SYNTHESIZED AM
BROADCAST
TRANSMITTER KIT
Ramsey Electronics Model No. AM25
Ever want to be your own disc jockey or talk show host ? Build
and run your own AM broadcast station with this exciting kit !
The AM25 transmitter offers a professional quality signal that is
free from frequency drift and programmable to anywhere in the
AM broadcast band !
• Great for transmitting your tape deck, CD player, or voice
throughout the house, yard, or car !
• Powerful enough for high school, college, or neighborhood radio
station - in use the world over !
• Easily programmed via DIP switch to any clear channel on the AM
broadcast band - from 540 to 1710 KHz.
• Operates on the same principle as the “big-boys”, learn how
commercial transmitters work as well !
• Superior performance - transmits up to 1/4 mile !
• Adjustable transmit power level, low pass filtered output.
• Unit runs on 9 - 15 volts DC.
• Handy information on FCC rules and antenna hints.
• Complete and informative instructions guide you to a kit that works
the first time, every time - enhances resale value, too !
AM25 •1
PARTIAL LIST OF AVAILABLE KITS
RAMSEY TRANSMITTER KITS
• FM25B, FM100B FM Stereo Transmitters
• AM1, AM25 AM Transmitters
• TV6 Television Transmitter
• FM100B Professional FM Stereo Transmitter
RAMSEY RECEIVER KITS
• FR1 FM Broadcast Receiver
• AR1 Aircraft Band Receiver
• SR2 Shortwave Receiver
• AA7 Active Antenna
• SC1 Shortwave Converter
RAMSEY HOBBY KITS
• SG7 Personal Speed Radar
• SS70A Speech Scrambler
• MX5, MX10 Mixers
• MD3 Microwave Motion Detector
• PH10 Peak hold Meter
• STC1 Stereo Transmitter Companion
RAMSEY AMATEUR RADIO KITS
• HR Series HF All Mode Receivers
• QRP Series HF CW Transmitters
• CW7 CW Keyer
• CPO3 Code Practice Oscillator
• QRP Power Amplifiers
RAMSEY MINI-KITS
Many other kits are available for hobby, school, scouts and just plain FUN. New
kits are always under development. Write or call for our free Ramsey catalog.
AM25 Synthesized AM BROADCAST TRANSMITTER KIT INSTRUCTION MANUAL
Ramsey Electronics publication No. MAM25 Revision 1.0a
First printing: December 1996
COPYRIGHT
14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.
1994 by Ramsey Electronics, Inc. 590 Fishers Station Drive, Victor, New York
AM25 •2
Ramsey Publication No. MAM25
Manual Price Only $5.00
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
SYNTHESIZED AM
TRANSMITTER AM25
TABLE OF CONTENTS
Introduction to the AM25 ....................4
AM25 Circuit Description ....................4
Schematic Diagram ............................7
Parts Layout Diagram .........................8
Parts List .............................................9
Assembly Instructions .........................10
Frequency chart ..................................18
Alignment Procedures ........................19
Troubleshooting ..................................20
FCC Information .................................20
Antenna Experimenting ......................22
Ramsey Kit Warranty ..........................23
RAMSEY ELECTRONICS, INC.
AM25 •3
590 Fishers Station Drive
Victor, New York 14564
Phone (585) 924-4560
Fax (585) 924-4555
www.ramseykits.com
INTRODUCTION
Many times we wish to be heard via wireless communications. Whether
transmitting music or voice, what is required is a stable transmitter tunable to
an unused frequency on the broadcast band. Here at Ramsey, we have
produced several models of FM transmitters and an entry level AM
transmitter. Recent requests for a synthesized AM type transmitter have
been answered with this kit.
The Ramsey AM25 transmitter is a true broadcast transmitter, which any
person may build and use in accordance with the rules of one’s national
telecommunications authority. For U.S. residents, that authority is the
Federal Communications Commission (FCC). The AM25’s low power
broadcasting capabilities make it practical for many uses, but one should
remember that this transmitter is definitely not a toy. We will refer to the FCC
regulations frequently in this manual and provide you with some information
necessary to enjoy the AM25’s capabilities in accordance with the law.
Typical uses for the AM25 include:
• Student operated school radio stations.
• Re-broadcasting received audio for shortwave or HAM radio use.
• Listening aid for auditoriums, churches, or other events.
• Short range experimentation.
We think that you will be very pleased with the transmitting range, audio
quality, and frequency stability of this build it yourself AM transmitter. If you
follow our assembly instructions carefully and use your AM25 in accordance
with applicable FCC rules, a whole new world of sharing news, views, and
music with friends and neighbors awaits you.
AM25 CIRCUIT DESCRIPTION
Before we dive into the circuit description of the AM transmitter, first we’ll
describe exactly what we mean when we say “AM”. An (A)amplitude (M)
modulated signal is actually a combination of two signals. The high
frequency carrier is the frequency that one will tune on the radio receiver’s
dial, from 530 to 1750 KHz. The modulation is the audio information that
rides “on top” of the high frequency carrier, resulting in a changing of the
level, or amplitude, of the output waveform. Take a glance at the schematic
and follow along at how we produce this AM waveform.
