Input Impedence Adjustment; Video Output Adjustment - Videotec TWRB Manuel D'instructions

6. Position switch 16 of SW2 in the ON position
7. Adjust trimmers TC1,TC2,TC3,TC4 to the minimum capacity values.

Input impedence adjustment

The input impedence has to assume different values according to the type of cable used for the video signal
transmission from TWT unit to TWRB/TWRR receiver :
Where to operate : Trimmer TP1
Adjustments :
It is necessary to know the impedance of the cable being used. So position an Ohmmeter between terminals A
and B on K3 and adjust trimmer TP1 until reading the correct cable impedence. If a Cat. 5 cable is used (see the
paragraph "Type of cable") the normal value is 100 Ohms.

Video output adjustment

Video output is adjusted using the trimmers and dip-switches, we recommend using an oscilloscope for the
adjustment procedure:
Where to operate : Trimmers TP1,TP2,TP3,TP4, Trimmers TC1,TC2,TC3,TC4, Dip-switches SW1, SW2, Jumper J1
Trimmer Function
TP1 Input impedance adjustment
TP2 Symmetry adjustment (CMRR)
TP3 Linear gain adjustment
TP4 LF gain adjustment
Adjustments :
1. Connect the power supply to connector K1
2. Connect the video monitor to the video outlet on connector K2
3. Connect the twisted pair cable from the TWT transmitter to connector K3
4. Power the unit. Unit power supply is present when LED2 is lit. If a video signal is input to the TWRB/TWRR
receiver, LED1 should be off
5. Power the monitor
6. Position jumper J1 excluding the -6dB loss (unless the distances are very short)
7. Regulate video output to 1 Vpp by adjusting linear gain (trimmer TP3)
8. Regulate HF gain (trimmer TC1) and LF gain (trimmer TP4) to regain Burst (colour) and to square the
synchronism impulse by positioning the trimmers for the best signal quality (on both the monitor and the
oscilloscope)
9. If signal quality is still poor or there is still no Burst, the dip-switches should be regulated starting from HF
(switches 1,2,7,8 of SW1) and finding the best combination (at this stage it may be necessary to set again trimmer
TC1 to get good signal quality); then pass to MF (switches 3,4 of SW1 and 9,10 of SW2) and to LF (switches 5,6
of SW1 and 11,12 of SW2 ) for which the same applies as for HF (in this case it may be necessary to set again
TP4).
If the signal quality is good skip directly to the next point (normally dip-switches need only to
be regulated for long distances).
10. Now make the fine adjustments using trimmers TC2, TC3, TC4. If fine adjustment is impossible, set the switches
13, 14, 15 of SW2 on ON and repeat the fine adjustment.
11. Now regulate minimum noise by adjusting trimmer TP2 (this adjustment optimises CMRR and is important when
there is more than one video signal along the same cable)
12. Check the video output signal amplitude at 1 Vpp again and if necessary set again it adjusting trimmer TP3
Note :
1. If there is too much noise in the video signal (especially over long distances), position jumper J1 on +10 dB in
the TWT transmitter (see the TWT manual ) and repeat the setting procedure in the TWRB/TWRR receiver.
2. If it is necessary to adjust the gain to a value lower than 6 dB ( in the case of short distances ), set SW1
switches from 1 to 8 at OFF, SW2 switches from 8 to 15 at OFF, SW2 switch 16 at ON.
Set then the jumper J1 at the value of -6 dB and carry on with the fine adjustment of the video output, as
described in the previous paragraph.
Trimmer Function
TC1
TC2
TC3
TC4
Pag. 5
HF fine gain adjustment
HF fine gain adjustment
MF fine gain adjustment
LF fine gain adjustment
TWRB/TWRR 2701