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Oops schematic mistake on first message. Correction made --> Help. Transistor circuit that connect or disconnect 75ohm terminator for video application

I breadboard separate two transistor circuits to compares the performance of switching in and out a 75 ohm resistor terminator to a video source. One circuit is based on a bi-polar 2N3904 and the other is a FET transistor(it's a DMOS N-channel MOSFET). Using a video generator that is set to sweep 1 to 30MHz and an oscilloscope to monitor the response, I observed that both type of terminators have issues. On the bipolar method, the frequency response is pretty flat to 30MHz when the 2n3904 is truned off, but when it is turned on, the frequency response has gain peaks and loss valleys. This cause the video sync to measure 40IRE (that's good), but the color burst measures 48IRE (too high). I need the frequency response to be flat to

10 MHz. The FET mehtod has the reverse effect. When the FET is on, the frequency response is pretty flat to 30MHz. When the FET is OFF, the frequency response has a number of gain peaks and loss valleys. Also, on both method, the video signal wobbles slowly slightly up and down (on or foo on both methods). I can't seem to get rid of the wobble. Must be due to the stray capacitance of both FET and bipolar. Any help will be appreciated.

Circuit of both termination method:

video source------------------*-------------------Oscilloscope | C

0V or 5VDC-->10k-------->B 2N3904 E | | 75 ohm | | | -5VDC

video source------------------*-------------------Oscilloscope | D

0V or 5VDC-->10k-------->G N-channel DMOS FET RON(MAX)=2ohm S for VGS= 5V | | 75 ohm | | | -5VDC
Reply to
LRCR
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I breadboard separate two transistor circuits to compares the performance of switching in and out a 75 ohm resistor terminator to a video source. One circuit is based on a bi-polar 2N3904 and the other is a FET transistor(it's a DMOS N-channel MOSFET). Using a video generator that is set to sweep 1 to 30MHz and an oscilloscope to monitor the response, I observed that both type of terminators have issues. On the bipolar method, the frequency response is pretty flat to 30MHz when the 2n3904 is truned off, but when it is turned on, the frequency response has gain peaks and loss valleys. This cause the video sync to measure 40IRE (that's good), but the color burst measures 48IRE (too high). I need the frequency response to be flat to

10 MHz. The FET mehtod has the reverse effect. When the FET is on, the frequency response is pretty flat to 30MHz. When the FET is OFF, the frequency response has a number of gain peaks and loss valleys. Also, on both method, the video signal wobbles slowly slightly up and down (on or foo on both methods). I can't seem to get rid of the wobble. Must be due to the stray capacitance of both FET and bipolar. Any help will be appreciated.

Circuit of both termination method:

video source------------------*-------------------Oscilloscope | | 75 ohm | | C

0V or 5VDC-->10k-------->B 2N3904 E | | -5VDC

video source------------------*-------------------Oscilloscope | | 75 ohm | D

0V or 5VDC-->10k-------->G N-channel DMOS FET RON(MAX)=2ohm S for VGS= 5V | | | | -5VDC
Reply to
LRCR 

--
With the base/gate always being at least 5V more positive than the
emitter/drain, how do you manage to turn the devices off?
Reply to
John Fields

Sounds like something's funny with your construction, can you post pictures? Also, why are you terminating to -5V?

Reply to
William P.N. Smith

Thanks John for you feedback,

The base to emitter is always forward bais regardless whether the base is 5v or 0v. True. The picture showing 0v is a mistake, since by turning off the lab power supply I really disconnected the base with no forward bais to the BE section. But the -5v is required to the emitter of the transistor since the video source or generator that I am using is AC coupled and the video generator that I am using is a video standard. The generator unfortunately does not allow DC baising and I want to keep the circuit on the T connector simple. On the bipolar, all I want to really see, how's the frequency response looks lke with the bipolar is on. What I really want to use is the FET design unless there is a strong argument in favor of the bi-polar. The FET is better in my mind for an AC coupled signal. On the FET diagram, the -5 to the source is really on the setup connected to 0v. For now, I am really concerned with the frequency response and the slow vertical motion that I see on the video scope analyzer. The vertical slow motion of the video signal tells me that the 75ohm resistor is not connected to ground effectively through thr FET. Also, on the peaks and valleys of the 1 to 30 MHz frequency response, as was replied by another helpful responder, FET switch/75 ohm resistor design probably make a poor terminator for a video 75 ohm coax line. The FET has all sort of stray capacitance with its small RDS resistance that is probably causing the frequency issues. Other vendor who sell video products with a swithable load to the 75 terminator use a relay. But I spoke to a video expert in the field (MAXIM) and he design a workable solid state switchable load for video line termination. I am assuming he used a FET switch to the 75 ohm resistor, but there is probably more to this simplied design. Any thoughts would be appreciated.

