Larson Davis System 824, Technical Reference Manual

Page: 561 / 673

9/29/09 C -15
FM Modulation Tests
The FM modulation test code computes the minimum and
maximum periods of the input waveform. The minimum and
maximum frequencies are the inverse of the average of the
last 8 periods. The carrier frequency is an exponential aver-
age of all waveform periods. Signal will be flagged as stable
if the carrier period varies less than 8 microseconds for 1
second.
SLM/RTA
The 1/3 octave filters used in the audiometric calibration
firmware are the same filters as are used in the SSA firm-
ware and meet the same specifications, (page D-8).
On level computation: After the “ON” state has
been detected, the
“ON” level is computed
as the average of 32
samples.
Overshoot computation: Overshoot is calculated
as the maximum
sample level minus the
averaged “ON” level
Measurement
conditions: Conditions that must be
met to produce output:
• ON/OFF modulation must be
greater than 20 dB (90%).
• The “ON” state must be stable
within ±0.5dB for at least 20
ms.
• Pulse transition from high to
low must occur within 2 sec-
onds of the transition from low
to high for the fall time to be
calculated. If this time is
greater than 2 seconds, the
824 will revert to looking for the
pulse rise time.
Carrier frequency (F
c
): 20 Hz
≤
f
c
≤
16000 Hz
Modulating frequency (fm): 3 Hz
≤
f
m
≤
100 Hz
Carrier to modulating frequency ratio: f
c
> f
m
* 5
Signal amplitude: Overload -35dB
το
Overload