Vectronics VEC-1220K - Manual (Page 32)

VEC-1220K/1230K/1240K/1280K Owner's Manual
30
THEORY OF OPERATION AND SPECIFICATIONS
Technical Circuit Description:
The VEC QRP-CW Transmitter is a simple three-stage oscillator/amplifier with
no tune-up requirement. Q1 is keyed on by logic-low input from a mechanical
key or electronic keyer to power oscillator Q2. Q1 also supplies turn-on bias to
driver Q3 and T/R switching diode D2. Q2 is a crystal-controlled series-tuned
colpitts oscillator modified for VXO operation (RFC1, C5). SW2 selects an
optional second crystal. Q3 drives PA Q4 through capacitive divider C13, C14.
Q4 is a simple class-C amplifier that remains powered at all times, but only
draws current when RF excitation is present. In transmit mode, the antenna line
is routed to Q4 via diode switch D2. In receive mode, it is routed to the receiver
port via D3. A pi-section low-pass filter (which includes C16) provides
impedance matching and harmonic suppression between Q4 and the 50-ohm
antenna port. CW output averages +32 dBm with -25 dBc of antenna-port
isolation.
Specifications:
RF Power Output:............................1.25 watts at 14 MHz
Suppression: ....................................35 dBc or better
Receiver Isolation:...........................-25 dBc
VXO tuning range: ..........................8 kHz at 14 MHz (less at lower frequencies)
Stand-by-current:.............................10-15 mA
Transmit current: .............................250-350 mA at 13.8 Volts DC