Ramsey Electronics FX Series User manual

220 MHZ AMATEUR PLL
SYNTHESIZED FM
TRANSCEIVER 220 - 240
Ramsey Electronics Model No. FX-223
Published in Three Sections:
• FX-series General Reference Information
• Kit Assembly Phase I: Circuit Stages A through F
• Kit Assembly Phase II: Circuit Stages G through TX
Plus:
• Fold-out Schematic Diagram
• Synthesized - no crystals to buy !
• Perky 5 watt RF output
• Dual conversion sensitive receiver with crystal and ceramic IF filters
• PACKET ready ! Dedicated packet interface connector on back!
• 12 Channels, expandable to as many as you want - programmed with
diodes !
• Easy 4 evening assembly - need only a voltmeter and another rig for
testing
• Fantastic manual teaches as you build, rig is assembled in bite-sized
sections that are tested as you build - your kit will work first time !
• Rig operate over 20 MHz of band, great for snooping out of band !

A DEDICATION
The Ramsey FX-series FM Transceiver Kits are writing a truly NEW chapter
in the annals of the ham radio story. Growing numbers of today's radio
amateurs ARE willing to build, understand, adjust and maintain modern VHF
gear capable of digital frequency programming and data communication as
well as FM voice. How do we know? We know it because, during 1991,
thousands of hams worldwide built and are using our pioneering FTR-146 for
two meters. The FX Transceiver design was developed in immediate
response to those builders' suggestions, wish lists and inquiries. This
publication is dedicated with deep thanks to all those FTR-146 builders!
FX-223 VHF FM Transceiver Reference and Kit Assembly Manual
Ramsey Publication No. M223FX
First Printing: December, 1991
Copyright 1991,1994 by Ramsey Electronics, Inc. All Rights Reserved.
Revision number 2.0
Published by Ramsey Electronics, Inc. Printed in the United States of
America
RAMSEY ELECTRONICS, INC.
Amateur Radio and Hobby Kits Dept.
793 Canning Parkway Victor, New York 14564
[phone] Fax: [phone]

TABLE OF CONTENTS:
The Ramsey FX-223 Transceiver Manual is organized into three sections:
General Reference Information, Kit Assembly Phase 1, and Kit Assembly
Phase 2.
FX-223 GENERAL REFERENCE INFORMATION
Notice Regarding FX-223 Frequency Coverage ................................4
FX-223 Technical Specifications........................................................5
Introduction to Transceiver Kit Assembly...........................................6
A Message to Ham Radio Beginners .................................................8
DC Power Supply Considerations......................................................9
Enclosure and Hardware Considerations.........................................10
Ramsey CFX Transceiver Enclosure Kit..........................................12
Guide to PC-board I/O Connections.................................................14
Microphone & Speaker Information..................................................16
A Few Antenna Considerations........................................................16
VHF Packet Data Operation.............................................................17
FX-series Transceiver Block Diagram..............................................18
FX-223 Circuit Overview...................................................................19
Quick-Reference Programming Guide .............................................27
FX-223 Programming Worksheet.....................................................31
Using Computer BASIC as a Programming Aid...............................32
How to Activate an Auxiliary (non-Standard) Offset.........................35
Programming "Minus" Offsets...........................................................36
FX-223 Programming Summary.......................................................38
Basics of Externally-Controlled Frequency Switching......................38
FX-series GLOSSARY of Terms ......................................................40
Troubleshooting Guide .....................................................................46
Note on Replacement Parts..............................................................49
RAMSEY ELECTRONICS FX Kit Warranty Terms..........................51
FX-223 Master Component Index ....................................................53
KIT ASSEMBLY: PHASE 1
Assembly and Explanation of Circuit Stages A through F
(including Kit Parts List, kit-building tips and preliminary receiver & VCO
test)............................................................................................60 - 86
KIT ASSEMBLY: PHASE 2
Assembly and Explanation of Circuit Stages G through TX (includes all
Frequency Programming directions plus alignment and transmitter tuneup.)
................................................................................................ 87 - 125
Optional Helical Filter Installation...................................................126
Technicians Notes..........................................................................127

