MFJ-8621 Data Radio
3
SIMPLIFIED TECHNICAL DESCRIPTION:
Your MFJ-8621 was especially designed for packet-only operation. MFJ has
eliminated costly circuitry that you don't need: such as squelch, repeater offsets, PL
tones, DTMF pad, PLL synthesis, memory, and the speaker-amplifier. At the same
time, we engineered in essential packet circuits like true-FM direct modulation,
unsquelched AFSK, PIN-diode switching, data-passband IF filtering, and a tailored 0-Vu
DC-coupled line driver to ensure top-notch performance. There are no user-adjustable
controls to set incorrectly, and the MFJ-8621 works with most popular TNCs.
MFJ-8621 TECHNICAL DESCRIPTION:
General:
The MFJ-8621 is a dedicated-channel 2-Meter FM-AFSK transceiver designed for use
with
DCD equipped
TNC-2 style modems at data-rates through 9600 baud.
Receiver:
Referring to Figure 1, incoming signals are preselected by the lowpass and bandpass
filters. Low-noise preamp Q1 enhances weak signal performance. U1 is a monolithic
dual-conversion narrowband-FM receiver IC that provides all mixer, IF amplification,
limiting, and audio functions. The 1st-LO, Q2, is an external 3rd-overtone oscillator-
tripler and the 2nd LO is built into U1. U1 and Q2 remain powered during transmit to
ensure seamless RX recovery and self-monitoring capability. IF passband filtering is
provided by ceramic filters FL1 (10.7 MHz) and FL2 (455 KHz). FL2 is a 20-KHz data-
bandwidth filter which supports 9600 baud operation. Discriminator output is looped
to U1's auxiliary op-amp which is configured as a DC-coupled AFSK line driver. Gain is
set to provide 0-dBm AFSK output into a 600-Ohm load for approximately 3.0 KHz
deviation. A jumper configures the line driver feedback loop for HF noise reduction at
1200 baud AFSK reception. Test points at the 1st-LO, RSSI, and DISC outputs
facilitate alignment.
Transmitter:
FM signals are generated by monolithic FM-transmitter IC U2. Frequency control is
provided by 18-MHz crystal Y3 (oscillator/modulator output is multiplied by three
successive doubler stages to 145 MHz). 1200-baud AFSK is fed directly to the IC's
reactance modulator. 9600-baud FSK is fed through a varactor modulator for true-
FM modulation. Pre-driver Q3 and Driver Q4 operate in class AB and use tuned bifilar
4:1 interstage transformers to facilitate matching and bandpass filtering. The PA
stage (Q5) is a conventional class-C amplifier with a tuned-L network and pi-section low-
pass filter to provide Z-matching and harmonic suppression.
T/R Switching:
RF switching is executed by PIN diodes D8 (receive path) and D7 (transmit path). DC
switching is executed by Q6, Q7. Supply voltage to U1 is regulated at 5 volts, and
voltage to U2 is regulated at 8 Volts. The 1st-LO voltage is regulated at 6.8 Volts.
Receiver chip U1 remains powered during transmit for seamless T/R switching
characteristic. The MFJ-8621 is designed to operate at 13.8-Volts @ 1-Amp.
TNC DATA CARRIER DETECT COMPATIBILITY
