Sierra Video 10 Series - Manual (Page 16)

SIERRA VIDEO
10
501519 Differential Input Video DA
The 501519 eliminates the effects of power line hum caused by circulating ground
currents in the coaxial cable shield. The differential input circuitry also rejects unwanted
common-mode signals induced by stray magnetic fields and electrostatic interference.
Circuit Description
The 501519 is a plug-in module. Refer to the 501519 schematic diagrams in the
schematic section of this manual.
The 501519 is powered by low voltage AC which comes into the module through the
edge connector. When dual supplies are ordered, the two supplies enter the 501519
separately and are paralleled after the AC to DC rectification. This provides total in-frame
isolation between the two supplies. The rectified DC is filtered by C21 and C20. U4
provides regulated +8VDC and U3 provides regulated -8VDC. On the front edge of the
501519 separate LEDs indicate the presence of the + and – 8VDC levels.
Input video enters on the edge connector pin 2. The incoming signal’s shield enters on
pin 1.
Both signal and shield are AC coupled to the amplifier (C2 and C3). R2 and R4 provides
a DC reference for the signal to ground. R1 and C1 bypass high frequency shield noise to
ground. Q1 through Q8 comprise a differential amplifier. Q2 and Q3 are each configured
as constant current sources of about 3.5 ma. Q5 and Q6 operate differentially. Common
mode, undesirable signals present no voltage across R22. The voltage difference
between the center conductor of the input and shield is the desired voltage. The voltage
across R22 will cause a differential current swing at the collectors of Q5 and Q6. Q4 and
Q7 are a differential input to single ended output current mirror. The signal at the collector
of Q4 is a current equal to the differential current at the collectors of Q5 and Q6. By
varying the resistive load at this point the voltage gain of the overall amplifier is set. R23
is a 15 turn potentiometer gain adjustment located on the front of the 501519.
U2 is a low frequency amplifier used to correct the overall DC of the 501519. The voltage
at the output of the module is compared to ground. The required correction signal
changes the bias of the current mirror via Q8.
The remainder of the circuit on the 501519 is a high quality 10 load video distribution
amplifier. The basic element is a 120MHz amplifier U1 followed by a current boosting
stage.
Q9 and Q10 are inside the gain determining feedback loop of U1. D10 and D11 provide
class AB operating bias for Q9 and Q10. Q9 and Q10 are 200MHz 500ma devices. They
provide a very low output impedance (about 0.1). R36 through R43 are each precision
75ohm resistors. Each drives a separate output line.