Caesar

On Thu, 02 Feb 2006 06:54:46 -0600, John Fields wrote:

Reply to
LRCR

Thanks William for you feedback,

The base to emitter is always forward bais regardless whether the base is 5v or 0v. True. The picture showing 0v is a mistake, since by turning off the lab power supply I really disconnected the base with no forward bais to the BE section. But the -5v is required to the emitter of the transistor since the video source or generator that I am using is AC coupled and the video generator that I am using is a video standard. The generator unfortunately does not allow DC baising and I want to keep the circuit on the T connector simple. On the bipolar, all I want to really see, how's the frequency response looks lke with the bipolar is on. What I really want to use is the FET design unless there is a strong argument in favor of the bi-polar. The FET is better in my mind for an AC coupled signal. On the FET diagram, the -5 to the source is really on the setup connected to 0v. For now, I am really concerned with the frequency response and the slow vertical motion that I see on the video scope analyzer. The vertical slow motion of the video signal tells me that the 75ohm resistor is not connected to ground effectively through thr FET. Also, on the peaks and valleys of the 1 to 30 MHz frequency response, as was replied by another helpful responder, FET switch/75 ohm resistor design probably make a poor terminator for a video 75 ohm coax line. The FET has all sort of stray capacitance with its small RDS resistance that is probably causing the frequency issues. Other vendor who sell video products with a swithable load to the 75 terminator use a relay. But I spoke to a video expert in the field (MAXIM) and he design a workable solid state switchable load for video line termination. I am assuming he used a FET switch to the 75 ohm resistor, but there is probably more to this simplied design. Any thoughts would be appreciated.

Caesar

On Thu, 02 Feb 2006 08:00:54 -0500, William P.N. Smith wrote:

Reply to
LRCR

I'm not convinced that's true, try disconnecting the power supply output wires or looking at it with a voltmeter.

Reply to
William P.N. Smith

Reply to
LRCR

(view with a fixed-pitch font)

this one may work a bit better

--+--- +5 | 10K source | 2N3906 | 1N914 | | +-->|--+-------C E-------+---- -5 | | | B | \ 150R === 100pF |---[33k]-' \ / | 1K .---[10K]-- control X --+-- 0V | .2N3906 | (0 or 5V) / \ | C ' B / 150R === 100pF B---[10K]--+-C E-- 0V | | | E | `. | | | |--[33K]-. 47K 2N33904 | | | B | | | +--|

Reply to
Jasen Betts

Reply to
LRCR

Reply to
LRCR

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked at it, I like the idea how D1 and D2 are used to terminate through the voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then reverse D1 and D2 to no termination. Very clever. I ran this circuit on the simulator and toggle J1 between 5v and 0v. Unfortunately, I saw no change on the right side of R11. Checking on the circuit I notice that Q5 and Q1 can never turn on regardless if the right side of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets forward bais to turn the transistor. The same is Q5. I can't seem to see any bipolar configuration for Q2 to switch Q5 and Q1 on or off. Can you see if I redrew your schematic correct. I hope to get your schematic to work on Workbench first before I try to bread-board it. Your idea for the diode to switch on or off the termination make sense. I just have to get the logic controls to work proper. Please send me your feedback.

