Vectronics VEC-131K - Manual (Page 31)

VEC-131K Owner's Manual Aircraft Receiver Kit
29
Poor sensitivity:
Look for antenna problems. Also, check condition of L1, L2, and L3. If they
are improperly shaped or if a lead is broken, sensitivity will be poor.
If all else fails, and factory service is needed, consult the warranty information at
the beginning of the manual.
THEORY OF OPERATION AND SPECIFICATIONS
The VEC-131K is a single-conversion superhetrodyne AM receiver covering
from 118 to 136 MHz.
Capacitor C1 and inductor L1 form a series-resonant trap filter to reduce image
band responses. Two heavily coupled resonant stages (C3-L2, C5-L3) form a
broad bandpass filter for the 118 to 136 MHz range. Transistor Q1 (2SC2498)
is a microwave device, and is biased for best noise figure. This Low-Noise
Amplifier stage (LNA) amplifies incoming signals by about 15 dB.
The NE602 IC (U1) serves as the first mixer and tunable local oscillator stage.
The NE602 mixer section employs an active Gilbert-cell double-balanced
design. The local oscillator (LO) involves components C11, C12, C13, L4, C14
and varactor diode D1 in a Colpitts oscillator configuration. Capacitors C11
through C14 are multilayer styles for best RF performance and oscillator
stability. Tuning is done by applying an variable voltage to the varactor tuning
diode, D1. As the bias voltage across D1 changes, its capacitance is varied
accordingly. As the reverse bias is increased, the junction capacity decreases,
and the LO frequency increases. The tuning voltage is set by potentiometer R6,
the main tuning control. The tuning voltage source is regulated by zener diode
D2.
Because the IF is at 10.7 MHz, the LO is tunable over a range of 128.7 to 146.7
MHz for a tuning range of 118 to 136 MHz. The receiver uses high-side LO
injection for good reason—low-side injection would put most of the image
response in the FM broadcast band. The LO is “mixed” with incoming signals
in U1 to produce an IF output of 10.7 MHz. A ceramic IF filter (FL1) follows
the mixer output. FL1 helps set the overall bandwidth, or selectivity, of the
receiver.
The first IF amplifier stage is Q2, a 2N3904 transistor operating in a class A
common emitter configuration. This stage provides about 18-dB gain, and is
used to overcome the 6-dB insertion losses of filter FL1. The last IF stage uses a
Motorola MC1350 IC (U2). This stage has about 50-dB gain, and an AGC
control range of about 60 dB. The high-impedance output of U2—on the order
of 2,000 ohms—is fed to a 25:1 ratio IF interstage transformer. This impedance
transformation is needed to match the relatively low-impedance of diode D3.