds was promising, and the benefits have gotten more attractive as spurious, harmonics and other performance specifications get ever tighter. To meet these requirements, the principal technique for reducing noise level in a spectrum or signal analyzer is to reduce RBW, with noise floor dropping 10 dB for each 10x reduction in RBW.
Unfortunately, sweep time lengthens with the square of the RBW reduction. A 100x increase in measurement time for a 10 dB improvement in signal-to-noise is a painful tradeoff.
As has occurred in the past, clever algorithms and faster DSP have combined to improve measurements and relieve the tedium for the RF engineer:
These two measurements cover the same frequency span with the same resolution bandwidth. Option FS1 in the Keysight X-Series signal analyzers (bottom) improves measurement rate by about 50 times.
Fast ASIC processing in the signal analyzer corrects for the frequency, amplitude and bandwidth effects of sweeping the RBW filters at speeds up to about 50 times faster than the traditional minimal-error speed. This improvement applies to swept—not FFT—measurements and is most beneficial when RBW is approximately 10 kHz or greater.
While the speed benefits are obvious, another may be nearly invisible: narrower RBWs also [update: see note below] improve repeatability.
This graph compares the repeatability (vertical axis) of fast sweep and traditional sweep. The lower level and shallower slope of the blue line show both improved repeatability and less dependence on sweep time.
The magnitude of the speed improvement depends on measurement specifics and analyzer configuration, but they’re achieved automatically and with no tradeoff in specifications. If slow measurements are increasing your ambient level of tedium, find more information about this technique in our fast sweep application note.
Note: Improved measurement speed and repeatability are alternative benefits in this case, contrary to the implication of my original wording. You can use the same measurement time and get improved repeatability, or you can improve measurement time without improving repeatability. I apologize for the confusion.