Waveform LOG, Linear? - Page 1
Forum section: electronics / beginners

author: tony3d
author_rank: Frequent Contributor
timestamp: July 19, 2016, 09:10:08 pm
content: Hi All,Been learning my new Owon AG1022F Waveform Generator! Using it to help with amplifier circuits mostly, and filter tweaking. The only function I don't understand in Linear vs Logarithmic Sweeps. When I switch from linear to log I don't see any change in the sine wave. I think I understand the difference, but how can you see this on the scope? Also, why would I need log over linear? Never had a waveform generator before so this is kinda new.

author: rstofer
author_rank: Super Contributor
timestamp: July 19, 2016, 09:50:49 pm
content: When looking at frequency and phase plots, they are almost always in log scale. Put a filter on the end of you generator and sweep over a few decades. Maybe have a center frequency of 10 kHz and start the sweep from 100 Hz and run up to 1 MHz (5 decades). When you look at the graph, the roll off should be a straight line of xx dB perdecade. If you use a linear scale, the distance between 100 and 1000 Hz, 1 decade, won't be very much. The distance between 100 kHz and 1 MHz, again, 1 decade, will be enormous.

author: tggzzz
author_rank: Super Contributor
timestamp: July 19, 2016, 09:54:58 pm
content: Quote from: tony3d on July 19, 2016, 09:10:08 pmIt is basic electronics w.r.t. frequency response. Look up "bode plots", and understand why linear filters (combinations of R,L,C) look simple and recognisable in the double-log bode plots.

author: MosherIV
author_rank: Super Contributor
timestamp: July 19, 2016, 10:03:01 pm
content: QuoteThe key word is SWEEP. What this does is to change the frequency (rate) of the wave not the waveform.It is easier to describe linear, this changes the freq at a steady rate.Logrithmical changes the freq at a changing rate, the change is very similar to a square law.How can this be seen on a scope?Set the time base on the scope so that you can see 3 sine wave, then do not change the time base.Start the sweep function and you should see the number sine wave on the scope increase on screen.Linear should increase the number at a fixed steady rate.Logrithmical, the number increases at a faster and faster rate.Why is Log useful?Because many of our sense are perceived in a logrithmical maner, eg to hear a double in volumne, the electrial level must be doubled. On a Log scale, this doubling looks like a linear straight line.Try looking at data sheets and the freq response, the freq scale is always log.

author: tony3d
author_rank: Frequent Contributor
timestamp: July 20, 2016, 02:02:35 am
content: Thanks everyone. Great help!

author: Brumby
author_rank: Unknown
timestamp: July 20, 2016, 02:32:37 am
content: Time =0 sec.1 sec.2 sec.3 sec.4 sec.Linear100Hz200Hz300Hz400Hz500Hzieadda figure for each subsequent point - in this case, 100HzLog10Hz100Hz1kHz10KHz100KHziemultiply bya figure for each subsequent point - in this case, 10These are the equivalent of plot points on a graph. The frequency will change smoothly, according to the curve of best fit of these points. Sweeps can go down or up - depends on what your looking for.The logarithmic scale is how we perceive music. Each octave is 2x the frequency of the previous (lower) octave.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 20, 2016, 03:22:20 am
content: Tied in with the perception thing is a practical element.A linear sweep slow enough to distinguish between 20 and 200 Hz (say sweeping that range in a five seconds) would take interminably long to sweep even the entire audio range, roughly 10 hours, even longer for broader ranges. Because most perceptible changes in response (both from our senses and from typical filters) occur over octaves or decades of frequency the required resolution is lower as the frequency increases, and thus the practical sweep rate is higher. This is what the log sweep does, sweeping each octave or decade in frequency in the same time. So in the initial example a sweep from 20 to 200 Hz takes 5 seconds, from 20 to 2000 Hz takes 10 seconds, from 20 to 20000 Hz takes 15 seconds. Much more acceptable.

author: Brumby
author_rank: Unknown
timestamp: July 20, 2016, 06:02:52 am
content: Yes, the practicality issue is most significant.Another example that I was thinking of using before... Let's assume you have set up your sweep generator to cover the range of a piano ... and you are going to 'play along'.Let's start with alogarithmic'sweep'. If you were to play all the keys for one second starting from the bottom and going all the way to the top, it would take you 88 seconds. At the start, the change of frequency is around 1.7Hz per second and it finishes at about 235Hz per second.If you take thelinearapproach at, say, 2Hz per second, you will press the second key in less than a second. When you get to Middle 'C' you will have to wait 8 seconds before pressing the next key and when you get to the last but one, it will be almost 3 minutes before you can hit that last note. All in all, it would take you close to 35 minutes to complete the sweep - and for the last 10 minutes, you'll be wondering more and more if the frequency is actually changing.... (You might need a DFM to convince you it is.)

author: tony3d
author_rank: Frequent Contributor
timestamp: July 22, 2016, 02:13:59 pm
content: Thanks again everyone.