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AMT3000 and AMT5000 AM Radio Broadcast Transmitter Kits

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AMT5000 PRO AM TRANSMITTER KIT

Product Details

 

The AMT5000 is designed to equal or exceed the range and audio quality of all other FCC Part 15 AM transmitters currently being sold at any price. It retains the proven audio preamp/compressor/limiter and RF frequency synthesizer from the widely acclaimed AMT3000 while adding design innovations in the RF output, antenna tuning stages, audio input stages, and versatile power supply stage capable of operating from a 12 VAC wall adapter, 12-15 VDC wall adapter, or a 12 VDC battery.

RF Output and Antenna Tuning Stages

The RF output stage uses a low-on-resistance power MOSFET transistor operating in the Class E switching mode at 98% drain efficiency when tuned per instructions. Only 2% of the available 100mW input power is dissipated in the transistor. Power input to the final RF stage is adjustable to comply with the 100mW limit mandated by the FCC for Part 15 AM transmitters. 

 

A large custom toroid coil is employed for tuning the antenna in the 1350kHz to 1700kHz frequency range. The toroid has an iron-powder core selected for maximum efficiency in this frequency range. For accurate antenna tuning, inductance taps are selected by jumpers and an extremely low-loss air-piston trimmer capacitor provides fine tuning. The tapped toroid and trimmer combination provide the capability to tune a variety of antennas such as the supplied 118" wire, a 102" CB whip antenna, or a 118" copper or aluminum pipe antenna of various diameters.

 

For antenna tuning at frequencies in the remainder of the broadcast band below 1350kHz, physically smaller commodity inductors and an additional FILMTRIM trimmer capacitor can be selected by jumpers. Efficiency is reduced, but the RF output is still very strong and is more than adequate for in-home broadcasting or other shorter range applications.

 

Spurious emissions are 43 dB down at the second harmonic and even lower at higher frequencies. This greatly exceeds the FCC limit of 20 dB.

Frequency Synthesis

The AMT5000 employs a crystal-controlled frequency synthesizer to ensure accurate tuning and drift-free frequency stability. Frequency accuracy is .003%. The synthesizer tunes the entire AM broadcast band in precise standard-channel-spacing increments by means of an 8-position DIP switch on the board. A trimmer capacitor in the oscillator circuit allows fine-tuning the oscillator frequency to zero-beat with a broadcast station for precise frequency adjustment.

 

There are two options for channel spacing : 10 kHz for North and South America and 9 kHz for most 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.

Tuning

Meter test points are provided on the circuit board to measure RF stage input voltage and current with a user-supplied multi-meter. Just insert the probes of an inexpensive digital or analog multi-meter into the test-point holes on the circuit board.

 

Tuning steps:

 

  1. set the operating frequency on the 8-position DIP switch.
  2. set the RF output tuning range for your operating frequency by moving a jumper plug on the board as specified in the tables in the instruction manual.
  3. peak the air-piston trimmer capacitor for maximum RF input stage current, then rotate the trimmer 1 turn clockwise for peak Class E efficiency.
  4. set the RF input power to 100 mW by rotating the "RF INPUT POWER ADJ" potentiometer while observing the "RF INPUT CURRENT" and "RF INPUT VOLTAGE" test points. A table is provided in the instruction manual showing all combinations of current and voltage that result in 100 mW input power.

Antenna and Ground Connections

Screw terminals are provided on the board for antenna and ground connections. A gas-discharge surge arrestor and a bleeder resistor protect the board from static voltage buildup or surges induced on the antenna by wind or lightning in the vicinity.

 

A good earth ground connection is crucial for proper operation of a wire or vertical antenna. For indoor use, the included ground wire may be connected to the screw on the AC wall socket or a copper cold water pipe, whichever performs for your needs. For an outdoor vertical antenna installation, the ground connection serves two purposes: RF grounding and lightning protection. As a minimum, an 8 ft. copper-clad steel ground rod will serve for lightning protection and provide a minimal RF ground. The RF ground can be enhanced for much better range by adding a minimum of 4 10ft (or longer) radial wires fanning out from the ground rod on the surface or buried an inch or so to avoid tripping hazards.

Audio Inputs

Inputs are provided for both consumer audio and professional audio sources.

