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History
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The AMT3000 AM transmitter kit is the result of years of
refinements toward one goal: to create a low-power AM transmitter that
provides a high quality signal at an affordable price. As you read
through the following description and compare the features with all other
transmitters currently on the market, you will realize that the AMT3000
is exceptional in both performance and price.
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Frequency Synthesis
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The AMT3000 employs a crystal-controlled frequency synthesizer
to ensure accurate tuning and drift-free frequency stability. The
synthesizer tunes the entire AM broadcast band in precise standard
channel-spacing steps. There are
two options for channel spacing : 10 kHz for North and South America and
9 kHz for other regions of the world (option is selected when ordering). Frequency coverage is 530 kHz - 1700
kHz for the 10 kHz channel spacing option and 522 KHz - 1710 kHz for the
9 kHz channel spacing options.
Frequency accuracy is ± .003%.
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RF and Modulator Stages
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Power input to the final RF stage is 100 milliwatts,
the maximum allowed by the FCC for Part 15 operation without a
license. The RF output and modulator stages use a unique and
precisely engineered 5-transistor configuration that results in low
distortion modulation right up 100%. A tunable pi-network matches
the RF output to the 118" antenna (length limited by FCC
regulations) without the need for any additional loading coils.
Harmonic radiation is highly attenuated by the antenna tuning network.
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Easy Tuning
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Tuning is an easy 3-step process:
(1) set
the synthesizer frequency on the 8-position DIP switch
(2) set
the RF output tuning range on the 4-position DIP switch
(3) peak
the RF output trimmer capacitor.
A tuning meter circuit is included on the
board to assist peaking. Just connect an inexpensive digital or
analog multi-meter to the test terminals and peak the trimmer for maximum
DC voltage.
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High Fidelity Audio Modulation
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Audio Frequency Response
Audio response from the input jacks all the way through to the
modulated RF output is 20Hz to 20 kHz ±1 dB with the treble boost
jumper in the OFF position. AM
radio receivers generally limit high frequency audio response to about
5kHz because they must be designed to reject adjacent channels which are
spaced 10kHz apart. Due to receiver audio roll off, you may find that the
treble boost jumper option gives a more pleasing sound. If you have a Hi-Fi AM receiver or a wide pass band option on your
receiver, you will experience near FM audio quality.
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Audio Processing
All commercial AM broadcast stations process their audio with
"compression" and "limiting". Compression reduces the
audio level variation between loud and soft passages. This makes the
signal sound louder, gives it "punch" and automatically adjusts
for loud and quiet passages. Limiting prevents the audio from over
modulating the RF signal beyond 100%, which can introduce considerable
distortion.
The AMT3000 provides true compressor/limiter functionality
thanks to a very effective integrated circuit designed specifically for
this purpose.
Three front-panel controls allow you to set input gain,
modulation level and compression ratio. The modulation control
(which actually sets the limiting level) may be set at 100% (or lower if
desired). Signal levels that exceed the set modulation level
are limited by compressing at a hefty, fixed 15:1 dB ratio. The
compression control adjusts the compression ratio over a range of 1:1 to
5:1 dB for all input signal levels. This adjustment range is more
than you will likely ever want. At the highest compression setting (5:1),
the background noise level of your audio source will be quite noticeable
between songs. Typically a 2:1 compression setting is optimum.
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Hum Reduction
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Anyone familiar with low-power AM transmitters knows that many
are susceptible to hum problems. Often 60 Hz or 120 Hz hum is
introduced into the received signal from several possible sources: poor
power supply filtering, power supply rectifier non-linearities,
and hum introduced by interaction with AC building wiring. The
AMT3000 provides solutions for all of these potential problems. Two
voltage regulators ensure excellent voltage regulation and filtering
tailored to the AC wall adapter provided with the kit. The diodes
comprising the on-board bridge rectifier circuit are individually
bypassed with capacitors to provide a low RF impedance to the power line
regardless of the on/off diode conduction state. On-board RF filter
inductors can optionally be configured with jumper plugs to isolate the
transmitter ground from the power lines, and from paths to ground through
the audio inputs, to help prevent hum at the receiver due to interaction
with AC building wiring. A ground wire is included for connection
to a separate earth ground. The bottom line is that you have all
the options to alleviate any hum problems you may experience in your
unique situation.
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Kit Assembly Made Easy
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We devoted just as much attention to making your kit-building
experience easy and accurate as we did to refining performance. Our
assembly manual guides you, step-by-step, through the building process to
ensure completeness and accuracy. The manual helps you to identify
components and gives tips along the way. All components, including
controls and jacks, are mounted on the printed circuit board eliminating
difficult discrete wiring. There are no holes to drill or coils to
wind. The high-quality circuit board has silk screened component labeling
to assist identifying all component locations.
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