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Tilt Equalization Adjustment Procedure
Since the purpose of Low Frequency Tilt Equalization of an AM transmitter is to optimize its performance on
low frequency, clipped audio waveforms, proper adjustment of the Tilt Equalizer is mandatory if optimum
modulation performance (maximum loudness) is the goal. Overcompensation, by using too much tilt
equalization, is actually worse than no tilt equalization at all because it further taxes the transmitter’s modulator
and power supply. In fact, on some transmitters, it may be preferable to not completely cancel low frequency tilt
because of power supply and modulator headroom issues. Only field experience with the subject transmitter (or
experimentation) will reveal whether or not low frequency tilt can, or should be applied to the transmitter. Note
also that by nature of its operation, Low Frequency Tilt Equalization puts additional demands on the transmitter
which are beyond that created by the Omnia’s competitive audio processing alone. In fact, the combination of
these two stresses, especially when applied to older, plate modulated transmitters, may cause damage. The
reader is therefore forewarned to use care, caution, and common sense when adjusting the Omnia’s Tilt
Equalizer when it is being used on older, less adequate transmitters!
1.
Navigate to the Omnia-3a’s Input & Output Menu, and reduce the output levels to 0dBu on both Left and Right
channels.
2.
Connect a sine wave Audio Oscillator to the Omnia-3am audio inputs. Set the oscillator to provide a 50 Hz,
0dBu output.
3.
Adjust the Omnia’s input level controls so that at least 10 dB of gain reduction is seen on the Wideband AGC
bargraph meter.
4.
In the Limiter Mixer menu, increase the output of the LF Limiter to its +6dB maximum value.
5.
In the Clipper Menu, increase the Clipper Drive to the +6 dB setting. This will produce sufficient clipping on
the 50 Hz tone to allow the following adjustments to be performed.
6.
In the Output Menu, increase the Omnia-3am’s output level to produce no more than 50% to 75% modulation
as shown on the station’s modulation monitor.
7.
Connect a RF sample to the oscilloscope input and observe the signal envelope. The modulated carrier should
appear similar to Figure 1. The clipped 50 Hz waveform should have a
flat-top
if there is no low frequency tilt
in the transmitter. If Tilt is visible, as in Figure 2, then adjustment of the Tilt Equalization control is required.
8.
Use a mixture of the Tilt EQ and Tilt Freq parameters to change the appearance of the tilt in the waveform so
that the top becomes as flat as possible. A properly compensated transmitter’s modulation envelope should
appear similar to Figure 1. If too much Tilt Correction is applied, the modulated envelope will appear similar to
Figure 3.
9.
After optimizing the Tilt controls for your particular transmitter, the Tilt adjustment is complete. Normal
programming may be applied and the Omnia –3am’s output levels adjusted for the desired negative and positive
modulation. Please see the following page for oscilloscope photographs of three examples of tilt equalization.
