ACE9030MSLASHIWSLASHFP2Q datasheet(23/39 Pages) MITEL

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ACE9030

the example gives 0·045 V.

Removing the arbitrary 1 kHz, the gain at the exclusive-

OR gate is given by:

Gain = 2 x Delay in I.F. cycles x V

DD / I.F.

with the units of volts per Hertz of deviation.

To be strict, the pulse width is reduced for a positive

deviation, so the gain is negative, but this may be ignored as

the audio polarity is not of any relevance and also is inverted

several times before driving the earpiece.

Using the delay lengths from table 3 the range of gains (at

the exclusive-OR gate) can be listed and then compared with

the lowest as shown in table 5.

This shows a gain range of 5·6 dB which must be allowed

for in the later stages, but also gives absolute gains which

show the discriminator could cause saturation in the following

stage if a high deviation signal is received. For example

speech can be set to 8 kHz deviation so the maximum voltage

(with V

DD at 3·75 V) is 70·3 x 8000 µV = 562 mV peak. With ST

and SAT the total deviation can become 14·5 kHz, potentially

giving 1·019 V peak signal, or over 2 V peak-to-peak and

leading to possible saturation. There is also the full V

switching waveform to handle. Other supply levels will simply

scale the signals and not change the saturation problem. A

6 dB attenuator is included in the low pass filter that follows the

discriminator output driver to avoid any possibility of saturation

in the audio reconstruction filter. This attenuator is a simple 2:1

potential divider so will also halve the D.C. level of the signal.

A further effect to be considered is the D.C. offset that

results from using delays that are not ideal multiples of cycles

of the AFCIN frequency. It can be seen from the Timing

Diagram, figure 19, that when the delay is exactly an odd

number of quarter cycles each half cycle at one end of the

delay will symmetrically straddle an edge the other end of the

delay so an unmodulated input will give an output with a 1:1

mark:space ratio; this corresponds to a mid-supply level at the

attenuator input, see figure 20. The attenuator output will then

be centred at V

DD/4, so the nominal D.C. gain, 2 x, of the

bandpass filter will give the AUDIO output centred at mid-

supply.

When a mode is selected with an odd number of thirds of

a cycle the output mark:space ratio is offset, and approximat-

ing 2·363 as 7/3 or 3·677 as

11/

3 the effect can be seen in

figure 21.

The signal will now be centred on a level at 2/3 or

3 of

supply, at the attenuator input (giving 1/3 or

6 x VDD at its

output) and by adjusting the D.C. gain of the bandpass filter it

is possible to set the AUDIO to a mid-supply centre if required.

The components used in the bandpass filter feedback

circuit should be chosen to both set the pass band frequencies

(for example 50 Hz to 30 kHz) and also to set the A.C. and D.C.

gains to complement the discriminator’s gain and D.C. offset.

Typcal A.C. gain at mid-band is around 20 dB. This filter is not

of high enough order to remove all out of band noise without

distorting the speech channel so to get the final band limited

signal precision high order filters such as in the ACE9040 are

required.

The ACE9030 is specified with the following component

values (see fig 18):

R1 = 47k

Ω

R2 = 100k

Ω

R3 = 33k

Ω

C1 = 82pF

C2 = 100nF

Fig. 21 Demodulation With Odd Thirds Of A Cycle

Fig. 20 Demodulation With Odd Number Of Quarter Cycles

These values are compatible with AMPS using a 14.85MHz

reference crystal and 450kHz IF.

Resistors R2 and R3

determine the dc gain and can be used to compensate for the

dc offset of the demodulated output. Resistors R2 and R3, R1

determine the mid band ac gain of the band pass filter.

AFCIN

AFCIN DELAYED

BY 2 / CYCLES:

OUTPUT

(EX-OR)

MARK:SPACE

RATIO = 1:1

MARK:SPACE

RATIO = 1:1

MARK:SPACE

RATIO = 2:1

MARK:SPACE

RATIO = 1:2

AFCIN

AFCIN DELAYED

BY 7/3 CYCLES:

OUTPUT

(EX-OR)

OR BY 11/3

CYCLES:

OR BY 2 /

CYCLES:

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