SPM and Security Subsystem
FX651
1.6
1.6.1
Application Notes
Device Sensitivity versus V
DD
- Input Amplifier Gain
The device sensitivity is proportional to V
DD
because internal voltage references are derived from the
power supply. This means that the input amplifier signal gain should be adjusted to compensate.
Figure 2 shows components recommended for 5.0V and 3.3V operation.
Alias Responses - False Decodes
The FX651 is designed to work in telephone systems where certain ranges of input frequencies are
expected and other ranges are not. The 12kHz SPM channel works in the presence of speech and a
tone in the range 16kHz - 20kHz. The security tone detector functions in the presence of an interfering
tone in the range 11kHz - 13kHz. A significant tone at other frequencies may interfere with correct
decoding of valid transmissions and should be avoided.
A small range of input frequencies could cause false decoding when no valid tone has been received.
This is because of the frequency shifting property of the SPM channel and the SWC filters (i.e sampled
data filters) in both channels.
The alias frequencies which should be avoided are:
12kHz Channel
Input frequencies in the ranges:
5.708kHz - 6.078kHz
41.504kHz - 41.874kHz
1.6.2
23.606kHz - 23.976kHz
47.986kHz - 47.616kHz
29.718kHz - 30.088kHz
59.402kHz - 59.772kHz
In general, decodes may be caused by tones in the ranges:
((
n
×
17.898)
±
(5.708
−
6.078))kHz
where n = 0, 1, 2, 3......
Taking n = 1 and the minus sign represents valid SPM inputs in the range 11.82kHz -
12.18KHz. The other inputs represent false decodes. The response time and level sensitivity
for a false decode is the same as that for a valid decode, e.g. 42ms - 45ms of 5708Hz at 50mV
could cause a false decode at the "DETECT 12kHz" pin.
18kHz Channel
Input frequencies in the ranges:
52.511kHz - 54.671kHz
88.511kHz - 90.671kHz
In general, decodes could be caused by any tone in the ranges:
((
n
×
71.591)
±
(16.92
−
19.08))kHz
where n = 0, 1, 2, 3......
The case where n = 0 and the addition sign is used represents valid security tone inputs. The
other cases represent false decodes. The response time and level sensitivity for a false
decode is the same as that for a valid decode, e.g. 5.8ms of 52.511kHz at 40mV could cause a
false response at the "DETECT 18kHz" pin.
©
1997 Consumer Microcircuits Limited
9
D/651/4
CMLMICRO [ CML MICROCIRCUITS ]
