MFJ MFJ-8621 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: Optimize 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 crystals for the channel of your choice. Simple step-by-step instructions
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 µV 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-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

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 unsquelched 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 the 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 conventional 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 the (+) or red lead to the center 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).

MFJ-8621 Data Radio
5
Antenna:
For best packet performance, use a high-quality antenna adjusted for minimum VSWR
at your operating frequency. Omni-directional types--like the MFJ-1740 1/4-wave
ground plane, MFJ-1750 5/8-wave ground plane, or MFJ-1764 dual 5/8-wave Super
gain-should provide excellent area coverage. If you need added punch to gain reliable
access into a busy regional node, install a directional antenna--like the MFJ-1763 3-
element 2-Meter beam. When installing the antenna, be sure to feed it with high-
quality low-loss coax and keep the cable run as short as possible. In many cases, a
lower antenna fed with good cable will outperform a higher antenna fed with lossy
cable. To cut line losses to a minimum, avoid adding unnecessary in-line connectors,
power meters, and coax switches.
TNC Connection:
The MFJ-8621 plugs directly into any MFJ TNC starting with the MFJ-1270B. Any
other DCD equipped TNC, such as the PK-96 and many others, will also work with this
data radio. Use a 5-pin DIN-to-5-pin DIN cable, MFJ-5100, for connection to MFJ
TNCs or the PK-96.
Caution: Not All TNCs have DCD (Data Carrier Detect) circuitry, and your MFJ-8621 --
which has an unsquelched output---will not work with those TNCs.
Pin Function
1 AFSK IN (150 mV pp / 800 mV pp)
2 GND
3 PTT (ground or logic-low for TX activation)
4 AFSK OUT (2.2 V pp into 600 Ohms)
5 N.C.
RFI:
After completing initial adjustments, always reinstall the covers on your MFJ-8621 and
TNC. Without shielding, the receiver may be swamped with RF interference generated
by microprocessors in the computer and TNC. For added RFI protection, many
veteran packet operators recommend installing ferrite RFI chokes on interconnect and
power cables--and keeping the radio as far as possible from the TNC and computer.
The MFJ-701 RFI kit provides 4 snap-on chokes for this purpose, along with
instructions. Snap-on RFI chokes are also available at Radio Shack (273-104 and 273-
105).

MFJ-8621 Data Radio
6
SELECTING BAUD RATE:
Receiver Filter:
The MFJ-8621 receiver features a special noise-reduction filter to enhance 1200-
baud operation. When running at 1200-baud, this filter improves weak-signal
performance. To activate the 1200-baud noise-reduction filter, install a shorting plug
at JMP-1 (this plug must always be removed when operating at 9600-baud).
See Figure 3 on page 9 for placement of the jumpers on the pc board.
Transmitting:
The MFJ-8621 has two FM modulators selected by jumpers, JMP2 and JMP3. One is
a sensitive reactance modulator designed for mic-level AFSK signals. Since most
TNC's output mic-level signals at 1200 baud, the reactance modulator should be used
at 1200 baud. The other is a "true-FM" varactor modulator designed for line-level
FSK signals. At 9600 baud, most TNC's have a line-level output and should be used
with the varactor modulator. If your TNC has a line-level output setting for 1200 baud
TX-AFSK (usually set via a jumper inside the TNC), you may use the varactor modulator
at both data rates.
For 1200-baud AFSK operation, install JMP1 and set JMP-2 and JMP-3 in the
reactance modulator positions (left-hand and center pins of each header).
For 9600-baud FSK operation, remove JMP1 and set JMP-2 and JMP-3 in the
varactor modulator positions (center and right-hand pins of the headers).
See Figure 3 on page 9 for placement of the jumpers on the pc board.
IMPORTANT NOTE: JMP1, JMP2 and JMP3 are set for 1200 baud at the factory.
Always recalibrate the MFJ-8621 transmitter oscillator for frequency (L6)
after switching modulators (see calibration, page 10).
TRANSMITTER DEVIATION:
Setting Deviation:
The MFJ-8621 has no internal deviation control. Deviation is adjusted via the AFSK
output level control on your TNC (consult your TNC manual for specifics). If you don't
have access to a FM deviation meter, you can set deviation by measuring the peak-to-
peak AFSK signal output from your TNC:
With jumpers in the
reactance modulator (1200-baud) jumper positions, a 150-mV p-
p sine wave should produce approximately 3-KHz deviation. With jumpers in the
varactor modulator (9600-baud) jumper positions, a 800-mV p-p sine wave should
produce approximately 3-KHz deviation.
Virtually any calibrated oscilloscope will allow you to make this measurement. Connect
your scope probe to PIN-1 on the 5-pin DIN jack at either the TNC or radio end (look
for the TX-AFSK line if your TNC uses a different type of jack). Use the CALIBRATE
function on your TNC to generate a test-tone.