IMPORTANT NOTICE
The Ramsey FX-223 VHF FM Transceiver is capable of
TRANSMITTING as well as receiving on any frequency in the
range of 220 to 240 MHz, making it suitable for a wide range of
VHF communications requirements. Operation of the Transmit
function of this equipment requires an appropriate license
issued by the Federal Communications Commission (FCC) for
the class of operation intended. The FCC issued NOVICE
CLASS license or higher is required for operation in the
Amateur 220 MHz band (222 to 225 MHz). Proper licensing is
required for MARS or CAP operations. Amateur licensees are
required by the FCC to maintain strict control over their
equipment to prevent unlicensed operation either in the
amateur band or outside it. FCC regulations ENFORCE severe
penalties for unlicensed operation of radio transmitting
equipment and for interference with other communications
services, whether malicious or accidental. Ramsey Electronics,
Inc. sells the FX-223 transceiver solely for correctly-licensed
operation. It is the sole responsibility of builders and operators
of this RF electronics device, capable of emissions controlled
by FCC Rules, to understand and comply with those rules.

SPECIFICATIONS FOR THE RAMSEY FX-223
General:
Frequency Range: Any 20 MHz segment between 220 and 240
MHz
Tuning: Diode-programmable PLL synthesis 12 front
panel selected frequency pairs, easily
expandable by switches, microprocessors,
computers, etc.
Programming: 5 KHz steps with programmable offsets
Transmit Offset: Programmable: Simplex, +1.6, -1.6, Aux
Mode: NBFM
Packet (Data) Operation: All rates incl. 9600 baud . 5-pin DIN jack
(TXD, RXD, PTT, +12VDC, GND)
Packet RX Audio: Speaker, discriminator or true FSK
Power Requirement: 13.6V DC +/-10% (Negative ground)
Power Consumption: 1.0 A Transmit (for 5 watts RF output) 200
ma. (Receive, no signal)
Antenna Impedance: 50 ohms
Microphone Impedance: 600 ohms or high impedance
T-R switching: PIN diodes
PTT circuit: Solid State (for standard ICOM-type
speaker/mic connection)
Semiconductors: 10 IC's, 16 transistors, 24 diodes (plus
programming diodes)
Transmitter:
Final Power Output: 3-5 watts RF
Final Output Stage: MRF237 or equivalent
Modulation: True direct FM
Max frequency deviation: +/- 25 KHz, +/- 5 KHz NBFM
Modulation distortion: Less than 5%
Receiver:
Circuitry: Double-conversion superhet
First IF: 21.4 MHz
Second IF: 455 KHz
Sensitivity: 12 db. SINAD less than 0.35 uv
Selectivity: 7 KHz (-6db.), 15 KHz (-60db.)
Squelch sensitivity: Less than 0.25 uv
Audio output: More than 2.0 watts
Circuit access points: COR, PL tone input, FSK demod. +12V,
+8V, +5V, PLL programming.

