resistor values in circuit

[redfrog]

I have some question about the resistor values in circuit.

1) can I use somewhat different valued resistor in r2r ladder, if the resistor value ratio 2:1 is satisfied?
the resistor 768 ohm, 383 ohm are hard to find from my junk or neighbors.
any explanation is welcomed. i am a novice at circut design.

2) what is the purpose of the C3,R7 ?
i thought them as a filter.
but it does not seem like a typical lowpass filter and hipass filter.

hipass filter on a book,

Code: Select all

 
        VIN ---C------+--------VOUT 
                      |
                      |
                      R
                      |
                      |
             ---------+--------         

but uzebox circuit has,

Code: Select all

        VIN ----------+----C----VOUT 
                      |
                      |
                      R
                      |
                      |
             ---------+--------

[havok1919]

1) can I use somewhat different valued resistor in r2r ladder, if the resistor value ratio 2:1 is satisfied?
the resistor 768 ohm, 383 ohm are hard to find from my junk or neighbors.
any explanation is welcomed. i am a novice at circut design.

In general yes, the ratios can be modified, but you want to make sure that the final voltage at the pulldown resistor (the 75 ohm resistors to ground) does not exceed about 700mV. (A requirement of the NTSC encoder.)

2) what is the purpose of the C3,R7 ?
i thought them as a filter.
but it does not seem like a typical lowpass filter and hipass filter.

What schematic are you looking at? If it's the outputs of the RGB ladders, the purpose of the capacitor is to AC couple the video signal into the NTSC encoder. (Removing the DC offset and ground reference.)

-Clay