What is the role of a Log Amplifier in a Spectrum Analyser? - Page 1
Forum section: electronics / testgear

author: djsb
author_rank: Super Contributor
timestamp: January 03, 2024, 08:55:17 pm
content: I'm trying to understand what is the role of a Log Amplifier in a Spectrum Analyser? What does a Log Amplifier do exactly? I'd like to understand this as I attempt to fix a number of Log AMP error's on my HP8563E SA. Thanks.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: January 03, 2024, 10:11:12 pm
content: You want the y axis on your spectrum analyzer to be proportional to log(Vrms), not Vrms, because the input signals have huge dynamic range. They might be 0.000000001Vrms or .001Vrms or 1.0Vrms -- you want it all to fit on the same screen, so you must have that logarithm. The newfangled strategy is to sample a block of IF bandwidth with an ADC, take a FFT, and compute y=log(abs(fft(if_signal))). The log amplifier is the old school analog version of this: an analog IF filter plays the role of a RBW bin in the FFT and the circuit after it wants to take the log. This might mean mapping .000000001Vrms to y=0.0V, .00000001Vrms to y=0.1V, .0000001Vrms to y=0.2V, ..., 1.0VRms to y=0.9V. The "abs" typically happens later in this architecture, so technically those "V" should have been "Vrms" and the log-amplified signals get to keep their phase information for a teeny while longer until it gets discarded in the detector immediately afterwards, but we can conceptually group those together for the purpose of finding the y value for the spectrum analyzer graph. To be precise, though, the log amplifier takes a sin wave with huge dynamic range, say .00000001Vrms to 1.0Vrms, and maps that to a sin wave with, say, 0.1Vrms to 1.0Vrms dynamic range.Side note: the reason to log-amplify before taking abs in the analog world is that AC is kinder to small signals than DC. At DC, every weird physics effect and its dog wants to murder your signal. If you convert .000000001Vrms to .000000001V and then try to take the log, calibration will be a nightmare. Ask the volt-nuts how easy .000000001V is to measure. However, if you turn .000000001Vrms to 0.1Vrms to 0.1V, you make the signal non-fragile before you move to DC land, and that's a much better strategy.EDIT: Good point, names on the units were confusing, hopefully a bit less so now.

author: MarkT
author_rank: Frequent Contributor
timestamp: January 04, 2024, 12:26:33 am
content: It converts a voltage signal to a dBV (or dBu or dBm or whatever log-scaled value you have set). A decibel is one tenth of a bel, and a bel is just another way to say log10(x), when in power units, or 2*log10(x) if voltage or current units. So in voltage dB is 20*log10(x). The reference level depends on whether dBV/dBu/dBm/etc. If you set a linear voltage scale for the Y-axis you don't need a log amp, but you won't see anything smaller than about -40dBc on a typical screen (which is why dB scales exist in the first place).

author: slugrustle
author_rank: Frequent Contributor
timestamp: January 04, 2024, 05:29:20 am
content: Now that you have been given good answers, time for a bad one: A Log Amplifier helps you see the forest for the trees, but only after you cut them down.