Why is this ocilating? - Page 1
Forum section: electronics / beginners

author: lincoln
author_rank: Regular Contributor
timestamp: December 23, 2015, 10:03:42 pm
content: Hello,I got this circuit of of the time nuts list archive, I am trying to square up a 10~20 MHz input and drive digital logic with it. The transistors I am using are MMBT3906... The problem I am having is the output is oscillating as 65KHz, regardless of whether or not I am driving the input or not. My power supply is solid, I have bridged the bead and that had no effect. waving with my finger or grounded screwdriver did not change the output so I don't think is a coupling issue...What to you think?

author: dom0
author_rank: Super Contributor
timestamp: December 23, 2015, 10:48:39 pm
content: That a CMOS inverter is better at this.

author: lincoln
author_rank: Regular Contributor
timestamp: December 23, 2015, 10:54:02 pm
content: Quote from: dom0 on December 23, 2015, 10:48:39 pmA fair point, and will use less board area but are also terrible with phase noise. Well the one I built has plenty of noise as well....

author: KJDS
author_rank: Super Contributor
timestamp: December 23, 2015, 11:14:28 pm
content: Do you have a picture of the layout?

author: lincoln
author_rank: Regular Contributor
timestamp: December 24, 2015, 12:01:41 am
content: Quote from: KJDS on December 23, 2015, 11:14:28 pmThis is after working at it a bit, you will see adding extra capacitors in the signal path and the dc blocking cap on the termination has a bit of wire shorting it. Non of that had an effect.edit > Image here>the forum didn't like my picthank you

author: dmills
author_rank: Super Contributor
timestamp: December 24, 2015, 12:26:50 am
content: Is it squegging?Things to try:Split R327 (or add a 1K resistor to the top end) and connect 100nF or so to ground from the junction.Place a large value cap across the power rail with a deliberate few ohms of series resistance (Dampens any supply network resonances which can be a thing in the low ultrasonic region, low ESR is NOT always best).One nice gotcha with PNP stages like this is that the reference for the amplifier transistors is the positive supply, while the external RF is referenced to ground, if you are not careful you can end up with input and output currents sharing the same path between the two references. A cap close to the the input connector between the ground and the stage power supply is a good idea here (Same thing near the output).What is the load on the common base stage?Is there a ground plane under that board?Regards, Dan.

author: lincoln
author_rank: Regular Contributor
timestamp: December 24, 2015, 01:46:52 am
content: Quote from: dmills on December 24, 2015, 12:26:50 amI'm sorry, What? <reads Wikipedia> Umm, I don't think so, frequency and amplitude remain stable when scoping the output.Quote from: dmills on December 24, 2015, 12:26:50 amOn its own , didn't change anything.Quote from: dmills on December 24, 2015, 12:26:50 amOk, I looked at a 22u with a 27 ohm and then a 14, didn't really change anything.Quote from: dmills on December 24, 2015, 12:26:50 amHow close is close?Quote from: dmills on December 24, 2015, 12:26:50 amOriginally high impedance, directly tied to a FPGA clock input, also tried 300 ohm with a dc blocking cap. The 300 ohm just 1/2 the amplitude,Quote from: dmills on December 24, 2015, 12:26:50 amyes.

author: Marco
author_rank: Super Contributor
timestamp: December 24, 2015, 05:32:48 am
content: What does it look like on the scope with/without input?What a strange circuit BTW.

author: DuPe
author_rank: Regular Contributor
timestamp: December 24, 2015, 08:30:01 am
content: Quote from: lincoln on December 23, 2015, 10:03:42 pmsure it is 65kHz? Might it be mixing product of a much higher frequncy that you do not see doe to BW limits of your scope?Sometimes emitter followers oszillate in UHF range. Base resistor ~100ohm will help

author: dmills
author_rank: Super Contributor
timestamp: December 24, 2015, 12:57:20 pm
content: That's a good thought, and digital scopes of the cheap sort are notorious for not having anti aliasing filters that actually work.Regards, Dan.

author: lincoln
author_rank: Regular Contributor
timestamp: December 24, 2015, 08:35:24 pm
content: I don't think its aliasing, but that would make more sense, This is with no input, output terminated with 300 ohm load, 20 MHz Bandwidth limit.This is with a 10MHz sign wave, 1v pk-pk 50 ohm terminated input. with out the shmutz on top, It pretty well matches predictions that ltspice gave me.Now if i zoom out a little and take a single shot capture:And zoom out a little more...

author: lincoln
author_rank: Regular Contributor
timestamp: December 24, 2015, 10:25:53 pm
content: Well it stopped oscillating, well, or rather it never was it was amplifying some noise on the power plane. When I having problems with this i check out the power planes and didn't see anything apart form some low common mode junk.Opened the power jumper and went on working with the rest of the board. scoping a different power rail looks like crap, looks like switching noise but there are no switching regulators on this board. The lab power supply is a switcher but not nearly this bad. I should have grabbed a screen shot but i was in the moment.Shut off all the consumer electronics near the bench, overhead lights, usb hubs, all the test gear I could. Now the output is clean.Slowing turn things back on one by one, and noise doesn't return. Everything is back on and booted and the dut is still well behaved. Wut?