Strange high frequency push pull transformer action

As mentioned before I'm working with a design that has a 10MHz push-pull transmitter to drive a 100 ohm cat6 cable.

The cable is terminated with 100 ohms on both ends and the length is ~200 ft. This is a multi-drop implementation much like RS485 but needed custom drivers due to the environment.

The driver is configured as a push pull with 5V on the center tap. The

2n7000 transistors are on each leg of the driver and are driven by high speed logic 5V chips. What I expected to see during transmission on the centertapped "driver" side of the transformer is a waveform like this when measured across Q2.

  • _________

  • | | 2X Vin
  • | |
  • | |
  • | Q1 |
  • | on |
  • | |
  • | |
+----------| |------------| |----------- Vin
  • | |
  • | |
  • | |
  • | Q2 |
  • | on |
  • | |
  • | |
  • | |
  • |--------|
+====================================================

instead it looks like this where the drain voltage of Q2 drops to ~

1/2Vin when Q1 turns on. + + + + + +----------| |------------| |----------- Vin
  • | |
| |
  • | |
| |
  • | Q1 |
| |
  • | on |
| Q2 |
  • |_______| ~1/2 Vin | on |
  • | |
  • | |
  • | |
  • |--------|
+====================================================

This of course reduces the amplitude of the "bus" side signal since it is the difference between Q1 drain and Q2 drain.

Pulses are 30 - 60nS wide.

So instead of the expected +/- 8 - 10V on the bus I am getting +/- ~2

What I've already looked at.

1) the signals look like a current driven push pull push pull power supply. So I removed the transformer and shorted the two drain connections together and measured inductance. (singe nH range. < 1 ohm XL). I measure the CT voltage with a scope and it is rock steady, 10uF ceramic directly from there to ground to insure that.

2) Changed the frequency down the 5MHz to see if it is a reflection of the pulse. Signals look identical just 2x wide was expected. :(

3) checked insertion loss, with an impedance analyzer and it looked good. < 1dB

4) Q1 was removed and replaced with a ohm ranger (semi flyback style). and the lower the resistance got, the higher the amplitude seen on the bus side. until transformer saturation.

There is something going on with that transformer I'm sure of it. It was designed by a 3rd party that has RF experience and they are convinced that it is some kind of RF/transmission line matching issue. I tend to think its more simple than that.

Any thoughts?

Reply to
mook johnson
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It looks as if the transformer connections got mixed up, with 5V applied to one end of the primary and the transistor drains on the center tap and the other end of the primary.

Jeroen Belleman

Reply to
Jeroen

What's the open-circuit primary inductance from either side to CT AND across both primary windings? (3 numbers).

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
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Reply to
Spehro Pefhany

Like Jeroen I believe the most likely cause is that one of the primary windings is flipped. IOW, instead of connecting a dot side with non-dot they may have connected two dot sides or two non-dot sides.

Hang a 100ohms resistive load to the output. If still looking as in you

2nd pic I could almost bet it's indeed a half primary flip. As Forrest Gump said, ..it happens :-)
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Its a 1:1 transformer with a CT on the driver side.

Bus side with primary open: 200uH CT to leg with bus side open: 50uH (either leg)

CT to both legs tied together with bus side open: ~ 5nH

Nothing obviously wrong. Any fancy RF/transmission line stuff could be taking place here?

Reply to
mook johnson

Ahh that was my first though as well. Forgot the mention I checked for that.

I guess i should have added more waveform.

If I probe Q1 drain instead of Q2, I get the same result. where Q1 goes from Vin to ground and goes from Vin to ~ 1/2Vin when Q2 turns on.

Could it be some strange kind of saturation? I don't think so since it the waveforms immediately look that way. Typically when i have saturated a core the first part of the waveform looks right then when the inductance falls the waveform drops off at the end. This one doesn't do that unless it is at the very start and I can see it.

The wave shape looks pretty good with sharp edges and flat tops so it doesn't look too bad but your never know.

Reply to
mook johnson

Schematic of your source termination?

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
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Reply to
Spehro Pefhany

How did you check? Did you swap connections to one of the primaries and get the same waveform?

Did you measure inductances with the drains shorted, or the drain connections on the transformer shorted? If the drain connections on the transformer are connected together I would expect to only see negligible leakage inductance!