MFJ-8621 Data Radio
7
Figure 2 Monitoring Incoming and Outgoing Signals with a Scope
Two-Way Deviation Monitoring:
In addition to monitoring TNC output at pin 5, you may also monitor both incoming and
outgoing Packet signals passing through your station by connecting an oscilloscope to
the RX-AFSK line at pin-4. RX-AFSK also appears at the "monitor" or "speaker" jack on
most TNCs. Use 1-V/div V-sens and 1-ms/div H-time base. To set TX deviation while
monitoring RX-AFSK, first note the waveform height of average off-air signals (2.2 V p-p
is typical for 3-KHz deviation). Next, key the MFJ-8621 transmitter via the CALIBRATE
function on your TNC--and adjust TNC output (TX-AFSK) for a waveform of about the
same height.
Confirming Deviation:
If a node in your area uses the X-1J2 version of theNET, you can take advantage of that
station's built-in deviation meter to confirm the accuracy of your setting. Anytime you
experience consistent difficulty connecting with local stations, you should suspect
improper deviation level as a possible cause.

MFJ-8621 Data Radio
8
ORDERING CRYSTALS:
You may order crystals for other channels factory-direct. Crystal specifications are:
Receiver Crystal:
XTAL Freq. = Operating Freq. - 10.7 MHz
3
TYPE................................................. 3rd overtone (44-MHz range)
TOLERANCE................................... .003% Commercial Standard
TEMPERATURE............................ 26-Degrees C (room temperature)
LOAD................................................ Series
CASE................................................. HC-25U or FM-2
Transmitter Crystal:
XTAL Freq. = Operating Freq.
8
TYPE.................................................. Fundamental Mode (18-MHz range)
TOLERANCE.................................... .003% Commercial Standard
TEMPERATURE............................. 26-Degrees C (room temperature)
LOAD.................................................. 32 pF, Parallel
CASE.................................................. HC-25U or FM-2
Crystals for the MFJ-8621 may be ordered directly from:
JAN Crystals
P.O. Box 06017, 2341 Crystal Drive
Fort Myers, FL 33906-6017
Telephone [phone], FAX [phone]
When ordering from JAN, simply tell the sales person that you wish to order a pair of
crystals for the MFJ-8621 Data Radio and give the
operating frequency in MHz. JAN
has complete MFJ-8621 crystal specifications on file in their ordering system and no
additional information is required. Alternatively, you may purchase crystals from:
International Crystal Manufacturing Co., Inc. (ICM)
P.O. Box 26330, 701 W. Sheridan
Oklahoma City, OK 73123-0330
Telephone: [phone], FAX [phone]
For receiver crystals, order 471270-CRYSTAL FREQ (not the operating freq.)
For transmitter crystals, order 434275-CRYSTAL FREQ (not the operating freq.)
JAN and ICM both welcome telephone orders and accept MasterCard or Visa.

MFJ-8621 Data Radio
9
INSTALLING CRYSTALS:
To install new crystals in your MFJ-8621, refer to the diagram below:
[ ] Remove the MFJ-8621 cabinet lid (one screw on each side of the cabinet).
[ ] Locate and pull the existing transmit crystal (Y3) from its socket.
[ ] Carefully plug the new transmit crystal in its place.
[ ] Locate and pull the existing receive crystal (Y2) from its socket.
[ ] Carefully plug the new receive crystal in its place.
Figure 3 Crystal Locations and Calibration Adjustment Points.
For future reference, label your pull-out crystals for operating frequency and store
them in a safe place.
Anytime you install new crystals, you must readjust L4 and L6 for exact frequency
calibration on the new channel (see page 10). Do not attempt to operate your radio
until this calibration procedure has been completed.