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Strange high frequency push pull transformer action

Started by mook johnson November 4, 2012
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?













On 2012-11-04 18:45, mook johnson wrote:
> 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. [...] > 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 | > + | | > + | | > + | | > + |--------| > +====================================================
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
On Sun, 04 Nov 2012 11:45:43 -0600, the renowned mook johnson
<mook@mook.net> wrote:

>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? >
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 Embedded software/hardware/analog Info for designers: http://www.speff.com
mook johnson wrote:
> 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? >
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/
On 11/4/2012 4:21 PM, Spehro Pefhany wrote:
> On Sun, 04 Nov 2012 11:45:43 -0600, the renowned mook johnson > <mook@mook.net> wrote: > >> 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? >> > > What's the open-circuit primary inductance from either side to CT AND > across both primary windings? (3 numbers). > > > > Best regards, > Spehro Pefhany
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?
On 11/4/2012 4:23 PM, Joerg wrote:
> mook johnson wrote: >> 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? >> > > 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 :-) >
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.
On Mon, 05 Nov 2012 06:02:05 -0600, the renowned mook johnson
<mook@mook.net> wrote:

> > >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?
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 Embedded software/hardware/analog Info for designers: http://www.speff.com
On 05/11/2012 12:08, mook johnson wrote:
> On 11/4/2012 4:23 PM, Joerg wrote: >> mook johnson wrote: >>> 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? >>> >> >> 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 :-) >> > > Ahh that was my first though as well. Forgot the mention I checked for > that.
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!
> 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 very fact that the waveforms just look expanded at 5MHz suggests there are no saturation or other artefacts.
> The wave shape looks pretty good with sharp edges and flat tops so it > doesn't look too bad but your never know. >
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 www.videosolutions.ltd.uk
On Mon, 05 Nov 2012 06:02:05 -0600, mook johnson <mook@mook.net>
wrote:

>On 11/4/2012 4:21 PM, Spehro Pefhany wrote: >> On Sun, 04 Nov 2012 11:45:43 -0600, the renowned mook johnson >> <mook@mook.net> wrote: >> >>> 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? >>> >> >> What's the open-circuit primary inductance from either side to CT AND >> across both primary windings? (3 numbers). >> >> >> >> Best regards, >> Spehro Pefhany > > >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? > >
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 | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Mike Perkins wrote:
> On 05/11/2012 12:08, mook johnson wrote: >> On 11/4/2012 4:23 PM, Joerg wrote: >>> mook johnson wrote: >>>> 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? >>>> >>> >>> 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 :-) >>> >> >> Ahh that was my first though as well. Forgot the mention I checked for >> that. > > 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! >
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.
>> 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 very fact that the waveforms just look expanded at 5MHz suggests > there are no saturation or other artefacts. > >> The wave shape looks pretty good with sharp edges and flat tops so it >> doesn't look too bad but your never know. >> > > 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. >
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/