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 | | -5VDCvideo source------------------*-------------------Oscilloscope | | 75 ohm | D
0V or 5VDC-->10k-------->G N-channel DMOS FET RON(MAX)=2ohm S for VGS= 5V | | | | -5VDC