VEC-1320K/1330K/1340K/1380K Owner's
Manual
34
forces a frequency shift for CW offset. D1 and D2 isolate Q1's +T and +R
voltage sources.
The RF input and AF output ports of unidyne detector U1 are configured for
differential-mode to enhance common-mode noise rejection and provided
highest gain. RF gain control (R1) and high-Q preselector (T1/C1/C2) preceed
the mixer input. AF output response is rolled off at high frequencies by C6.
U2 is a two-stage AF-power op-amp configured in high-gain mode by feedback
capacitor C7. Loop-gain is further increased in the 600 Hz range by a second
frequency-selective feedback network (R3/C11/C12). MUS (minimum usable
sensitivity) exceeds 1 uV, while high-frequency AF products are rolled off for
QRM reduction. U2 provides over 100-mW AF drive into 8 ohms, which is
more than adequate to drive low-Z headphones and 8-ohm extension-type
speakers.
In transmit mode, dc-switch Q4 provides shaped keying of VXO Q1 and
supplies turn-on bias to driver Q2. PA Q3 operates in class-C and draws current
only when RF drive is present. Broadband matching techniques are used to
eliminate critical tune-up steps for the builder. A pi-section low-pass filter
provides impedance matching and harmonic suppression between Q3 and the 50-
ohm antenna port. CW output normally exceeds 1 Watt (+30 dBm) with -35
dBc or better harmonic suppression.
For simplicity, transceiver T/R functions are handled by a DPDT switch. A
second switch applies power to the radio, and crowbar diode D3 protects
transceiver circuitry against accidental reverse-polarity power connection. If
"on-board" CW sidetone generation is required, a small powered peizo-sounder
may be connected to key-switch Q4 through current-limiting resistor R10.
Typical Specifications:
RF power output:
1.2 watts @ 14 MHz (may be greater at lower frequencies)
Suppression:
-35 dBc or better
VXO tuning range: 9 kHz typical at 14 MHz (will be less at lower frequencies)
VXO T/R offset:
300-500 Hz @ 14 MHz (will be less at lower frequencies)
Receiver sensitivity: .3 uV minimum usable signal on all bands
Receiver current:
25 mA at 13.8 Volts DC
Transmit current:
250-350 mA at 13.8 Volts DC
SCHEMATIC