The very fact that the waveforms just look expanded at 5MHz suggests there are no saturation or other artefacts.

My money is still on the centre tap isn't really the centre!! Measure the inductance of each primary, then across the drain connections and come back here with the result. The inductance across the drain connections should be 4 x each winding.

--
Mike Perkins 
Video Solutions Ltd 
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Reply to
Mike Perkins

Have you verified that your gate drives are correct, with NO overlap? Though that might not guarantee that you aren't getting into saturation anyway.

Measure the current into the center-tap. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
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Reply to
Jim Thompson

Yup, I'd double, triple and quadruple-check that. One method is to ground the center tap, feed a square wave from a function generator into the secondary and see if opposite polarities come out the primaries.

If it was saturation the current intake would be high and the 2N7002 FETs would attempt to unsolder themselves.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

The amazing thing is that they sometimes succeed, and still survive.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

Mmmh. There are several things that do not fit my first thought:

One is that if indeed one of the FETs connects to the CT instead of either end, the remaining end goes below GND, which will forward-bias the other FET's body diode. He wouldn't have missed that, I presume.

Second, his inductance measurements appear to make good sense for a correctly connected transformer. (50uH from CT to either end and 200uH on the secondary with primary open. 5nH with both ends of the primary shorted together. Although that would imply a coupling factor of .9999, which is suspiciously good.)

Third, in the first waveform diagram, one cannot possibly have pulse widths of 60ns at a repetition rate of 10MHz. 30ns would be OK though.

Maybe it's one of those cases where what you think you see is not actually what is there.

You mentioned your twisted pair cable is doubly terminated. Do you actually see about 50 Ohms between the wires?

Jeroen Belleman

Reply to
Jeroen Belleman

You are old enough to remember the 2N3055, some of them had their TO3 cans adopt a blue tint from gross overheating, like the exhaust pipes of a well-seasoned Harley-Davidson. Yet many of those were still good for more rock sessions.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

That can also be the case if one of the primary halves is flipped.

That doesn't sound possible. If it is true then we have a viable Nobel prize candidate in our midst :-)

That might have just been a notation error. The plot he described looks suspiciously like flipped transformer connection. I've had that happen on the bench when I whipped up a transformer by hand a bit too fast, or the phone rang while wiring it up. So now I alwats keep a Sharpie in the bench drawer for marking, same as the Edding pens in Europe.

[...]
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Snip!

It's not impossible. I have this 1:4 impedance ratio transformer with a 6-decade bandwidth, which corresponds to a coupling factor of some 1e-6 below unity. Granted, it's a Guanella transmission line affair, using ferrite and Vitrovac loaded UT47 coax; not a wire-wound thing like I suppose we have here.

Jeroen Belleman

Reply to
Jeroen

I still have a few in the drawer, but haven't used one (even for a one-off) since about 1988. A lot of those rock sessions sound like a Sportster with a megaphone pipe--loud, repetitive, and completely unnecessary. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

ll

The

h

ssion on

 Vin
     |
     |
     |
  |
  |
     |
     |
     |

-|

========================= ====

 Vin
   |
   |
   |
|
     |
     |
     |

-|

========================= ====

~2

ohm

 10uF
t
I

.-------------> | .---> | | '-'-'-'-'-' ============= (A) .-.-.-.-.-.-. (B) | | | ||--' +5v '--|| |||| Q2 _||--+ Q1 +--||__ | | === ===

Not possible. If the transformer's right, the drain of the "off" FET has to rise when the other turns on, there's simply no other choice.

If the "off" FET's drain didn't rise to 10v, or spiked then decayed, etc.--that would be different. For Vds not to rise at all, but fall instead--that's not possible.

The transformer's wrong. Has to be.

--
Cheers, 
James Arthur
Reply to
dagmargoodboat

Mook seems to have gone very quiet. It would be good to hear the outcome even if he has to swallow a little humble pie.

--
Mike Perkins 
Video Solutions Ltd 
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Reply to
Mike Perkins

y

ot

ou

t

for

s

It's pretty safe to assume Mook's off trying to untangle those darn primary connections. Happens to the best of us.

--
Cheers, 
James Arthur
Reply to
dagmargoodboat

yeah well, that would help out in the replacement process :)

Jamie

Reply to
Jamie

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