Improving settling time on signal switch - Page 1
Forum section: electronics / projects

author: soerenbnoergaard
author_rank: Contributor
timestamp: May 17, 2015, 06:17:58 pm
content: Hello everybodyAfter having followed the EEVblog for so long, this is my first post in the forumI have a problem with a circuit I am building. The purpose of the circuit is to "chop" the output of my signal generator so I can create bursts of sine waves. I do this by using a diode as a switch and an oscillator based on a comparator to turn the bias of the diode on and off (see the attached schematic).As seen on the scope-shots below, I have some settling time when I flick the switch and I would like to minimize this. I found it helps to lower the 2.2 nF output capacitor, but doing so shifts the lower cutoff so low frequency signals get attenuated.I would like the following:3 dB lower cutoff at 20 Hz (for audio purposes)Settling time < 1 ms3V coin cell battery operationOn the scope-shots:Yellow = Input signalPink = Output signalBlue = Open-collector output of the right-most comparator = anode of the diodeAny suggestions? Thanks!

author: Marco
author_rank: Super Contributor
timestamp: May 17, 2015, 06:23:07 pm
content: Use a second switch diode (and another bias resistor) so you don't pull down the output.Or just use an analog multiplexerPS. this will only be able to supply a signal to a very high impedance DUT.

author: soerenbnoergaard
author_rank: Contributor
timestamp: May 17, 2015, 06:44:40 pm
content: Quote from: Marco on May 17, 2015, 06:23:07 pmWhere should the diode be?Yea, the output should probably be buffered...

author: Marco
author_rank: Super Contributor
timestamp: May 17, 2015, 06:54:03 pm
content: To the right of the other diode, anode to anode, with a bias resistor from cathode to ground. Same idea as the other diode switch, you just use two switches.A 74HC/CD4051 has 10 uA range supply current BTW.

author: soerenbnoergaard
author_rank: Contributor
timestamp: May 17, 2015, 07:09:36 pm
content: Quote from: Marco on May 17, 2015, 06:54:03 pmThis doesn't seem to improve the settling time but it clips the signal.I'll have a look at that chip -- thanks!I tried different values for the bias resistor with no luck in changing the overshoot.

author: onlooker
author_rank: Frequent Contributor
timestamp: May 17, 2015, 07:53:53 pm
content: The settling time is determined by the RC constant, 1uF*(100kR||100kR) = 50ms. To stay with the original schematics, you may try to reduce this RC constant. For example use 0.2uF and 10kR. Then buffer/amplify the output if needed.

author: soerenbnoergaard
author_rank: Contributor
timestamp: May 17, 2015, 08:12:00 pm
content: Cool, I'll try that tomorrow. Thanks!

author: onlooker
author_rank: Frequent Contributor
timestamp: May 17, 2015, 08:33:34 pm
content: On a 2nd look, simply reducing RC constant will damp your signal too much since your sine signal is also showing a period of 1ms (1kHz). It is not separate enough from your 1 ms settling time requirement.You may just get rid of the 1uF cap all together (and replace it with a small resistor). It serves no purpose from what I see(?).Edit: Unless the level is already shifted.

author: soerenbnoergaard
author_rank: Contributor
timestamp: May 18, 2015, 06:22:56 am
content: I put the input cap there so the dc shifting would not go back into the signal generators output stage (push pullhttps://www.google.dk/url?sa=t&source=web&rct=j&ei=4oJZVZCDNIynsAHT1oH4CQ&url=http://www.pobot.org/IMG/pdf/philips_pm51a6ce.pdf&ved=0CBsQFjAA&usg=AFQjCNGz-TlIhi1nzpH_i8dXDW424xgcNQ&sig2=jhweKCGmEPu8RW48Cvx4JA)I actually don't know if it is necessary?

author: soerenbnoergaard
author_rank: Contributor
timestamp: May 31, 2015, 12:18:31 pm
content: I decided to try out a new design. Apparently, a JFET works quite well as a switch and no DC bias is required so no coupling capsNow I just have to build it up.I included some scope-shots and the new schematic, if someone would find it useful. Thanks for the help, everyone.// Søren

author: dom0
author_rank: Super Contributor
timestamp: May 31, 2015, 01:20:09 pm
content: You could also use a classic diode bridge. Built with 1N4148 and some standard transistors to control the current sources it should work well into the MHz region (those things get _really_ fast with the right diodes).

author: soerenbnoergaard
author_rank: Contributor
timestamp: May 31, 2015, 01:22:10 pm
content: Quote from: dom0 on May 31, 2015, 01:20:09 pmThanks for the suggestion -- I will consider that if I would need to use the circuit above audio frequencies.

author: soerenbnoergaard
author_rank: Contributor
timestamp: June 04, 2015, 05:32:11 pm
content: Here are some pictures of the build -- should have been a bit more generous with the copper, but it works -- as long as I don't have to turn the potThank you all for suggestions! I think I might need the 4051 for another project