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monitoring. For the
valid
measurement of total modulation,
over the entire baseband signal range (10Hz–100kHz), the 531N
requires 150µV (55dBf) for a total-mod reading that’s no more
than 2% in error, and 250µV (60dBf) for less than a 1% error.
The front-panel
SIGNAL LEVEL
bargraph is a
relative
display
and should not be considered linear, either in terms of field
strength or transmission power. Generally, at least three or
four segments must be lighted for valid total-mod readings.
The
LOW SIGNAL
alarm comes on when a total-mod reading
would be compromised by more than 1%.
Multipath
Distortion
Multipath effects arise from the reception of a radio signal
from more than one direction (multiple paths). The primary
path is generally ‘direct’ (line-of-sight), and subsequent arrivals
of the same signal come along after it has bounced off build-
ings or natural terrain. This multiplicity of arrival times com-
promises certain parameters of the otherwise robust FM trans-
mission. Long before its effects can be heard, multipath can
invalidate total-mod measurements.
The Model 531N incorporates a multipath distortion detector
and bargraph readout. Used together, the
MULTIPATH
and
SIGNAL LEVEL
displays can be an aid in aiming the receiving
antenna for best reception. A multipath alarm comes on when
more than a couple of the
MULTIPATH
bargraph segments are
lighted. The associated alarm is preset to the maximum value
of multipath distortion that can be tolerated for an accurate to-
tal-mod reading.
Composite
Clipping
A small amount of well-regulated “hard clipping” of the com-
posite waveform is a common technique with broadcasters to
secure maximum modulation efficiency and, as a result, the
highest ‘perceived loudness.’ Modern composite clipping cir-
cuits perform their action prior to injection of the stereo pilot,
or they may ‘detour’ the pilot signal around the clipper to pre-
vent its being clipped along with the program signal. This does
not mean, however, that composite clipping does not generate
harmonics at 19kHz. These can add to and subtract from the
legitimate pilot tone, essentially manifesting as ‘pilot modula-
tion.’
Because the Model 531N translates a measurement of pilot am-
plitude modulation to an equivalent degree of multipath distor-
tion, excessive composite clipping may show up on the
MULTI-
PATH
display, even when no secondary signal path is present.
This could be the case in a direct, high-level RF connection as
well. This does not mean that a multipath alarm should be ig-
nored when no possibility of multipath exists. If excessive
composite clipping does indeed cause a multipath indication,
chances are that occupied bandwidth limits are being exceeded
despite proper constraints on peak deviation.