[SOLVED] How to understand and remove ringing in the board

[ankitvirdi4]

Hello All,

I have a display board which is basically to display messages controlled through a microcontroller.

Total of 12, 7 segment digits common cathode.**broken link removed**

Problem Statement: The board starts ringing when Seg A and CC4 are on. There is noise on the entire board, 6V rail has a peak to peak noise of 4V; 3V3 has a noise of 1.2 Vp-p; even on the same ground point this noise is seen, say digital ground track between uC and 74HC595.

The frequency of the noise is 70-80 MHz and it goes away as soon as any of SegA or CC4 are switched off completely.

Thus, This noise repeats every 8mSecs and stays for 1ms same as the CC4.

How do I analyse and identity the source of this oscillation? Is it track inductance resonating with some capacitor or with stray capacitance? Or feedback due to capacitive coupling? or impedence mismatch causing reflections?

what should be the pointers to attack on inorder to get rid of the ringing?

Thanks!

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[FvM]

Sounds like you have build a nice oscillator with the segment or digit driver transistors. Long PCB traces are a prerequisite to make it oscillating. There's a problem in your circuit, not directly related to the "ringing" problem which should be fixed first. The npn transistor segment drivers are only partly turned on by the 3.3V base voltage. Should be e.g. changed to pnp transistor drivers.

Saturated driver transistors are less likely to oscillate, if it's still happening, you can try ferrite beads in the collector lines or small (e.g. several 100 pF) blocking capacitors against ground.

 

[Easyrider83]

This also will never work ))))
Incrorect transistors are used. This is funny day for me :laugh:
Going around the topics and looking for guys, who don't understand ohm's law)))

 

[ankitvirdi4]

This also will never work ))))
Incrorect transistors are used. This is funny day for me :laugh:
Going around the topics and looking for guys, who don't understand ohm's law)))

Thank you for your comment

The circuit may be wrong but it is somehow working. I mean although there is noise which is disturbing the analog signals from the sensor. The display is shows exactly what it should as per software.

Although, as y'all say this circuit is wrong I would want to understand why? so I never repeat the same mistake again and get a better understanding of electronics.

 

[Easyrider83]

Show your real schematics and pcb layout. Somehow it should not work.

 

[ankitvirdi4]

The npn transistor segment drivers are only partly turned on by the 3.3V base voltage.

Saturated driver transistors are less likely to oscillate, if it's still happening, you can try ferrite beads in the collector lines or small (e.g. several 100 pF) blocking capacitors against ground.

Okay, so the BC847B are incorrect and are not being saturated but operating in the active region or am I misunderstanding? Why can't NPN be used I am sorry I did not understand that part. Or the BC817 should be changed to PNP transistors?

But now it is clear that the transistor are causing oscillations. Thank you.

I see waveforms on the emitter of the BC847 that I cannot understand, I'll try to post the image in a while.

 

[KlausST]

Hi,

I also think the high side drivers (in combination with layout and other schematic problems) will cause the problem.

Besides the high side drivers i'd like to see GND plane of the pcb. And the power supply electrolytic bulk capacitors and fast ceramic capacitors at each IC.

Klaus

 

[FvM]

This also will never work ))))
Incrorect transistors are used. This is funny day for me
Going around the topics and looking for guys, who don't understand ohm's law)))

Sounds rather like you have a really bad day. Sorry for that.

The original circuit ís in fact expected to work, but with very dim display. See below the idea of a PNP segment driver replacing the BC847 transistor. Segment control signals to be inverted.

 

[ankitvirdi4]

Hi,

I also think the high side drivers (in combination with layout and other schematic problems) will cause the problem.

Besides the high side drivers i'd like to see GND plane of the pcb. And the power supply electrolytic bulk capacitors and fast ceramic capacitors at each IC.

Klaus

Hello Klaus,

https://www.edaboard.com/threads/347781/

This thread has the powersupply. Also, one more observation is if the 6V rail voltage is increased as it is linear power supply the amplitud eof the oscillations also increase proportionally.

How do the post the layout? which format?

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Sounds rather like you have a really bad day. Sorry for that.

The original circuit ís in fact expected to work, but with very dim display. See below the idea of a PNP segment driver replacing the BC847 transistor. Segment control signals to be inverted.

View attachment 124468

Thank you, I will immediately start implementing this and post results as soon as I have them.

I still do not understand why not to use BC 847 as you said. I would be very grateful if you shed some light on it. It will help understand better. Meanwhile I'll go buy a good book today (Please suggest one if you feel like) so I read more and improve my basic fundamentals otherwise I will turn out to be a very bad analog designer when I grow up.

 

[FvM]

I still do not understand why not to use BC 847

Why not calculate the circuit voltages? BC847 base voltage is 3.3V, so emitter voltage won't be higher than 2.7 V. This can work for red displays with low forward voltage, but small forward voltage changes result in large current variation. It's better to use a saturated driver and a higher series resistance value.

 

[KlausST]

Hi,

I don't want to see a power supply circuit.
I want to see yout display pcb (layout). And the Gnd plane on this pcb and the capacitors on this pcb.

Regarding NPN PNP...
Your circuit run the NPN in linear mode. The output voltage is lower than the base voltage. So the 6V are just useless.
Ant the linear mode (instead of saturation mode) makes the circuit to oscillate.

Klaus

 

[ankitvirdi4]

Why not calculate the circuit voltages? BC847 base voltage is 3.3V, so emitter voltage won't be higher than 2.7 V.This can work for red displays with low forward voltage, but small forward voltage changes result in large current variation. It's better to use a saturated driver and a higher series resistance value.

I checked the datasheet of the display module (Red)
Forward voltage per segment is typ 2V and max 2.6V

I had a word with the design engineer(I am an intern here) who's design was the reference for this design as his is similar only smaller. He said that BC847 was used in emitter follower configuration to achieve regulation ,the forward current is limited by 47Ohms in my case. It was my misconception that display drivers are being opeated as a switch but they are not. Regulation was necessary because 6V is unregulated output of a linear supply because if regulated, power dissipation will be a problem as there is not enough space for a heatsink in the housing.


He told me find out which particular transistor is going into oscillation and use small capacitors there also clear out the layout for that part.

 

[KlausST]

Hi,

If there is no switching power supply, then power dissipation is independent of any regulation mode.

Klaus

 

[FvM]

Forward voltage per segment is typ 2V and max 2.6V

Means that you get about zero display current with maximum forward voltage. As said, it can work, but display brightness isn't well controlled.

I generally don't get the idea behind emitter follower configuration in this case. Specifically it's bringing up the oscillation problem. But anyhow, you have all necessary information about the circuit, so proceed as you like.