TX 150 Suitable for Backup, LPFMs

Just as we have all gleaned benefits from Space-Age spinoffs over the years, many times other advances beget products and ideas that have more wide-ranging benefits.
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Just as we have all gleaned benefits from Space-Age spinoffs over the years, many times other advances beget products and ideas that have more wide-ranging benefits.
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Simple "plug & play" installation

Self-contained

Small size

Lightweight, switching power supply

Low heat output

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Front-panel RF and mod monitor jacks

No momentary remote on/off

Poor recovery from momentary power outage

Reach Progressive Concepts at (630) 736-9822. For information on the TX 150, visit the Broadcast Warehouse Web site at www.broadcastwarehouse.com.
This has certainly been true in the broadcast industry, where advances in computer technology have revolutionized our audio chains and transmitter controls. It is also true of the communications industry, where developments in solid-state power amplifier devices design have carried over to broadcast transmitters.

Frequency agile

A couple of young radio engineers who had for several years been involved in the English low-power radio scene have utilized this new technology to develop a self-contained, compact, frequency-agile FM transmitter suitable for standalone LPFM or emergency backup use or to drive an external power amplifier for high-power applications.

The TX 150 is manufactured by Broadcast Warehouse, headquartered in Wimbledon in the United Kingdom. Progressive Concepts, the exclusive U.S. agent for Broadcast Warehouse, sent us a TX 150 to evaluate.

The transmitter is a two-rack-unit chassis that weighs about three pounds. Rear-panel external connections include AC power input, left and right audio inputs, composite input/loop, RF out and control logic. On the front panel are connections for RF monitor and modulation monitor.

An automatic-mode switching power supply is employed, giving the unit the capability of operating on an AC power source of anywhere from 90 to 265 volts without user configuration. In this respect, it quite literally is plug and play.

The user has a lot of options for feeding audio to the TX 150. Using the internal stereo generator and limiter, balanced left/right audio can be connected via RF-shielded XLR connectors anywhere from -10 to +18 dB. If it is desirable to use an external stereo generator, baseband audio can be connected via a rear-panel BNC jack.

Stereo gen

If the built-in stereo generator/audio limiter is used, the output can be looped through an external device, such as an RDS encoder, SCA generator or composite clipper.

The internal audio limiter is dual-mode and can be jumper selected to hard or soft action. The entire audio limiter section can be disabled, or the limiter left in the circuit with the internal clipper disabled. The unit can be operated mono from either a single-channel source or in a summing mode from two channels, with or without pilot. Pre-emphasis is selectable at 0/50/75 uS.

Frequency is selectable by two means. One is by internal binary switches. The frequency simply is dialed in on the four switches in 100 kHz increments. A 12.5 kHz offset is selectable.

The other means is by the front-panel LCD menu system and pushbuttons. By setting 4440 on the internal binary switches, front-panel frequency selection is enabled. Any 100 kHz frequency from 87.5 to 108 MHz is selectable. RF output is inhibited until the dual-speed PLL has locked.

Power level is front-panel adjustable (using a greenie or other diddle-stick) from 30 to 200 watts. The power amplifier uses a dual-gate Gemini packaged FET and employs an internal harmonic filter. Power control is achieved by varying the drain voltage to the PA FET. The PA is temperature- and VSWR-protected.

During our evaluation, we operated the unit for several hours at the 150-watt level with no discernable problems. Even at that power level, the unit ran cool to the touch.

A single three-inch rear-panel muffin fan provides cooling for both the PA and the switching power supply. The fan generates some noise, so the TX 150 normally could not be mounted in a studio rack where microphones are employed.

On-screen info

The front-panel LCD readout provides operating and configuration information in several screens.

The main parameter screen shows frequency, power and peak deviation. These indications appeared to be quite accurate. Power metering was very close to the Bird 43 wattmeter that we had installed in line with the output.

An RF power screen displays forward and reflected power. A peak deviation screen provides a bar-graph indication of deviation along with an alphanumeric indication of peak deviation. An alarm is displayed when peak deviation exceeds 75 kHz.

Another screen shows the amount of gain reduction (0 to -24 dB). Other screens show power supply voltage and PA temperature.

We found that the TX 150 performed well. The sound was transparent, although at higher input levels the limiter action was somewhat audible.

A problem occurred when we simulated a brief power interruption. When this happened, the front-panel LCD screen backlight would flash on and off repeatedly and the unit would not come on. To restore operation, the power had to be turned off for several seconds, then restored. This indicates that a UPS should probably be used with the unit.

This is a versatile, compact, rugged little transmitter. LPFM broadcasters will find that it is all they need between their studio equipment and transmission line to get on the air.

Commercial and NCFM broadcasters will find that the TX 150 is a ready-to-use exciter and in some cases may be able to replace the exciter and IPA transmitter stages. Another application may be a studio auxiliary transmitter, providing a third level of backup should the main and aux transmitters fail.

The TX 150 lists for approximately $2,495 and is available domestically from Progressive Concepts in Streamwood, Ill.

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