Help. Transistor circuit that connect or disconnect 75ohm terminator for video application

[LRCR]

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

 

[Robert Baer]

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

Err..a signal source of 75 ohms travelling down what pretends to be a
transmission line and that line not terminated *will* give "peaks and
valleys" due to reflections off the end of the line.
Also, nothing is said as to how the signal magically goes to the
scope; virtually any probe used for that purpose will alter a resistice
75 ohm termination, to the detriment of the signal.
Methinks you need to go back to square one and think this over
completely.

 

[LRCR]

Robert,

You do make a good point. I am using from the source to the
oscilloscope an RG59 cable with a T at the scope. The circuit is at
the T. Also. I attached a large capacitor 220uF (Crazy) from the
gate to ground(the gate source is from a lab Power Supply) and it
appears to help. Maybe that a clue.


Caesar