ACE9030
the example gives 0·045 V.
Removing the arbitrary 1 kHz, the gain at the exclusive-
OR gate is given by:
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 VDD/4, so the nominal D.C. gain, 2 x, of the
bandpass filter will give the AUDIO output centred at mid-
supply.
Gain = 2 x Delay in I.F. cycles x VDD / I.F.
with the units of volts per Hertz of deviation.
When a mode is selected with an odd number of thirds of
a cycle the output mark:space ratio is offset, and approximat-
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.
7
ing 2·363 as /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 1/3 of
1
1
supply, at the attenuator input (giving /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
circuitshouldbechosentobothsetthepassbandfrequencies
(forexample50Hzto30kHz)andalsotosettheA.C. andD.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.
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 VDD 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 VDD
switching waveform to handle. Other supply levels will simply
scale the signals and not change the saturation problem. A
6 dBattenuatorisincludedinthelowpassfilterthatfollowsthe
discriminatoroutputdrivertoavoidanypossibilityofsaturation
intheaudioreconstructionfilter.Thisattenuatorisasimple2:1
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
The ACE9030 is specified with the following component
values (see fig 18):
R1 = 47kΩ
R2 = 100kΩ
R3 = 33kΩ
C1 = 82pF
C2 = 100nF
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
3
AFCIN DELAYED
1
OR BY 2 /
4
BY 2 / CYCLES:
4
CYCLES:
OUTPUT
(EX-OR)
MARK:SPACE
RATIO = 1:1
MARK:SPACE
RATIO = 1:1
Fig. 20 Demodulation With Odd Number Of Quarter Cycles
AFCIN
OR BY 11/3
CYCLES:
AFCIN DELAYED
BY 7/3 CYCLES:
OUTPUT
(EX-OR)
MARK:SPACE
RATIO = 1:2
MARK:SPACE
RATIO = 2:1
Fig. 21 Demodulation With Odd Thirds Of A Cycle
23
MITEL [ MITEL NETWORKS CORPORATION ]
