Sierra Video 10 Series - Manual (Page 17)

SERIES 10 DA
11
501019 Equalizing Video DA
The coaxial cable used in distributing video signals is not a perfect transmission medium.
For signals of primary importance, lengths of 15 meters or more should be equalized to
restore the higher frequency detail in the video signal and to preserve signal quality. The
adjustable equalization range of the 501019 is up to 300 meters of RG59.
Circuit Description
The 501019 is a plug-in module. Refer to the 501019 schematic diagrams in the
schematic section of this manual.
The 501019 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 501019
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
501019 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 the 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 3.5ma. 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. This 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
501019.
The network consisting of R14, R15, R16, R17, C7, C8, C9 and C10 is a four pole
equalizing network. The amount of effect the network is allowed to have on the overall
circuit is adjusted by R22. At one end the network is shorted out to itself and the input
circuit gain is flat with respect to frequency. At the other end of R22’s adjustment range
the network is connected across the emitters of Q5 and Q6. The frequency response of
the input circuit is then frequency dependent. This produces the desired response to
counteract the response loss of long lengths of input cable. The amount of cable the
501019 compensates for is continuously adjustable by the setting of R22.
U2 is a low frequency amplifier used to correct the overall DC of the 501019. 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 501019 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 ohm). R36 through R43 are each
precision 75 ohm resistors. Each drives a separate output line.