INTRODUCTION
to FX-series VHF Transceiver Kit Assembly
For the 1990's, Ramsey Electronics has adopted a "Learn As You Build"
philosophy for ALL our electronics kits. We feel that licensed ham operators
should know about the equipment they use, and also should have the desire
to understand how their gear works. Additionally, it has been our corporate
response to all those urgings by public officials that both students and their
parents need to become sharper in science and math. This "Learn as You
Build" approach to electronics hobby kits is now evident in all Ramsey
Electronics build-it-yourself kits from our under-$5 student kits up to this
synthesized VHF transceiver suitable for ham radio and public service
applications alike.
We think that "learning (and UNDERSTANDING) as we build" is especially
essential in a more sophisticated project such as the Ramsey FX-series VHF
transceivers. In fact, we are so convinced of this basic need that this kit
instruction manual departs from the traditional scheme of separating
assembly directions from a "theory of operation."
The FX-series of Ramsey VHF/UHF Transceivers puts today's FM 2-way
radio technology back in YOUR hands at a budget price. Our idea of
"budget" looks far beyond the modest purchase price to our goal that you
can maintain your FX- unit in good operating readiness with no need for
expensive shop service. On the other hand, we also have made the
transceiver design as abuse-proof and rugged as possible. "Alignment,"
traditionally an intimidating many steps process is very easy, quick and fool-
proof in this circuit design.
Instead of separate stage-by-stage assembly directions plus separate theory
information, these FX- instruction booklets highlight your transceiver's
operational theory, often a single component at a time, with actual
construction steps provided as follow-up after each explanation. The
assembly sequences are easy to find in the following pages. You indeed
have the freedom to solder first and read all about it later. We hope, though,
that you'll take it easy, learning as you build, and then enjoy the reliability of
your Ramsey FX Transceiver for a long time to come.
The "style" of our kit-building directions presumes that you are peeking at
our multi-color parts layout sheet while seeing that the very same parts
outlines are imprinted on the component side of your FX- PC-board. Our
smaller kits do not justify any need for on-board imprinting (silk-screening).
Therefore, such kits provide more detailed published explanations for
identifying correct locations for inserting and soldering parts.
You'll install EVERY FX- part perfectly by using our simple step-by-step kit
building process. And you'll know the WHY of most assembly steps, if not all
of them. Before you start, THINK about what you'll create from those bags of

parts as a finished product! For a minimal investment of your time as well as
your well-earned money, you will have a VHF FM voice-data transceiver that
you will truly own. Real "owning" ultimately means knowing how to maintain
and understand something that we have, in contrast to merely possessing a
thing because you spent the bucks to do so. You'll have the flexibility of 12
channels chosen by YOU with the easy ability to change or expand. You'll
have both FM voice and high-speed data capability. When you're ready, you
can experiment with many different enhancements, concentrating on those
truly useful to you. If there's ever a problem, you won't think twice about
digging in and fixing it. Whenever you decide you could use still another
VHF/UHF FM/data transceiver at a budget price, you'll know with confidence
that an FX-series kit is the right way to go.
What's faster: turning your FX- Transceiver to any one of 12 possible
channels programmed by you, or trying to remember again exactly how to
use the memory pre-sets of your HT, or your HF rig, or the VCR, or the
microwave?
Let's learn about and build up an FX- VHF FM Transceiver!

A MESSAGE TO HAM RADIO BEGINNERS:
If you have just earned your Novice or Technician license, or are studying for
either of them right now, we'd like to say a special Thank You for choosing
this Ramsey VHF/UHF transceiver as part of your ham radio beginnings. We
have tried to make this instruction manual as clear as possible. However,
there are some VHF radio "basics" covered by the FCC question pools for all
ham license study guides that we must presume that you have studied and
understood.
Here is a simple guide to selected Technician Class questions to help with
any review you wish to make before building:
FCC Subelement 3AA (Selected Rules): 4.2 11-1.1 12.5 15.2
FCC Subelement 3AB (Operating Procedures): 2-1.1 2-1.2 2-1.3 2-1.4
2-1.5 2-2.1 2-2.2 2-3.1 3.2 6-3.1
FCC Subelement 3AC (Propagation): It's up to you to understand the
characteristic differences among HF
(shortwave), VHF and UHF
communications.
FCC Subelement 3AD (Amateur Radio Practice): All of this is fundamental
know-how for hams. In working on this
project, be especially familiar with: 1-1.1 1-
1.2 1-1.3 7.1 9.1 through 9.5 (dummy
loads)
FCC Subelement 3AE (Electrical Principles) 3AF (Circuit Components)
Please know ALL of this.
FCC Subelement 3AG (Practical Circuits): 4.21
FCC Subelement 3AH (Signals and Emissions): 1.1 2-1.1 2-4.1 2-6.2
2-7.1 4.1 6-1.2 7-1.1 7-2.1 7-2.2
FCC Subelement 3AI (Antennas and Transmission Lines): You will want
to know all of this, if you don't want to take
all your savings from building your own
transceiver and spend it on a commercially-
built antenna. Very good VHF antennas are
easy and inexpensive to build yourself!

