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AMT3000 and AMT5000 AM Radio Broadcast Transmitter Kits

Setting the Quality Standard for High Quality, Low Power, FCC Part 15 Transmitters

 

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AMT3000 Detailed Description

 

 

History

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.

Frequency Synthesis

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%.

RF and Modulator Stages


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.

Easy Tuning


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.

 

High Fidelity Audio Modulation

 

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.

 

 

 

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.

 

Hum Reduction

 

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. 

Kit Assembly Made Easy

 

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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