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