SA614A

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Product data sheet

Rev. 4 — 14 February 2014

13 of 28

NXP Semiconductors

SA614A

Low power FM IF system

2. The demodulated output demonstrates a threshold. Above a certain input level, the

limited signal begins to dominate the regeneration, and the demodulator begins to

operate in a normal manner.

There are three primary ways to deal with regeneration:

1. Minimize the feedback by gain stage isolation.

2. Lower the stage input impedances, thus increasing the feedback attenuation factor.

3. Reduce the gain. Gain reduction can effectively be accomplished by adding

attenuation between stages which can also lower the input impedance. Examples of

impedance/gain adjustment are shown in Figure 11. Reduced gain results in reduced

limiting sensitivity.

A feature of the SA614A IF amplifiers, which is not specified, is low phase shift. The

SA614A is fabricated with a 10 GHz process with very small collector capacitance. It is

advantageous in some applications that the phase shift changes only a few degrees over

a wide range of signal input amplitudes.

13.3 Stability considerations

The high gain and bandwidth of the SA614A 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:

1. supply lines and ground

2. stray layout inductances and capacitances,

3. radiated fields, and

4. 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 VCC 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 W. 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.