Vectronics VEC-1120K - Manual (Page 32)

VEC-1120K/1130K/1140K/1180K Instruction Manual
30
THEORY OF OPERATION AND SPECIFICATIONS
Technical Circuit Description:
Potentiometer R1 serves as a continuously variable RF attenuator and is the
receiver gain control. Using the RF gain control to set volume is advantageous
as it also reduces strong in-band signals that could otherwise overload the
receiver front end.
Series inductors L2 and L3, and trimmer capacitor C18, provide front-end band-
pass filtering, and an impedance match to 50 to 75-ohm antenna systems. IC U1
is an NE602/612 mixer and Local Oscillator in an 8-pin dip package. The
mixer section is an active Gilbert cell design for good conversion gain and low-
noise figure. The LO section uses heavy capacitive loading to minimize
frequency drift. Tuning is capacitive, using a modern Hi-Q miniature plastic
variable.
Since this is a direct conversion receiver, the LO is tuned to the carrier frequency
of Single Sideband Signals, or to an difference of 300 to 800 Hz on CW signals,
to produce an aural output that is differentially coupled to a LM386 audio IC.
LO stability is enhanced by a 78L05 voltage regulator.
Specifications:
Sensitivity.................................... 3-uV or better
Modes.......................................... CW, RTTY (FSK), SSB, AM
Audio output................................ 50 mW typical
Frequency coverage..................... Portions of the 80, 75, 40 and 30 meter bands
Power Source............................... 9 Volt transistor battery, alkaline preferred.
PC Board ..................................... 4.000" x 4.700"