DC POWER SUPPLY CONSIDERATIONS :
Your Ramsey FM Transceiver is designed to operate from any stable DC
voltage source in the 12 to 15 volt range, from typical car, boat or plane 12V
systems to a wide variety of battery packs or AC-powered DC sources. In a
pinch, you can get on the air for quite a while with 8 to 10 ordinary "D" cells!
Our lab tests show only a .93 amp current draw for 5 watts of RF output. We
have just a few points of advice and caution:
1. Your DC supply should be able to provide a minimum of 1.0 amperes in
continuous service.
2. Any battery setup capable of supplying 12-15VDC will serve quite well.
3. Use of wall plug power supplies is NOT recommended. Obviously,
12VAC output is not suitable. Most DC output units do not have
adequate voltage regulation.
4. Turn your transceiver OFF before re-starting the vehicle in which it has
been installed.
5. Replace F1 only with a 1 amp fuse.
6. If you power your transceiver from the +12V accessory voltage available
from other equipment, be sure that source is rated for the 1 amp
required.
7. Your transceiver circuit includes noise suppression at the DC input and
additional filtering at the VCO, primarily to prevent ignition/alternator
noise from being introduced into the FM modulation. If you hear ignition
noise in the receiver, the vehicle has a serious general problem. Check
your transmitted signal on another receiver before mobile operation.
Radio Shack sells a variety of noise-suppression capacitors and
chokes. The ultimate solution, which has been tested, is to run the
transceiver from a smaller accessory battery.
If you plan to build a power supply for fixed-station use, there are numerous
construction articles in ham and electronics hobby publications. A
convenient new book featuring easy-to-find components and clear
explanations is Building Power Supplies (Radio Shack 276-5025.)

ENCLOSURE & HARDWARE CONSIDERATIONS:
The companion CFX case and knob kit is sold as a separate option ONLY
as an accommodation to those radio hams who have their own ideas or
resources for the "finishing touches."
However, a proper case for your FX-series is much more than a "finishing
touch," since the controls and jacks are panel mounted and proper RF
shielding is required.
The CFX case measures 9-3/4"L X 6"W X 1.5"H. 9" x 6" dimensions are
minimum for accommodating the PC board. The height may vary if you wish
to include an internal speaker, accessory PC boards, additional front panel
controls or indicators, etc. If you are new at all this and do not already own a
suitable enclosure plus that collection of hardware and knobs that every ham
seems to accumulate, here is what you need to know if you are hesitant to
purchase the CFX case kit:
A. "Blank" electronics enclosures have become among the most
expensive hardware in the industry, especially if you are buying just one
unit. This is because they are sold mainly to engineers and designers
for prototyping. A blank metal enclosure even slightly comparable to the
CFX case kit will run $30 to $80 or more. Even a plain aluminum
chassis box/cover will be around $15.00.
B. If you think there's any chance you someday may wish to sell or trade
your transceiver, you should be aware that units mounted in odd boxes
may have even less value than the bare circuit board with
documentation alone.
C. Many distributors have a minimum mail order of $25.00.
D. Your best chance for finding an inexpensive alternative case is to have
access to a lot of ham friends who tend to "collect stuff," or to browse
the catalogs or showrooms of electronic surplus dealers. You just might
find a gorgeous new box originally intended for somebody's ingenious
Ultra Modem, external disk drive or other dream gadget from two years
ago.
Are we trying to discourage you? No, not really! We know you can see that
there are good reasons to consider calling Ramsey Electronics and getting
your CFX case on its way while you work on the PC-board and PLL
Programming. However, we do not want you to feel "stuck" with our
recommended CFX enclosure, so we have worked up a detailed shopping
list for getting what you will need to make as attractive a finished unit as
possible with a single trip to the neighborhood Radio Shack store.

