Vectronics VEC-422K - Manual (Page 24)

VEC-422K Owner's Manual SCA Decoder Kit
22
The XR2211 is a Phase Locked Loop (PLL). Tuning control R16 varies the
VCO (Voltage Controlled Oscillator) section of the XR2211 over the range of
frequencies used by the SCA services. When a subcarrier is detected, the VCO
will lock to the subcarrier frequency once the tuning control is in range. Lock is
achieved by the Phase Detector generating an error correcting voltage (steering
voltage) and feeding it back to the VCO to keep it locked on frequency.
The subcarriers are FM modulated, that is the frequency of the subcarrier varies
according to the program material. The phase detector sees these frequency
variations, and continually adjusts the steering voltage to keep the VCO in lock.
These voltage variations faithfully mimic the program material, and by simply
coupling an audio amplifier into the tuning voltage line, the program material
may be recovered and monitored.
VCO
P.D.
SCA
signals Audio
amplifier
Error correcting
voltage.
Specifications:
SCA subcarrier range ......................50 to 100kHz
Power requirements .........................9-volt transistor battery, alkaline preferred
Audio output J2 ...............................line level output
Audio output J3 ...............................4 to 32 ohms, 50mW max.
PC Board .........................................4.000" x 4.700"