Vectronics VEC-101K - Manual (Page 24)

VEC-101K Owner's Manual Shortwave Converter Kit
22
antenna. If the signals are coming from out-of-band stations using a preselector
in line with the antenna will help to reduce the level of the interfering signal.
Image problems: This problem will be more pronounced on the higher
shortwave bands. The best solution is using a preselector in line with the antenna
to reduce the level of the interfering signal.
Signals drift out of tune: This could be caused by a failing battery. Also, the
converter should be located away from heater or air-conditioning ducts! Rapid
temperature changes will cause drifting.
AM Broadcast stations are heard across the dial: The balance mixer design
and input tuning give good rejection of AM broadcast signals; but, it is possible
for very strong signals to leak around the converter. This is the nature of the
design. A preselector may help reduce the problem to an acceptable level. If the
offending station prevents reception of a favorite shortwave station, adjust the
local oscillator trimmer so the shortwave station falls on a quiet channel.
If all else fails, refer to the Warranty for factory repair options.
THEORY OF OPERATION AND SPECIFICATIONS
Technical Circuit Description:
Input selectivity is provided by LC combinations L1/C10 and L2/C11. A single
tuned stage is used to achieve the needed 1.06 MHz tunable IF bandwidth.
Coupling capacitors C1 and C2 are used for the impedance transformation
needed between the antenna and tuned band-pass circuit, and to the 2000-ohm
input impedance of the NE602 mixer. The NE602 is an active doubly-balance
Gilbert cell mixer. The doubly-balanced mixer enhances attentuation of IF band
signals.
A broadband L-type matching circuit (L5, C3) is used between the 2000-ohm
mixer output and the receiver IF. output jack J2.
The NE602 also contains the local oscillator. All fixed capacitors in the
oscillator section are monolithic types for good stability. LC combinations
L3/C12 and L3/C13 form the oscillator tank circuits.
Specifications:
I.F. Range: ................................. 530 to 1710 kHz (limited by receiver)
Coverage: .................................. two 1 MHz wide bands between 3 and 22 MHz
Bypassing: ................................. unit bypassed when power is off
Antenna: .................................... 60 mW into 8-ohm load
Impedance: ................................ matched for hi Z automotive antenna systems