MFJ MFJ-8621, MFJ-8621X2 User Manual
Table of Contents
Features.........................................................................................................................................................1
Technical Specifications (Typical Unit):..............................................................................................2
Receiver:.........................................................................................................................................2
Transmitter:..................................................................................................................................2
Simplified Technical Description:.........................................................................................................3
MFJ-8621 Technical Description:......................................................................................................3
General:...........................................................................................................................................3
Receiver:.........................................................................................................................................3
Transmitter:..................................................................................................................................3
T/R Switching:.............................................................................................................................3
TNC Data Carrier Detect Compatibility............................................................................................4
Connecting To Your MFJ-8621:..........................................................................................................4
Power Supply:...............................................................................................................................4
Antenna:..........................................................................................................................................5
TNC Connection:..........................................................................................................................5
RFI:.....................................................................................................................................................5
Selecting Baud Rate:................................................................................................................................6
Receiver Filter:..............................................................................................................................6
Transmitting:.................................................................................................................................6
Transmitter Deviation:.............................................................................................................................6
Setting Deviation:........................................................................................................................6
Two-Way Deviation Monitoring:............................................................................................7
Confirming Deviation:................................................................................................................7
Ordering Crystals:......................................................................................................................................8
Receiver Crystal:..........................................................................................................................8
Transmitter Crystal:...................................................................................................................8
Installing Crystals:......................................................................................................................................9
Crystal Oscillator Calibration:................................................................................................................10
Transmit Oscillator:....................................................................................................................10
Receiver Oscillator:.....................................................................................................................10
More About 9600-Baud FSK Packet Signals:...............................................................................11
Sending And Receiving 9600-Baud Data:.......................................................................................11
In Case Of Difficulty:...................................................................................................................................12
1. Unable To Copy Off-Air Signals:........................................................................................12
2. Unable To Connect:...............................................................................................................12
3. Transmitter Erratic Or Inoperative:...............................................................................12
Important Operator Notes:....................................................................................................................12
Appendix.........................................................................................................................................................13
Technical Assistance.................................................................................................................13
Replacing The Fuse Link...........................................................................................................13
MFJ-8621 Data Radio
1
MFJ-8621 2-Meter Packet Only
Data Radio
Congratulations on your choice of the MFJ-8621 Packet Only Data Radio. Please read
this manual thoroughly before attempting to operate your unit. Let's begin with a look
at some useful MFJ-8621 Features:
FEATURES
Packet-Only Performance: The MFJ-8621 is designed from the ground up for packet.
This means you'll get uncompromising performance at all data rates--on AFSK or FSK--
for a fraction of what you'd pay for a converted voice radio.
Cool Running: The MFJ-8621 draws only 25 mA on receive, and less than 1 A on
transmit. Run it 24 hours a day 365 days a year, or pack it with your lap-top and take
it on the road.
Sensitive: Motorola-IC based receiver circuitry recovers data from weak signals for
better throughput and fewer collisions.
Made-For-Data Filters: Optimi ze your receiver for wide 9600-baud signals or narrow
1200-baud signals. No other radio does this!
Rugged 5-Watt Transmitter: All the power you need to get the job done without
creating needless interference at distant nodes.
Clean Unsquelched Data Output: The MFJ-8621 uses a wide-response DC-coupled
line driver rather than a speaker amp for flawless signal recovery.
Note: Your TNC must have full DCD (Data Carrier Detect) in order to work with radios
that have full unsquelched AFSK output.
Lightning-Fast TXD: Set TXD low! PIN-diode switching, continuous-running receiver,
and crystal control deliver ultra-fast switching.
Twin Modulators: Varactor-modulation for true-FM at 9600-baud and reactance
modulation for mic-level signals at 1200 baud provide ultra-clean FSK and AFSK.
Simple Setup: The MFJ-8621 is compatible with virtually all up-to-date TNC-2 modems.
Easy To Rechannel: Run your MFJ-8621 with supplied crystals on 145.01 MHz, or
order custom cry stals for t he channel of your choi ce. Simple step- by-step instr uctions
illustrate how to exchange crystals and set on frequency without expensive test
equipment.
The Right Tool For The Job: Your MFJ-8621 was designed from scratch for fast,
clean packet communication and nothing else! Whether you are monitoring the local
DX cluster or breaking new ground at 9600, you can look forward to years of hands-off
service.