The RF oscillator consists of Q4, Q6 and associated components. The
frequency of operation is determined by selecting the proper values for L5,
AM25 •4
Low Frequency Audio Waveform
High Frequency Radio Carrier
Amplitude Modulated S ignal
L7 and varactor diode D2. The Phase-Locked-Loop portion of the circuit
controls the capacitance of the varactor diode to keep the frequency exactly as
selected. We’ll examine that in more detail later. A buffer amplifier, Q5
increases the output of the oscillator enough to drive final RF amplifier Q3.
The audio input path is routed from J2, the audio input source, to opamp U1
which both amplifies and level shifts the incoming signal. Transistors Q8, Q9
are used to increase the current output capacity of the opamp in order to
modulate the final RF amplifier stage. These transistors are biased to be
operating in the linear region with a no signal output level of exactly one-half
the supply voltage as set by pot R23. The incoming audio signal is therefore
amplified undistorted (for great sounding audio). The audio input level to the
amplifier can be adjusted using R31, the input level adjustment.
Power FET transistor Q3, is the “power amplifier” section of the circuit. Its drain
voltage is supplied through matching auto-transformer L4 from Q8 and Q9,
thus producing an AM output waveform. This signal is then low pass filtered
using C1 - C7 and inductors L1 - 3.
Notice that the audio information is applied at the power amplifier stage. This is
referred to as “high level” modulation, and is commonly used for high power
AM broadcast stations. The distinct advantage to this is that the RF amplifier
need not be biased for linear operation, which is not only very inefficient but
somewhat complicated also. It is much easier and cheaper to manufacture a
linear amplifier for the relatively low frequency audio range, than to perfectly
linear amplify a low level AM RF waveform to a high power RF level. The main
disadvantage of high level modulation is that the audio modulator’s power must
be half that of the final transmitter, not too tough for our low power kit, but try to
imagine the amplifier for a 50,000 watt AM broadcast station! Boy, that audio
amp would sure crank the ‘ol car stereo!
It should also be stated that, due to the linear operation of the amplifiers in this
circuit (transistors Q8 and Q9 biased partly “on”), this circuit will consume a fair
AM25 •5
amount of power. It is not recommended that a common 9V battery be used
to power this kit. Instead, a battery “pack” consisting of eight 1.5 volt cells, a
12V sealed battery, or other external 12V DC supply may be used.
The Phase-Locked-Loop portion of the circuit is responsible for maintaining
the frequency of the transmitter. U2 is the brains of the whole circuit. This
chip looks at the settings of each switch in the dip switch S1 and from this
information, calculates the desired frequency. These switches are
programmed in Binary, you simply add up the closed switch positions: 1, 2,
4, 8 all the way up to 128 to make any number between 0 and 255. The final
output frequency will be the programmed number times 10 KHz. For the AM
broadcast band, 540 - 1710 KHz, we’ll only need to program between 54 and
171. For example, if a broadcast frequency of 550 KHz if desired, we’ll need
a programmed number of 55 (55 times 10 KHz equals 550 KHz). Closing
switches 32, 16, 4, 2 and 1 all adds up to 55, our desired number. As
another example, what would we need for 1520 KHz? Our desired program
number is 152 and we’ll have to close switches 128, 16, and 8 which adds
up to 152! It really is quite easy and fun to find the right switches to close!
The phase locked loop (PLL) synthesizer IC takes the 10.24 MHz crystal
frequency and divides it internally by 1024 to obtain a stable reference
frequency of 10 KHz for the PLL’s phase detector. U1 also takes the output
frequency from the VCO (Voltage Controlled Oscillator) and divides it by a
number that we’ll call ‘N’. N is the frequency data that was sent by U2, and N
is always equal to the desired frequency in megahertz times 10.
Using the example from above, a frequency of 550 KHz gives an N of 55, so
the frequency from the oscillator is divided by 55 and then compared with the
reference frequency of 10 KHz. If the desired frequency is less than the
reference frequency U2 increases the output voltage on pin 7. This increases
the voltage across diode D2, a varactor diode. As the voltage across the
varactor increases, it causes a decrease in capacitance (Increasing reverse
bias essentially increases the distance between the capacitor’s plates by
increasing the depletion region in the diode (C = kA/d). The decrease in
capacitance causes an increase in the VCO’s RF frequency (f = 1/[2π (LC)
½
]), bringing the AM25’s output frequency back on frequency. If the desired
frequency is higher than the reference, pin 7 is driven low. If the frequency is
just right then pin 7 becomes a high impedance, basically disconnecting it
from the circuit so it will cause no change in the voltage on D2. The voltage
changes on pin 7 are filtered by R25, C26, R28, and C28 to provide a
steady, noise free tuning voltage for D2. In this way the output frequency of
the oscillator is "locked" to that desired by U2. When the frequency is locked,
U2 will cause led D3 to be brightly lit. If D3 is dim or off there is a problem
and the frequency is not locked. If the frequency starts to drift for any reason
(such as a temperature change) then U2 instantly corrects the tuning voltage
to bring it back to the proper frequency.
AM25 •6
AM25 •7
PARTS LAYOUT DIAGRAM
AM25 •8
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