This $16 to $18 (plus tax) in basic hardware also presumes availability of all
needed drill bits and/or a reamer or punches of sufficient size to make the
needed access holes for the rear panel jacks. You'll also want to figure on
spray paint as well as a clear finish to protect the panel labels. Tools and
supplies, if not on hand, could cost much more than the CFX enclosure kit
itself.
In addition, this style of case will have to be utilized upside-down and also
length wise rather than as designed. This means that the top (black)
becomes the foundation for mounting the PC board with the standoffs as
well as securing the SO-239 antenna connector assembly. You may wish to
repaint the white bottom which now becomes the top. And, to use the project
labels (black lettering), you'll probably wish to repaint both pieces.
This adaptation of standard Radio Shack hardware is adequate for indoor or
occasional use but not recommended for mobile operation. We are happy to
provide this suggested alternative to the rugged case, knob and hardware kit
custom designed for the FX-series transceivers. The choice is yours!
Quantity RS Part No. Description 1992 Price ($)
1270-272/74 Deluxe Project Enclosure 8.79 or 10.79
1 set 274-section Pkg. of 3 or 4 knobs 2.00-3.00
1 set 270-201 Rub-on project labels 2.99
2 sets 276-195 PC-board standoffs/ 2.38

THE RAMSEY CFX TRANSCEIVER ENCLOSURE KIT:
The CFX Kit is very obviously a fair value and solid investment for the long
term performance and worth of your transceiver. As we have mentioned, we
make it "optional" ONLY because some of our ham customers have
specialized applications requiring only the basic PC board kit. We try
whenever we can to accommodate that important do-it-yourself spirit of ham
radio.
ENCLOSURE KIT PARTS LIST
Please check the boxes after the components have been identified, and it is
also handy at this time to “sort” the like components into groups or bins (an
egg carton does nicely) to avoid using the wrong component during
assembly.
 1 Steel bottom shell with 5 threaded PC board standoff posts
 1 Steel top shell
 1 Front control panel
 1 Rear panel with access holes to PC mounted jacks
 2 Steel side rails
 5 PC board standoff spacers
 5 #4 nuts to secure PC board on standoffs
 8 #4 screws to mount front and rear panels
 8 #6 screws for securing top and bottom to side rails
 1 large knob for Channel Selection Switch
 2 smaller knobs for Volume and Squelch controls
 4 self-adhesive rubber feet
CFX CASE ASSEMBLY PROCEDURE:
The purpose of these hardware parts is largely self-evident. We offer the
following suggestions for your convenience and to minimize wear and tear
on your factory-fresh CFX enclosure.
 1. Since the bottom shell might be used for several "test fittings" during
transceiver assembly, install the rubber feet right away to protect its
finish.
 2. Obviously, the PC board cannot be secured permanently to the
bottom section until after installation of L9, R32 and L10 AFTER
Alignment and before Transmitter tuneup.
 3. Install the strain-relief grommet supplied with the transceiver kit in its
rear panel hole. Pass the black ground wire through this grommet. The
red, fused wire will have to be unsoldered from S1 and then carefully

resoldered after passing through the grommet. Snap in the locking
section of the relief grommet only AFTER both the red and black wires
are in place.
 4. Remove the two screws from the SO-239 antenna connector, gently
bend the lugs as needed to match the rear panel holes, then secure the
jack and lugs to the rear panel.
 5. Since the top shell will not be needed until you're ready to go on the
air, keep it wrapped in protective material until you're really ready to use
it.
 6. The side rails may be installed to the bottom section at any time. To
prevent loss of the screws for the top, keep them loosely threaded in the
side rails.
 7. There is no point in securing the front panel controls and jacks to the
panel permanently until AFTER wiring the Channel Selection switch.
Note the locking hole for the switch in the front panel which mates the
tab on the front of the switch.
 8. Use care and a well chosen pair of pliers to secure the microphone
and speaker jacks to the front panel, so as not to scratch the panel.
 9. Bend the leads of the TX LED so that their tension presses the front
of the bulb against its front panel hole.
 10. Whenever you find it necessary to remove the top shell, "store" the
screws back into their holes in the side rails.
 11. If you decide to install a ribbon cable in the diode matrix for external
programming control, route the cable (folded at a right angle) on the
synthesizer and receiver side of the board so that it exits the case
between the rear panel and top cover, right above the DC power cord.
Do NOT route any such cable across the VCO and transmitter side.
 12. After the PC board is secured to the case bottom and front/rear
panels, it is a good idea to neaten up the wires to the controls and
jacks, bundling them at two or three points with tie wraps or cord.

