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Old 8th October 2012, 07:03 PM   #1
akis is offline akis  United Kingdom
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Default PCB layout problems

Hello

I have been working on the following design for some time now, and after various modifications over time I have settled on the attached schematic. I have built this circuit on three different breadboards from different manufacturers, and with mixing and matching components. There are a few nuances, but the circuit works just fine.

I have subsequently designed a PCB and have tried to assemble it. There have been a few problems, and would like your help and suggestions.

As you can see there is a Wien bridge oscillator at 200KHz and that feeds into a current amplifier in order to drive a load of 8-25R.

My PCB does not have great spans of copper plains, however there are individual ground tracks all leading to a central point. The track widths are adequate for the currents involved.

The first problem is the instability of the Wien bridge oscillator. While it is true that the Wien bridge in general will either clip to the rails or not oscillate at all, the topology I have presented works quite reliably on all three types of breadboard I have tried. However, on the PCB it is almost impossible to stabilise it. As soon as it gets connected to the current amplification stage it produces myriads of much faster oscillations.

This instability has been corrected with the introduction of small feedback capacitor C11 and more importantly with capacitors C3 and C8. On the breadboards I have only used C3 and C8 at 100pF. On the PCB I had to go up to 600pF to tame the unwanted oscillations.

Another problem I have observed is that the introduction of an aluminium sheet under the breadboard, or under the whole of the PCB, with a wire connecting it to the circuit's ground seems to stabilise the oscillator, and the current amplification stage under load.

I have no experience in designing PCB at 200KHz but I am sure normal audio practices would also apply. The beneficial effects of the aluminium sheet under the PCB indicate to me that perhaps I need a lot of ground planes on the PCB, but I am also considering simply mounting the PCB over an aluminium board, since the whole assembly will be inside a plastic box.

I would appreciate any comments and suggestions on the circuit but most importantly the PCB design. I am also going to attach a photo of the current PCB so you can see the layout of the components so far.

Edit: I have attached a picture of the PCB. I have only assembled the oscillator and current amplification stage so far. There are other parts of the circuit not shown on the schematic.
Attached Images
File Type: jpg ttf-100-simple.JPG (377.4 KB, 225 views)
File Type: jpg IMG_20121008_190724s.jpg (780.7 KB, 209 views)

Last edited by akis; 8th October 2012 at 07:15 PM.
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Old 8th October 2012, 07:14 PM   #2
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A picture of the pcb layout, not just a photo, is crucial to enable good suggestions.
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Old 8th October 2012, 07:15 PM   #3
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I don't understand why you have complicated with two capacitors (C4 and C6) because I think one capacitor was enough. But fine, let's get over it ...
It's very important here to treat PCBs way, the interaction between your circuit and power supply. Have you thought to conceive and power supply and if so, how? The answer to this question is very important because it can influence, more or less, the design of the PCB.
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Old 8th October 2012, 07:25 PM   #4
akis is offline akis  United Kingdom
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Quote:
Originally Posted by cliffforrest View Post
A picture of the pcb layout, not just a photo, is crucial to enable good suggestions.
It is a two sided PCB, how can I present it here ?
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Old 8th October 2012, 07:28 PM   #5
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post the gerber files.

Most pcb layout programs allow display / print / export of the copper sides.
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Old 8th October 2012, 07:29 PM   #6
akis is offline akis  United Kingdom
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Quote:
Originally Posted by donpetru View Post
I don't understand why you have complicated with two capacitors (C4 and C6) because I think one capacitor was enough. But fine, let's get over it ...
It's very important here to treat PCBs way, the interaction between your circuit and power supply. Have you thought to conceive and power supply and if so, how? The answer to this question is very important because it can influence, more or less, the design of the PCB.
The second capacitor, on the breadboard has a very small beneficial effect on the amplitude of the wave on full load. On the PCB however it is another story: the second cap seemed to cause millions of harsh oscillations that were only stopped with the addition of C3 and C11 at 600pF (on the breadboard it was 100pF each).

The power supply will be two 12V batteries, but for the time being I am using my bench PSU.
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Old 8th October 2012, 07:41 PM   #7
akis is offline akis  United Kingdom
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Quote:
Originally Posted by cliffforrest View Post
post the gerber files.

Most pcb layout programs allow display / print / export of the copper sides.
Ok here are the gerber files the same ones I sent to the PCB manufacturer. I also attach the full schematic because the one I posted above is not the full thing.
Attached Files
File Type: zip MainBoard.zip (53.1 KB, 24 views)
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Old 8th October 2012, 08:04 PM   #8
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Come on!

I cannot display a .sch file. What is that?

You post the wrong schematic with no details of decoupling caps and expect comments on stability?

Please do the work and present the files as .jpg like everyone else!

I can already see from your photo that many components are not fitted to the pcb. How is one to comment intelligently on what is going on?

I'm out.
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Old 8th October 2012, 09:31 PM   #9
akis is offline akis  United Kingdom
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Quote:
Originally Posted by cliffforrest View Post
Come on!

I cannot display a .sch file. What is that?

You post the wrong schematic with no details of decoupling caps and expect comments on stability?

Please do the work and present the files as .jpg like everyone else!

I can already see from your photo that many components are not fitted to the pcb. How is one to comment intelligently on what is going on?

I'm out.
The .sch is a P-CAD ASCII file. I thought it was some kind of standard, but obviously not. I am trying to produce jpeg files of the schematic but it is not easy.

I trust you can view the Gerber files OK?
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Old 8th October 2012, 11:51 PM   #10
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You probably have a couple of problems.
That can lead to confusion if they interact.
1. It is essential to decouple the power supply to the WB Oscillator section.
Had practical experience with that.
2. The CFP output that you use is notorious for instability.
If you are not already aware of the issues then there are multiple threads to search. I have just started to study this because fixes seem to be more trial and error than I trust.
Avoid the problem with EF unless you really need CFP.
If so then you can try base stoppers on the outputs, the drivers, between the output collectors and the driver emitters.

Best wishes
David


BTW. Hard to keep everyone happy but many people don't like JPGs either and prefer a lossless format.

Last edited by Dave Zan; 9th October 2012 at 12:02 AM. Reason: JPG
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