Analogue Application Information ......
The Gain Control System
EXTERNAL INTEGRATION COMPONENTS
R
5
C
8
VOGAD
IN
R
6
R
12
V
SS
V
DD
VOGAD
OUT
HI
PEAK
DETECTOR
HI/LO
PEAK DETECTOR
Tx
DRIVES
MIC. OUT
MIC. IN
Tx
MIC. IN
VOLTAGE
CONTROLLED (VOGAD)
AMPLIFIER
MAIN PROCESS
Rx
DRIVE
Tx INPUT PROCESS
To EXTERNAL AUDIO PROCESSES
PROCESSED AUDIO IN
CAL INPUT
Fig.3 “VOGAD” Sensors and Timing Components – (part of Fig.4)
Tx gain control of the FX806A is by 1 of 2 selectable signal
peak detectors whose output is fed via external integrating
components to the Voltage Controlled Amplifier positioned
in the Tx Input Process Path.
The integrated level to the VOGAD In pin causes the Voltage
Controlled Amplifier gain to be reduced. VOGAD attack
and decay calculations are described at the foot of the
proceeding page.
The FX806A automatically chooses the appropriate peak
detector when the signal path is set by a Mode Command.
The Hi/Lo Peak Detector is employed when external audio
processes are used.
The Hi Peak Detector is employed when external audio
processes are not used.
25
Limiter Only
20
Output Distortion (%)
15
Internal Path with Pre-emphasis
Circuit Elements set to 0dB
Input Level for 0dB
= 71.0mV p-p
Input Frequency
= 1.0kHz
Output Deviation
= 60% = 0dB
“Hi-Peak” VOGAD & Limiter
10
60%
Output Deviation
308mVrms
5
0.0
+10.0
0
0.0
0.071
+20.0
0.4
0.6
Input Level (dB)
Mic. Input Level (Vp-p)
0.2
Fig.4 Distortion vs Mic. Input Level
Suggested Calibration Methods
To effectively null all internal microcircuit tolerances, the following initial calibration routine is suggested:
Tx Calibration :
From Mic. In to Modulator Drives Out
Disable Peak Detectors (Mode Command).
Set Transmitter Drives to 0dB (Mod Levels Set).
Pre-emphasis may be employed as required (Control
Command).
Set Input Level Amp to 0dB (Control Command).
(1) Mic. In = 250mVrms at 1kHz; Set Process Gain Amp for
output of 1440mV p - p (100% deviation).
(2) With Process Gain Amp set as (1); Mic In = 25mVrms at
1kHz, set Input Level Amp for output level of 308 mVrms
(60% deviation).
Rx Calibration:
From Rx Audio In to Audio Output
Set Audio Output Drive to 0dB (Volume Set).
Leave Process Gain Amp set as In (1) (above).
(3) With Rx Audio In level of between 154mVrms and
308mVrms (see Specification page), at 1kHz, set the
Input Level Amp for an output level of 308mVrms.
5
CMLMICRO [ CML MICROCIRCUITS ]