Caesar

Reply to
LRCR

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked at it, I like the idea how D1 and D2 are used to terminate through the voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then reverse D1 and D2 to no termination. Very clever. I ran this circuit on the simulator and toggle J1 between 5v and 0v. Unfortunately, I saw no change on the right side of R11. Checking on the circuit I notice that Q5 and Q1 can never turn on regardless if the right side of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets forward bais to turn the transistor. The same is Q5. I can't seem to see any bipolar configuration for Q2 to switch Q5 and Q1 on or off. Can you see if I redrew your schematic correct. I hope to get your schematic to work on Workbench first before I try to bread-board it. Your idea for the diode to switch on or off the termination make sense. I just have to get the logic controls to work proper. Please send me your feedback.

Caesar

Reply to
LRCR

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked at it, I like the idea how D1 and D2 are used to terminate through the voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then reverse D1 and D2 to no termination. Very clever. I ran this circuit on the simulator and toggle J1 between 5v and 0v. Unfortunately, I saw no change on the right side of R11. Checking on the circuit I notice that Q5 and Q1 can never turn on regardless if the right side of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets forward bais to turn the transistor. The same is Q5. I can't seem to see any bipolar configuration for Q2 to switch Q5 and Q1 on or off. Can you see if I redrew your schematic correct. I hope to get your schematic to work on Workbench first before I try to bread-board it. Your idea for the diode to switch on or off the termination make sense. I just have to get the logic controls to work proper. Please send me your feedback.

Caesar

Reply to
LRCR

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked at it, I like the idea how D1 and D2 are used to terminate through the voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then reverse D1 and D2 to no termination. Very clever. I ran this circuit on the simulator and toggle J1 between 5v and 0v. Unfortunately, I saw no change on the right side of R11. Checking on the circuit I notice that Q5 and Q1 can never turn on regardless if the right side of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets forward bais to turn the transistor. The same is Q5. I can't seem to see any bipolar configuration for Q2 to switch Q5 and Q1 on or off. Can you see if I redrew your schematic correct. I hope to get your schematic to work on Workbench first before I try to bread-board it. Your idea for the diode to switch on or off the termination make sense. I just have to get the logic controls to work proper. Please send me your feedback.

Caesar

Reply to
LRCR

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked at it, I like the idea how D1 and D2 are used to terminate through the voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then reverse D1 and D2 to no termination. Very clever. I ran this circuit on the simulator and toggle J1 between 5v and 0v. Unfortunately, I saw no change on the right side of R11. Checking on the circuit I notice that Q5 and Q1 can never turn on regardless if the right side of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets forward bais to turn the transistor. The same is Q5. I can't seem to see any bipolar configuration for Q2 to switch Q5 and Q1 on or off. Can you see if I redrew your schematic correct. I hope to get your schematic to work on Workbench first before I try to bread-board it. Your idea for the diode to switch on or off the termination make sense. I just have to get the logic controls to work proper. Please send me your feedback.

Caesar

Reply to
LRCR

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked at it, I like the idea how D1 and D2 are used to terminate through the voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then reverse D1 and D2 to no termination. Very clever. I ran this circuit on the simulator and toggle J1 between 5v and 0v. Unfortunately, I saw no change on the right side of R11. Checking on the circuit I notice that Q5 and Q1 can never turn on regardless if the right side of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets forward bais to turn the transistor. The same is Q5. I can't seem to see any bipolar configuration for Q2 to switch Q5 and Q1 on or off. Can you see if I redrew your schematic correct. I hope to get your schematic to work on Workbench first before I try to bread-board it. Your idea for the diode to switch on or off the termination make sense. I just have to get the logic controls to work proper. Please send me your feedback.

Caesar

Reply to
LRCR

yes the FET is better for AC coupled signals

make sure you don't leave the gate OPEN when it is off, it is VERY high Z and will pick up 60 Hz hum, that may be what you are seeing... connect the gate to the source (ground) when you want the FET off..

if the 75 Ohm video cable is not temrinated, thee WILL be peaks and valleys in the frequency respsonse due to the relfections adding constructivley and descructivley,,,thats why we need terminations....

an N ch JFET with the source at ground, the drain to 75 Ohms to the signal, and you control the gate voltage + for on and 0 for off should work well.

Mark

Reply to
Mark

Reply to
LRCR

Set whatever you are using for a news reader to use Courier font.

--
JosephKK
Gegen dummheit kampfen Die Gotter Selbst, vergebens.  
--Shiller
Reply to
Joseph2k

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