 

There are two RCA jacks for Consumer Audio inputs. The signals from the two jacks are mixed equally to mono for connecting a stereo audio source such as a CD player, MP3 player, or the audio output of a computer. They can also be used to equally mix two mono sources.

 

A screw-terminal block on the board allows connecting the individual wires from a Pro Audio source. The inputs are static surge protected. Pro Audio sources are suited to long cable runs and noisy electrical environments. They employ a balanced, differential signal carried on a twisted-pair and a separate audio ground wire. Since noise and hum are induced equally on the twisted-pair conductors, they are cancelled by the differential receiver. Pro Audio source devices typically have XLR or TRS (tip-ring-sleeve) outputs.

Audio Processing

All commercial AM broadcast stations process their audio with "compression" and "limiting". Compression (also known as "automatic level control") tends to maintain a fairly constant audio level through variations between loud and quiet levels. Limiting prevents the audio from exceeding a certain preset level on peaks. This prevents over modulating the RF signal beyond 100%, which would cause considerable distortion.

 

The AMT5000 provides true compression and limiting functionality by using a very effective commercial IC chip.

 

Three front-panel controls allow you to set input GAIN, MODULATION level and COMPRESSION ratio.  The GAIN control allows optimizing the input audio level to the best value for the input to the preamplifier circuit. The MODULATION control sets the peak limiting level which produces 100% modulation. Signal levels that exceed the set MODULATION level are limited by compressing at a very high, 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. Typically a 2:1 or lower compression setting is optimum. If external audio processing is desired, both compression and limiting can be individually disabled by control adjustment (MODULATION = maximum, COMPRESSION = minimum).

Modulation Stage

The modulation stage employs a power MOSFET transistor operating as a common-source class A audio amplifier, providing the DC voltage source with low-distortion audio to the final RF output. Maximum modulation is 100%, but there is ample headroom to allow positive modulation up to 130%. Positive modulation exceeding 100% normally causes unpleasant distortion, except when you use an external audio processor capable of asymmetrical limiting (separate controls for positive and negative peak limiting).

 

Audio response from audio input to the final modulation transistor output:

20 Hz - 20 kHz 0.5 dB

Jumper selects flat response or NRSC (National Radio Systems Committee) standard pre-emphasis.

 

Modulated RF 3-dB Response:

20 Hz - 16.5 kHz audio response) with 20-30 ohm RF ground (typical for a reasonably good RF ground).

Modulated audio response is higher when supplemental inductors are jumper-selected at frequencies below 1350 kHz.

 

Audio response from the input terminals to the final modulation transistor output is 20Hz to 20 kHz 1 dB. The actual modulated carrier audio frequency response is down 3 dB at 16.5 kHz due to bandwidth limiting effect of the high-Q toroid output circuit. Also, AM radio receivers generally limit high frequency audio response to about 5 kHz because they must reject adjacent 10kHz channels. A jumper option enables treble-boost similar to the NRSC curve, but with mid-point boost lowered to 1.7 kHz instead of 3.5 kHz. The lower mid-point boost produces a better balance of bass and treble for the AMT5000 for typical AM receivers. The maximum boost is 9.5 dB at 10 kHz. Think of this option as "NSRC boost" with added audio equalization. There is no "brick wall" cutoff at 10 kHz as specified in the NRSC standard.

 

If you have a Hi-Fi AM receiver or a wide pass band option on your receiver, you will experience near FM audio quality. It may sound better with or without the NRSC boost, depending on the receiver.

Power Conditioning

An extremely clean, well regulated DC power supply is an important requirement for any AM transmitter. To ensure this requirement is met, all rectification, regulation and filtering is done on the board. This guarantees clean power to the circuits even with an unregulated power wall adapter.

 

The standard wall adapter supplied with the kit is a 120 VAC-to-12 VAC transformer. A bridge rectifier on the board converts the 12 VAC to DC, which is then fed to the on-board 12 VDC and 5 VDC voltage regulators. Jumper plugs on the board can be configured to enable operation from a 12-15 VDC wall adapter (unregulated or regulated) or a 12 VDC battery.

 

To comply with FCC power line conducted emissions rules, an RF filter is included on the board at the power input point. Jumper plug options allow bypassing the filter.

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 make component location easy.

 

 

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