ACE9030
An output to drive the phase comparator is generated
fromtheN1loadsignalatthestartofeachcycle, givingapulse
every (N1 + N2) counts and to help minimise phase noise in
the complete synthesiser this pulse is re-timed to be closely
synchronised to the FIM/FIMB input. During the N1 down
count the modulus control MODMP is held HIGH to select
prescaler ratio R1 and during the N2 up count it is LOW to
select R2, so the total count from the VCO to comparison
frequency is given by:
to 8 bit numbers, 0 to 255 and to simplify the logic a set value
of 0 will give a count of 256. If a value of 0 for N2 is wanted then
N2 should be set instead to R1 and the value of N1 reduced
by (R1 + 1), also equal to R2.
To ensure consistent operation some care is needed in
the choice of prescaler so that the modulus control loop has
adequate time for all of its propagation delays. In the
synthesiser there is propogation delay TDELAY from the
FIM/FIMB input to the MODMP/MODMN output and in the
prescalertherewillbeaminimumtimeTSET-UP fromthechange
in MODMP/MODMN to the next output edge on FIM/FIMB, as
shown in figure 27. For predictable operation the sum
TDELAY + TSET-UP must be less than the period of FIM/FIMB or
otherwise, if the rising and falling edges of MODMP/N are
delayed differently, the prescaler might give the wrong bal-
ance of R1 and R2. This will often set a lower limit on the
frequency of FIM than that set by the ability of the counter to
clock at the FIM rate. For 900 MHz cellular telephones the use
of a ÷ 64/65 prescaler normally ensures safe timing.
Fractional-N mode operates by forcing the
MODMP/MODMN outputs to the R2 state for the last count of
the N1 period whenever the Fractional-N accumulator over-
flows, effectively adding one to N2 and subtracting one from
N1, and so increases the total division ratio by one for each
overflow. The effect of this is to increase the average division
ratio by the required fraction.
NTOT = N1 x R1 + N2 x R2
R2 = R1 + 1
NTOT = (N1 + N2) x R1 + N2
but
so
It can be seen from this equation that to increase the total
division by one (to give the next higher channel in many
systems) the value of N2 must be increased by one but also
that N1 must be decreased by one to keep the term (N1 + N2)
constant. It is normal to keep the value of N2 in the range 1 to
R1 by subtracting R1 whenever the channel incrementing
allows this (i.e. if N2 > R1) and to then add one to N1. These
calculations are different from those for many other synthesis-
ers but are not difficult.
The 12-bit up/down counter has a maximum value for N1
of 4095 and to give time for the function sequencing a
minimumlimitof3isputonN1. Thereisnoneedforsuchlarge
values for N2 so its range is limited by the programming logic
PROGRAMMING EXAMPLE FOR BOTH SYNTHESISERS
Toillustratethechoiceofprogrammingnumbersconsider
the ETACS system as now used in the UK.
Eachchannelis25kHzwidebutasthechanneledgesare
put onto the whole 25 kHz steps the centre frequencies all
have an odd 12·5 kHz. This is not ideal for the synthesiser but
does give the maximum number of channels in the allocated
band.
The mobile receive channels are a fixed 45 MHz above
thecorrespondingtransmitfrequency,asisthecasewithmost
cellular systems. In the ACE9030 the intention is to use the
mainsynthesisertogeneratethereceiverlocaloscillatoratthe
first I.F. above the mobile receive carrier, and to then mix the
auxiliary synthesiser frequency with this to produce the trans-
mit frequency. A typical I.F. is 45 MHz, leading to an auxiliary
frequency of 90 MHz and a crystal of 14·85 MHz with a tripler
for the second local oscillator, and a final I.F. of 450 kHz.
The channel numbers and corresponding frequencies
are shown in table 6.
This began as TACS with 600 channels (numbered 1 to
600) from 890 to 905 MHz (mobile transmit) with the provision
toexpandby400channels(601to1000)from905to915MHz.
This additional spectrum was given over to GSM use before
TACSneededexpanding,soTACSwaslaterextendedby720
extra channels from 872 to 890 MHz, to form ETACS and
leaving a somewhat odd channel numbering system. The
channel numbers are stored as 11-bit binary numbers and are
listed here as both negative numbers to follow on downwards
from channel 1 and also as large positive numbers as these
are the preferred names. These two numbering schemes are
really the same, as the MSB of a binary number can be
interpreted as either a sign bit (2’s complement giving the
negative values) or as the bit with the highest weight (1024 for
an 11 bit number, giving the large positive values).
CHANNEL
MOBILE TRANSMIT
FREQUENCY (MHz)
872·0125
...
MOBILE RECEIVE
FREQUENCY (MHz)
917·0125
...
MAIN VCO
(MHz)
NUMBER
1329 or –719
962·0125
...
...
2047 or –1
889·9625
889·9875
890·0125
890·0375
890·0625
...
934·9625
934·9875
935·0125
935·0375
935·0625
...
979·9625
979·9875
980·0125
980·0375
980·0625
...
0
1
2
3
...
600
904·9875
949·9875
994·9875
Table 6
29
MITEL [ MITEL NETWORKS CORPORATION ]