GUIDE TO PC BOARD I/O CONNECTIONS:
In addition to primary interconnections required for jacks and controls, etc.,
your FX-series Transceiver PC board provides additional access to
operating voltages and circuit features to make later customizing as neat
and easy as possible. All these points are plainly marked on the board itself
and highlighted on the facing page, with a few other components for

1. PRIMARY CONNECTIONS:
PWR (near L20): +12-15 volts DC from S1.
GROUND: - DC from battery or power supply.
SPEAKER: both connections near U6 and U2.
MIKE: both connections (IN & GND) near notched end of U1.
SQUELCH: three connections near L1.
VOLUME: three connections near VR1 and C42.
ANTENNA: Center of SO-239 connected at "RF OUT" near C63.
CHANNEL SWITCH: Row of holes numbered 1 through 12. The switch
wiper (moving contact) is wired to +5V near "1."
EXT AUDIO: must be jumpered per options to enable pin 4 of J1.
2. TEST POINTS:
+12V, +8V, +5V, +8R, +8T permit checking for presence of those voltages.
+8R = Receive mode. +8T = Transmit.
TP1: For checking of VCO control voltage during alignment.
TP2: For checking VCO frequency with counter.
TP3: For checking U3 prescaler output with counter.
3. OPTIONS:
+12V, GND, +8V, +8T, +8R: provide supply voltage for accessories or
modifications designed by you. +8T and, +8R are near L9 and U4.
Jumper Options for Packet Operation: SPKR to EXT AUDIO: Speaker level
audio for packet (J1). DISC to EXT AUDIO: FM discriminator output for
packet.
COR: "Carrier Operated Relay" output from U1.
Binary Programming Holes: The row of holes alongside the Binary
Programming labels permits installation of ribbon cable for external
programming devices designed by the innovative amateur radio community.
switches or interfaces, or a row of internal DIP switches.

MICROPHONE & SPEAKER INFORMATION
The FX transceivers with the hardware supplied are designed to accept
standard ICOM or ICOM-compatible speaker-mikes such as MFJ-284. The
most conveniently available such unit is Radio Shack No.19-310.
Be aware that the receiver audio amplifier is capable of supplying a husky 2
watts or more of audio power and will drive full-size communications
speakers to excellent volume levels.
The PTT switching circuit can be activated simply by introducing a
resistance (e.g. 10K) from the microphone input to ground. This resistance is
enough to trigger the PTT circuit without interfering with the microphone
audio input.
Consider these factors in selecting microphone, speaker and/or speaker-
mike for FM voice operation. If your microphone and speaker preferences
differ radically from the use of a speaker/mike, and you do not wish to alter
the front panel, remember that you also have very easy access to mike and
speaker lines via J1, the packet connector.
A FEW ANTENNA CONSIDERATIONS
The idea of building your own transceiver is to save money and enjoy your
hobby. Effective VHF and UHF antennas are easy and inexpensive to build,
whether for fixed or mobile use. There are plenty of off-the-shelf antennas to
buy, but don't hesitate to "roll your own."
If you are a newcomer to ham radio, you'll discover many strong opinions
about the "best" antenna to use. A home-built ground plane vertical or even
a dipole can provide very satisfying results. Whether you need a gain factor
or directivity in your antenna depends on your operating goals. Regardless
of the style of antenna, it really pays to give serious attention to electrically-
solid, weatherproof connections of the coaxial cable to the antenna
elements. A simple antenna in good condition will outperform a fancy one
that's been neglected.
Popular ham magazines and ARRL publications provide plenty of antenna
building ideas. MFJ Enterprises offers good value in simple ready-to-use
antennas for 220 MHz.

