SA604A_15 Datasheet, PDF (14/31 Pages) NXP Semiconductors – High-performance low-power FM IF system

NXP Semiconductors
SA604A
High-performance low-power FM IF system
13.3 Stability considerations
The high gain and bandwidth of the SA604A in combination with its very low currents
permit circuit implementation with superior performance. However, stability must be
maintained and, to do that, every possible feedback mechanism must be addressed.
These mechanisms are:
• supply lines and ground
• stray layout inductances and capacitances
• radiated fields
• phase shift
As the system IF increases, so must the attention to fields and strays. However, ground
and supply loops cannot be overlooked, especially at lower frequencies. Even at 455 kHz,
if the supply line is not decoupled, using the test layout in Figure 17, instability occurs. To
decouple, use two high-quality RF capacitors, a 0.1 F monolithic on the V_CC pin, and a
6.8 F tantalum on the supply line. An electrolytic is not an adequate substitute. At
10.7 MHz, a 1 F tantalum has proven acceptable with this layout. Every layout must be
evaluated on its own merit, but do not underestimate the importance of good supply
bypass.
At 455 kHz, if the layout of Figure 17 or one substantially similar is used, ceramic filters
can be connected directly to the input and between limiter stages with no special
consideration. At frequencies above 2 MHz, some input impedance reduction is usually
necessary. Figure 11 demonstrates a practical means.
As illustrated in Figure 12, 430  external resistors are applied in parallel to the internal
1.6 k load resistors, thus presenting approximately 330  to the filters. The input filter is
a crystal type for narrowband selectivity. The filter is terminated with a tank which
transforms to 330 . The interstage filter is a ceramic type which does not contribute to
system selectivity, but does suppress wideband noise and stray signal pickup. In
wideband 10.7 MHz IFs the input filter can also be ceramic, directly connected to pin
IF_AMP_INPUT.
430 Ω
16 15 14 13 12 11 10 9
430 Ω
SA604A
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aaa-012850
Fig 12. Crystal input filter with ceramic interstage filter
SA604A
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 4.1 — 30 March 2015
© NXP Semiconductors N.V. 2015. All rights reserved.
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