AX-ConfigurationUtility User Manual
40
12.2.
Background information
Without frequency tracking the worst case frequency deviation between RX and TX is
)
(
RX
TX
carrier
ppm
ppm
f
f
+
•
=
,
Where
TX
ppm
and
RX
ppm
are the precisions of the crystals used in the transmitter and
the receiver. The frequency tracker must be allowed to track this deviation, thus
the “MAX AFC range” field in the
AX-ConfigurationUtility PHY panel (7) should be set to
this value,
Max AFC Range: =
)
(
RX
TX
carrier
ppm
ppm
f
f
+
•
=
.
Note: AX-ConfigurationUtility performs the inverse calculation. Given Max AFC Range, the
necessary crystal precision is displayed. AX-ConfigurationUtility assumes identical crystals
are used in RX and TX, thus the displayed value is XTAL precision =
2
/
)
(
RX
TX
ppm
ppm
+
.
The above setting allows the tracker to walk away
f in the wrong direction while there is
no signal (see Figure 14). In the worst case the receiver frequency can therefore be f°_RX
when packet transmission starts, that is 2●∆f away from f_TX. For successful tracking a
fraction of the TX spectrum should fall into the RX BW. Thus the yellow shaded region
should be non-e
mpty: (TX BW + RX BW)/2 ≥ 2●∆f.
AX-ConfigurationUtility issues a
warning if this constraint is not fulfilled. Inserting
RX BW = TX BW =
deviation
BR
BR
h
•
+
=
•
+
2
)
1
(
into the overlap condition leads to
1
2
min
−
•
=
BR
f
h
.
Irrespective of the overlap condition
5
.
0
h
is required.
For optimal sensitivity RX BW
opt
=
BR
h
•
+
)
1
(
should be chosen. If values
5
h
emerges,
increasing the bitrate should be considered as the RX BW for large h values has to be
increased beyond RX BW
opt
to achieve stable operating conditions.
