Microchip Technology SA5 Series, User Manual

Page: 17 / 63

Installation
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2019 Microchip Technology Inc. DS50002938A-page 17
RF Noise Mitigation
If the system that the MAC is designed into is sensitive to RF or microwave frequencies
(especially 3.417
GHz and its harmonics), care must be taken to dampen those fre-
quencies at locations that might be adversely affected. One way to do this is to deter-
mine the appropriate capacitor that has its "zero-ohm" characteristic at the frequency
of interest and use that capacitor to effectively short that frequency to ground where it
might a problem. For example, it has been determined that at 3.417
GHz, the
"zero-ohm" capacitor value for an 0603-sized SMT component is 4.3 pF; for an
0402-sized capacitor, the value must be 6.8 pF; and for an 0201-sized capacitor, 8.2 pF
is ideal.
From this, it is recommended to place the appropriate-valued capacitor at the node of
interest. It is important that this cap be exactly on the node where the noise must be
squelched, and is grounded right at the cap as well. In addition, a good RF ground
plane is required, otherwise the improvement can be nullified.
The nulling capacitors are placed as close to the MAC I/O pins as possible. Because
the 3.417
GHz frequency is used within the MAC, the above mentioned nulling capac-
itors are recommended to be installed on pins 1,3,5-8.
EMI and Noise Considerations
When a user has an application where the phase noise and spur integrity are crucial,
the SA5X must be provided with a clean source of DC power (free of spurious current
or voltage noise). Connecting fans, heaters, and other switching devices to the DC sup-
ply powering the SA5X can result in degraded performance.
This noise is coupled through the power line to cause modulation spurs on the output
signal. Special care must be taken to avoid noise at 100
Hz and its harmonics (roughly
up to the tenth harmonic).
If power line filtering is added at the power input pin of the SA5X, this filtering cannot
have any resonance points greater than the specified impedance of less than 0.1
Ω
from DC to 100 kHz in order to avoid the potential for noise peaking or oscillations in
the internal power regulators.
In addition, the input operating voltage range specified for the SA5X during turn-on
must continue to be met during operation of the unit. For example, using a 0.3
Ω DC
source resistance for the input supply line may not be appropriate because the voltage
drop resulting from this resistance (caused by the turn-on current or quiescent operat-
ing current) could cause the input voltage to drop below the specified allowable value.
Signal Connections
The SA5X pinout is shown in Figure 2-3 .
FIGURE 2-3: Pinout.