Sub-Audio Signaling Processor
Page 8 of 24
MX805A
Recommended External Component Notes:
1. Xtal/Clock circuitry shown in inset are recommended in accordance with the MX-COM’s “Standard and
DBS 800 Crystal Oscillators” application note.
2. Resistor R8 is a System Component. Its value is chosen together with the MX806A Modulation Summing
Amplifier to provide a subaudio signal level of –11.0dB to the system modulator.
3. Figure 3 and Figure 4 illustrate alternative input component configurations.
4. The values for R2 and R5 are dependent on the input signal level. Values given are for the specified
composite signal (reference page 14). R4 add hysteresis to the comparator and is not always required.
5. The values used for C1 and C2 are determined by the frequency of X1.
C1= C2 = 33pF for X1< 5.0MHz
As a guide:
C1= C2 = 18pF for X1> 5.0MHz
If the on-chip Xtal oscillator is to be used, then the external components X1, C1, C2, and R1 are required
as shown in Figure 2 (inset). If an external clock source is used these components are not required; the
input should be connected to the Xtal/Clock pin and the Xtal pin unconnected.
6. Resistor R7 is used as the DBS800 system common pull-up for the C-BUS Interrupt Request (IRQ ) line.
The optimum value of this component will depend upon the circuitry connected to the IRQ line.
7. The level at this point should be approximately 900mV
8. Silicon small signal
P-P.
3.1 Input Configurations
Figure 3 shows an input configuration that is generally for use for CTCSS signal and NRZ data reception.
Input coupling capacitor C3 is required because the RX Sub-Audio Input is held at V
during all powered
BIAS
conditions of the MX805A. Diodes D1 and D2 can be any silicon small signal diode.
The output resistance (open loop) of the on-chip Rx Amp is = 6kΩ. In the configuration shown in Figure 3, the
(Rx Amp) RC time constant is therefore 90ms. If this period is too long for some systems, i.e. those using half
duplex, short data burst, and external amplifier should be considered in place of the on-chip Rx Amp.
MX805A
RX AMP
MX805A
RX LPF
MX805A
COMPARATOR
D.C. RESTORATION
+
_
HYSTERESIS (OPTIONAL)
17
14
NOTE 7
RX AMP IN
13
15
R4
R3
16
D1
D2
R2
R5
C3
20
+
21
COMPARATOR IN
19
RX SUB-AUDIO
INPUT
_
COMPARATOR
OUT
C4
Figure 3: MX805A Input Components
3.1.1 Using and External Op-Amp
For DC coupling the MX805A to the receiver’s discriminator output when using burst mode NRZ
communication, it is recommended that an additional, external Op-Amp is employed as configured in
Figure 4. This configuration will quickly compensate for sudden shift of DC input bas.
Components R9, R10, and R11, should be calculated to provide an accurate potential of 2.5VDC (equal to
V
BIAS
) at pin junction 15/16when using a discriminator input and 900mVP-P at the output of the external op-
amp. Note that the MX805A LPF has a 6dB gain. If additional filtering is required, C9 should be used, it
should be calculated with R9 to provide a lowpass cutoff frequency (f ) of 500Hz.
CO
ã2001 MX-COM, Inc.
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480116.006
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