SPM and Security Subsystem

FX651

© 1997 Consumer Microcircuits Limited

9

D/651/4

1.6

Application Notes

1.6.1

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.

1.6.2

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

23.606kHz - 23.976kHz

29.718kHz - 30.088kHz

41.504kHz - 41.874kHz

47.986kHz - 47.616kHz

59.402kHz - 59.772kHz

In general, decodes may be caused by tones in the ranges:

((

17.898) (5.708

6.078))kHz

n

×±

−

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:

((

71.591) (16.92 19.08))kHz

n

×±

−

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.