VHF PACKET DATA OPERATION
Your FM transceiver was planned and designed to accommodate easy and
reliable VHF packet radio operation.
The J1 Packet I/O port can be quickly connected to many modern TNC's and
the Ramsey P-IBM or P64 Packet Modems with Radio Shack's shielded DIN
cable (42-2151). Otherwise, your first step is to prepare a reliable 5-
conductor cable with a 5-pin DIN plug (RS 274-003) at one end, and the
correct connector needed by your packet TNC at the other end. If you
salvage a "ready made" 5-pin DIN cable from something like a discarded
computer joystick, be sure that there are indeed 5 wires, or at least the ones
that are required by your TNC!
Consult your Packet TNC or Packet Modem documentation for ALL details
on hookup and operation. Pin 4 of J1 offers a choice of amplified and
squelched (speaker level) audio output or low-level (discriminator) output.
This choice is set up by the jumper wire positions clearly visible on the PC
board.
Packet RX Audio Jumper: Some TNC's require low-level audio from the
discriminator output of the FM detector, while others will accept speaker
output. Either is available in the FX transceivers. Simply install a jumper in
the appropriate location near VR1, to connect "EXT AUDIO" to either
"SPKR" for speaker audio or "DATA" for discriminator audio. Use scrap
resistor wire to make the jumper. You also can choose to wire these three
points to a miniature SPDT switch which you can mount on the rear panel
near the packet connector. OR, use a PC mount switch in the jumper area
itself.
Use this space to diagram your TNC cable connection:
DRAW YOUR TNC

RAMSEY FX-SERIES FM TRANSCEIVER
(With emphasis on the PLL and VCO)
MC13135

FX-223 CIRCUIT OVERVIEW
The FX-223 VHF FM Transceiver circuit theory is explained in progressive
stages and in some detail as part of our "Learn As You Build" approach to
electronic kits. Builders are encouraged to study and learn about a stage or
section, build it and then test it before going to the next stage. The circuit
explanations are necessarily written for people with all levels of experience,
starting with and FAVORING beginners.
Following is a straight and "minimally chatty" synopsis or overview of FX-
series technical information provided in the building stages. However, we'll
still follow the same stage-by-stage designations of the building process.
A: DC Power Input
Much of the circuitry operates on the regulated 8 volts supplied by voltage
regulator VR1. "+8R" or "+8T" are points where the regulated 8V output is
switched for Receive or Transmit by the PTT circuitry (Q12, U4c, U4d, Q13,
Q14).
The Receiver IC (U1) and the digital frequency synthesis circuit are powered
by +5 volts regulated by VR2. The op amps used in the circuit (U4 and U5)
operate from this single supply through the use of voltage divider networks
at the respective IC's. The full 12-15 volt input is supplied to the transmitter
RF output section and to the audio amplifier (U2).
Components L20 and C42 provide ignition noise filtering. The 5-pin DIN
Packet I/O Jack (J1) has pinouts corresponding to current conventions for
TNC's. Receiver audio to pin 4 may be taken from the amplified speaker
output, or from the FM discriminator output or from the true FSK data output
of U1. Selection is by a jumper wire on the PC-board.
B: Receiver Audio Amplifier
The LM380 is a self-contained general purpose audio amplifier capable of
over 2 watts audio output with a voltage gain of 50. Audio from the FM
discriminator (U1) is fed through C7 through the 10K volume control (R7) to
pin 2, the amplifier input. The amplified output at pin 8 is available through
C34 to both the speaker jack and pin 4 of the Packet I/O Jack. C41 in series
with R108 across this amplified output are good practice recommended to
prevent self-oscillation of the IC. Pin 1 is bypassed to ground through C48 in
normal operation.
If pin 1 is grounded directly, the internal bias of the LM380 is upset, and the
amplifier is silenced. Q6 is a simple switch. When 8 volts is applied through
R107 and D22 to the base of Q6, the transistor collector grounds pin 1 of
U2, thus silencing the receiver during transmit. The COR output of U1 (pin
16) also mutes the amplifier.