MFJ-8621 Data Radio
2
TECHNICAL SPECIFICATIONS (Typical Unit):
Receiver:
Frequency Coverage................. 144-148 MHz
Sensitivity....................................... 0.6
µ
V for 12 dB Quieting
Image Rejection.......................... -45 dB or better
1st IF................................................ 10.7 MHz
2nd IF.............................................. 455 KHz
1st LO............................................. Crystal , 3rd overtone
2nd LO............................................ 10.245 MHz
Selectivity....................................... -6 dB at 20 KHz (data-passband filter)
AFSK Output................................. Unsquelched, 0dBm @ 3.0-KHz deviation
Current Drain.............................. 25 mA
Transmitter:
Frequency Control.................... Crystal, 18-MHz x 8
FSK/AFSK Input level.............. 150 mV p-p @ 1200 baud, .8 V p-p @ 9600
Deviation........................................ 0-5 KHz, adjustable at TNC
RF Power Output....................... 4.0 - 5.0 Watts into 50 Ohms
VSWR Tolerance....................... 3:1 Max.
Current Drain.............................. 1.0 Amp @ 13.8 Volts
Figure 1 Block Diagram of MFJ-8621 Data Transceiver
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-no tch 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 sign al 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 MH z) 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 t he 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
MFJ-8621 Data Radio
4
As a "packet-only" radio, the MFJ-8621 is specially designed to transmit and receive all
data rates through 9600 baud. However, to accomplish this, the MFJ-8621 must
deliver
unsquelched
AFSK or FSK data to the TNC. The MFJ-8621 does not provide
squelched AFSK because conventional squelch circuits are far too slow to be useful
above 1200 baud. In order to process unsqu elched data from the MFJ-8621 (or any
high speed data radio), your TNC must have a data carrier detect (DCD) circuit built in.
All MFJ model TNCs -- starting with t he MFJ-1270B-- and virtually any other TNC that
operates over 1200 baud should have one as well (PK-96, DPK-9600 for example).
Unfortunately, many TNCs, like the PK-232 and even some low cost mini-TNCs, lack
DCD because they are made exclusively for use with conventio nal FM voice radios to
run at 1200 baud. If your TNC does not have DCD, it cannot distinguish between
limiter noise (the strong rushing noise FM radios produce with no signal present) and
data signals. Since the TNC sees the limiter noise as incoming signal, it will not allow
the TNC to transmit due to the TNC's data collision protection function. Consequently,
the radio will appear incapable of transmitting! Again, your TNC
must
have DCD to
work with the MFJ-8621.
If your TNC
does
have a DCD, make sure the function is enabled in your particular unit.
Read your TNC documentation carefully to confirm what type of carrier detection--if
any--exists in your unit. If you
do
have DCD, activate it.
Important Note:
Some TNC's have software DCD circuits that may also require internal adjustments
(check your TNC manual and follow activation instructions carefully). Other TNCs may
not have a DCD at all or have a DCD indicator that is really nothing more than a mis-
labeled "signal-present" indicator. Separate "universal" add-on DCD circuits are
available for TNCs that lack a DCD. PacComm offers add-on DCD circuits for many
applications and has information about choosing the right DCD to fit your needs.
Refer to the appendix for notes on specific TNCs.
CONNECTING TO YOUR MFJ-8621:
Power Supply:
The MFJ-8621 will operate from any well-filtered DC power source delivering 13.8
Volts at 1 Amp. The MFJ-4110 wall-adapter supply is a light-weight voltage-regulated
unit especially suited for powering the MFJ-8621. "CB" type supplies or battery packs
capable of 1-A output will also work (anticipate lower RF output when using a 12-volt
source). The MJF-8621 accepts a standard 5.5mm x 2.1mm coaxial-type power plug
(Radio Shack 274-1569). Connect t he (+) or red lead to the cent er pin, and connect
the (-) or black lead to the outer sleeve. Use a 2 amp fast-blow fuse with any power
supply other than the MFJ-4110. Your MFJ-8621 features "crowbar" protection to
prevent damage from incorrect power supply polarity. If power leads are accidentally
reversed, the crowbar-diode conducts and immediately opens the radio's line fuse,
cutting off power (for line-fuse replacement instructions, see the Appendix).
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