Vectronics VEC-1290K Manual - page 22
Technical Circuit Description:
RF oscillator section:
One section of six-section inverter CMOS IC package is
used for the oscillator (UID). The oscillator tank coil is made of four inductors
in series. Two sets of shorting jumpers allow the inductance value to be set
from 122 uH to 552 uH. Each of the three possible jumper configurations
covers a portion of the 530 to 1710 kHz AM broadcast band. Fine tuning to an
exact channel is done via a 100-pF ceramic trimmer across the tank coil.
Buffer and driver stages:
The oscillator output is buffered by section UIB of
the 4049 inverter chip. The output of UIB drives two inverter gates in parallel-
U 1 D and U 1 E.
The paralleled operation insures sufficient drive level the final transistor for
Class C operation.
Final amplifier stage:
A 2N3904 transistor is used for the power amplifier
stage. The transistor is biased for Class C 'operation. ``Class C operation
permits the use of high-level modulation for best linearity and efficiency. Note
that many inexpensive AM transmitter kits use modulated oscillators, with
resulting
FM'ing of
the carrier and a maximum modulation level
of
perhaps 25
to 30 percent.
Audio Amplifier and Modulator stage:
Low level microphone or line-level
audio is amplified by a LM386 audio amplifier IC. The audio level to the IC is
set by gain control R1. The output from the IC is directly applied to the RF
power stage, the 2N3904 transistor.
The DC output
of
the LM386 is normally at
''/s
of
the supply voltage with no
signal. The DC level follows the audio signal; at maximum modulation (100%)
the instantaneous output voltage will vary from near zero to almost the full
supply voltage. In a true Class C stage, doubling the supply voltage will
produce a peak power level (PEP)
of
four times the carrier power.
Direct coupling:
After C1, all audio stages up to the PA transistor collector are
DC coupled, with no interstage coupling capacitors or modulation transformers
to limit or tailor the audio response.
Antenna and matching:
The VEC-1290K is supplied with an attached 6-foot
wire radiator. This antenna meets current FCC Part 15 requirements for
unlicensed RF transmitters. The antenna is extremely capacitive reactive at
broadcast band frequencies, and presents a very high impedance to the
transmitter. Capacitor C8, inductors L3, L4 and L5, and trimmer capacitor C11
form a pi-network impedance transformation network to match the
2