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AM-55 / Sep 2015
A M - 5 5 G U I
LF Tilt EQ
The Low Frequency Tilt (LF Tilt EQ) is used to correct (either fully or partially) for
the low frequency response inadequacies of primarily older style AM transmitters. In
such transmitters, inter‑stage transformers and/or coupling capacitors limit the frequency
response at low frequencies due to their high‑pass, DC‑blocking nature.
Because high waveform fidelity is required for good peak control, and because
transmitters with such inadequacies do not have frequency response extending down to
near DC, the clipped waveforms generated by the audio processor will not pass through
such transmitters without undergoing low frequency losses. These losses cause the wave
‑
form to tilt, which can cause the top of the clipped waveform to exceed 100% modulation
at low frequencies.
In order to prevent over modulation on such waveforms the audio level into the transmitter
must be reduced, and this results in a loss of loudness and/or coverage.
LF Tilt EQ is designed to pre‑distort the shape of the waveform generated by the processor
in order to make it complimentary to what the transmitter will do to the waveform later.
In essence, LF Tilt EQ is a specialized, extremely low frequency equalizer that tilts the
waveform in the direction OPPOSITE to how the transmitter will tilt it later and the result
is flatter tops of the clipped waveform and more accurate modulation of the transmitter.
Note that the LF equalization generated by the tilt equalizer is NOT intended to boost
low frequencies in program material to make it more audible. Rather, the LF Tilt EQ occurs
in the subsonic audio region, typically between 10‑50Hz.
Adjusting The LF Tilt EQ
Adjusting the AM LF Tilt EQ is a simple process. The first step is to determine if the
transmitter will require Tilt correction in order to modulate well at low frequencies and
the following procedure will determine this:
1. Connect the AM‑55 to the audio input of the transmitter to be tested.
2. Connect the transmitter to either a dummy load of rated power or to the antenna
system.
3. Turn on the AM‑55’s test oscillator and adjust it to 75Hz. Adjust the
Test Oscillator
Level
control and the TX 1 or TX 2 output (as appropriate) until approximately
60% modulation is indicated on the station’s modulation monitor. Ensure that the
LF Tilt EQ L (level) controls are set at 0dB.
