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#1
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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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 | | 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 |
#2
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application
On Wed, 01 Feb 2006 23:33:26 -0500, LRCR wrote:
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 --- With the base/gate always being at least 5V more positive than the emitter/drain, how do you manage to turn the devices off? -- John Fields Professional Circuit Designer |
#3
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application
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: On Wed, 01 Feb 2006 23:33:26 -0500, LRCR wrote: 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 --- With the base/gate always being at least 5V more positive than the emitter/drain, how do you manage to turn the devices off? |
#4
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application
On 2006-02-02, LRCR wrote:
video source---------------*-------------------Oscilloscope | | 75 ohm | | C 0V or 5VDC--10k--------B 2N3904 E | | -5VDC (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 | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3904 10K | --+--- -5 TTL-swittched broadband terminator concept. Bye. Jasen |
#5
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application
Jasen,
Thank you for your response. I am having trouble reading your ascii schematic. Also "Fixed font" I don't understand. I am excited to understand your circuit. You have to excuse me since I am new in reading ascii schematic. As you can see my ascii schematic is not very good. Is there any way to send an attachment with a picture, pdf or help me understand your ascii schematic? Luis On Fri, 03 Feb 2006 07:00:30 -0000, Jasen Betts wrote: On 2006-02-02, LRCR wrote: video source---------------*-------------------Oscilloscope | | 75 ohm | | C 0V or 5VDC--10k--------B 2N3904 E | | -5VDC (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 | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3904 10K | --+--- -5 TTL-swittched broadband terminator concept. Bye. Jasen |
#6
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application
Disregard previous message. The key is as you said fixed pitched such
as new courier. Everything lines up. Thanks. Caesar On Fri, 03 Feb 2006 08:07:43 -0500, LRCR wrote: Jasen, Thank you for your response. I am having trouble reading your ascii schematic. Also "Fixed font" I don't understand. I am excited to understand your circuit. You have to excuse me since I am new in reading ascii schematic. As you can see my ascii schematic is not very good. Is there any way to send an attachment with a picture, pdf or help me understand your ascii schematic? Luis On Fri, 03 Feb 2006 07:00:30 -0000, Jasen Betts wrote: On 2006-02-02, LRCR wrote: video source---------------*-------------------Oscilloscope | | 75 ohm | | C 0V or 5VDC--10k--------B 2N3904 E | | -5VDC (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 | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3904 10K | --+--- -5 TTL-swittched broadband terminator concept. Bye. Jasen |
#7
Posted to sci.electronics.design,sci.electronics,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
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 |
#8
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
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 On Fri, 03 Feb 2006 08:25:46 -0500, LRCR wrote: Disregard previous message. The key is as you said fixed pitched such as new courier. Everything lines up. Thanks. Caesar On Fri, 03 Feb 2006 08:07:43 -0500, LRCR wrote: Jasen, Thank you for your response. I am having trouble reading your ascii schematic. Also "Fixed font" I don't understand. I am excited to understand your circuit. You have to excuse me since I am new in reading ascii schematic. As you can see my ascii schematic is not very good. Is there any way to send an attachment with a picture, pdf or help me understand your ascii schematic? Luis On Fri, 03 Feb 2006 07:00:30 -0000, Jasen Betts wrote: On 2006-02-02, LRCR wrote: video source---------------*-------------------Oscilloscope | | 75 ohm | | C 0V or 5VDC--10k--------B 2N3904 E | | -5VDC (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 | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3904 10K | --+--- -5 TTL-swittched broadband terminator concept. Bye. Jasen |
#9
Posted to alt.electronics
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
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 |
#10
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
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 On Fri, 03 Feb 2006 07:00:30 -0000, Jasen Betts wrote: On 2006-02-02, LRCR wrote: video source---------------*-------------------Oscilloscope | | 75 ohm | | C 0V or 5VDC--10k--------B 2N3904 E | | -5VDC (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 | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3904 10K | --+--- -5 TTL-swittched broadband terminator concept. Bye. Jasen |
#11
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
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 On Fri, 03 Feb 2006 07:00:30 -0000, Jasen Betts wrote: On 2006-02-02, LRCR wrote: video source---------------*-------------------Oscilloscope | | 75 ohm | | C 0V or 5VDC--10k--------B 2N3904 E | | -5VDC (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 | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3904 10K | --+--- -5 TTL-swittched broadband terminator concept. Bye. Jasen |
#12
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
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 On Fri, 03 Feb 2006 07:00:30 -0000, Jasen Betts wrote: On 2006-02-02, LRCR wrote: video source---------------*-------------------Oscilloscope | | 75 ohm | | C 0V or 5VDC--10k--------B 2N3904 E | | -5VDC (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 | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3904 10K | --+--- -5 TTL-swittched broadband terminator concept. Bye. Jasen |
#13
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
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 On Fri, 03 Feb 2006 07:00:30 -0000, Jasen Betts wrote: On 2006-02-02, LRCR wrote: video source---------------*-------------------Oscilloscope | | 75 ohm | | C 0V or 5VDC--10k--------B 2N3904 E | | -5VDC (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 | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3904 10K | --+--- -5 TTL-swittched broadband terminator concept. Bye. Jasen |
#14
Posted to sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
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 |
#15
Posted to sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
Thanks Mark for your feedback.
On 3 Feb 2006 18:47:29 -0800, "Mark" wrote: 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 |
#16
Posted to alt.electronics
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application - Circuit2.pdf (0/1)
On 2006-02-04, LRCR wrote:
Hi Jasen Attached is you suggested circuit redrawn in Workbench. It fell off on the way here. I don't have workbench here. 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. Not my invention, I copied that idea from the RF switch of a games console. 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. I've had another look at my drawing, the top and bottom transistors were the wrong types, swap them. sorry. --+--- +5 | 10K source | 2N3904 | 1N914 | | +--|--+-------C E-------+---- -5 | | | B | | 150R | | | | | | |---[33k]-' \ | === 100pF | \ / | 1K .---[10K]-- control X --+-- 0V | .2N3906 | (0 or 5V) / \ | C ' B / | === 100pF ^B---[10K]--+-C E-- 0V | | | E | `. | | | | | 2N33904 | | | |--[33K]-. 47K | 150R | | | | | | | B | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3906 10K | --+--- -5 TTL-swittched broadband terminator concept (rev1.1). Bye. Jasen |
#17
Posted to sci.electronics,sci.electronics.design,alt.electronics,sci.electronics.components
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Oops schematic mistake on first message. Correction made -- Help. Transistor circuit that connect or disconnect 75ohm terminator for video application
LRCR wrote:
Jasen, Thank you for your response. I am having trouble reading your ascii schematic. Also "Fixed font" I don't understand. I am excited to understand your circuit. You have to excuse me since I am new in reading ascii schematic. As you can see my ascii schematic is not very good. Is there any way to send an attachment with a picture, pdf or help me understand your ascii schematic? Luis On Fri, 03 Feb 2006 07:00:30 -0000, Jasen Betts wrote: On 2006-02-02, LRCR wrote: video source---------------*-------------------Oscilloscope | | 75 ohm | | C 0V or 5VDC--10k--------B 2N3904 E | | -5VDC (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 | | | +--|--+-------C E------+--+--- +5 | 1N914 | scope | 2N3904 10K | --+--- -5 TTL-swittched broadband terminator concept. Bye. Jasen Set whatever you are using for a news reader to use Courier font. -- JosephKK Gegen dummheit kampfen Die Gotter Selbst, vergebens. --Shiller |
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