ACE9030MSLASHIWSLASHFP2N datasheet(22/39 Pages) MITEL

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ACE9030

Fig. 19 F.M. Discriminator Example Timing Diagram

7, D6

Delay as

I.F.

Absolute Gain

Relative Gain

I.F. cycles

kHz

µV/Hz,

DD = 3·75 V.

DD = 4·85 V.

0, 0

2·742

450

45·7

59·1

1·8 dB

0, 0

2·363

450

39·4

50·9

0·5 dB

1, 0

4·266

455

70·3

90·9

5·6 dB

1, 0

3·677

455

60·6

78·4

4·3 dB

0, 1

3·252

450

54·2

70·1

3·3 dB

1, 1

2·251

455

37·1

48·0

0 dB

Table 5

phase effect of f.m. increases as more cycles are exam-

ined. The modulation lines are drawn as the phase change on

the real input waveform but the separation from the nominal

edge position can also be interpreted as the probability of a

whole cycle shift on the sampled version of the signal as in the

ACE9030.

Only six delay comparisons are shown, stepping across

at the sampling rate, but the effect of the pattern can easily be

seen by continuing the sequence, and two conclusions can be

drawn:

1) The output waveform is at twice the frequency of the input

- this is true for all delay-and-multiply schemes.

2) The output high time is modulated by the phase modulation

accumulated over the delay duration and the output period is

only modulated slightly by the instantaneous input phase

shifts so the effect is to modulate the duty cycle. Due to the

sampling of the I.F. input the modulation will really be

quantised so the width of the modulation box should be read

as a probability of a whole cycle shift in edge position rather

than as a phase shift but the effect is the same when averaged

over many cycles.

By converting the modulation from a phase shift to a duty

cycle all that is then needed to recover the original baseband

signal is to smooth the discriminator output to remove the

900 kHz sampling, leaving an analog signal proportional to the

original frequency deviation and to the delay length. The low-

pass filter in ACE9030 is first order and has its – 3 dB point at

approximately 70 to 80 kHz to significantly reduce the clock

level; the audio bandpass filter then further reduces this level

to give a cleaner audio output to drive the ACE9040 where it

is again filtered.

The demodulation gain can be determined by considering

the effects of a 1 kHz frequency offset on the input signal, and

using I.F. = 450 kHz and Delay = 2·742 cycles and

DD = 3·75 V in the example:

A 1 kHz frequency offset will give a phase change in the

I.F. signal, during each cycle, of 2

π x (1 kHz / I.F.) radians or

as a fraction of a cycle, (1 kHz / I.F.) which in this example is

1/450 of a cycle.

If the discriminator delay is measured in I.F. cycles the

change in output pulse width for each of the two output pulses

is: (Delay in cycles) x (phase change per cycle)

= Delay x (1 kHz / I.F.) in I.F. periods, which in this example

is 2·742 x 1/450 periods of 450 kHz, or 13·5 ns.

Output pulses occur at a rate of 2 x I.F., so the change in

output pulse width measured in output periods is 2 x (change

measured in I.F. periods), giving 2 x Delay x 1 kHz / I.F. The

duty cycle is defined as pulse width / period so the result is also

the change in duty cycle. For this example the change is 0·012.

The output is a logic signal so its amplitude is V

DD for all

settings and for all modulation, thus the final effect of a 1 kHz

offset is an output change of 2 x Delay x 1 kHz x V

DD / I.F. and

INPUT

ROLLING

PAIRS OF

SAMPLES

TO EX-OR

GATE FOR

COMPARISON

OUTPUT

NOMINAL CYCLES

3/4

NOMINAL CYCLES

3/4

NOMINAL CYCLES

3/4

NOMINAL CYCLES

3/4

NOMINAL CYCLES

3/4

NOMINAL CYCLES

3/4

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