SA638 データシートの表示（PDF） - Philips Electronics

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SA638

IS-54 IF receiver

Philips Electronics 

Philips Semiconductors

IS-54 IF receiver

Preliminary specification

SA638

CIRCUIT DESCRIPTION

The Mixer

The mixer section converts signals up to 150MHz to a 2nd IF of up

to 2MHz (typically around 455KHz) with a power gain of 15dB, input

IP3 of –10dBm and NF of 8.5dB. This section is comnlon to both

modes of operation. An on–chip oscillator is provided (for use with

an external tank, varactor, and synthesizer). The LO section has a

buffer to drive the mixer differentially.

After external bandpass filtering, the signals enter the chip again into

the IF amplifier section. One input of the IF amp is biased to Vccmid

(which is Vcc/2 generated internally and by–passed externally) and

AC-coupled to the input signal externally while the other IF amp

input is fed back from the amplifier output and AC by–passed to

ground externally. This minimizes low frequency offsets since the

amp is DC-coupled with very high gain.

(Note that, compared to our standard 6xx IF chips, we are freeing

quite a few pins by using only 1 decoupling pin and by NOT splitting

up the IF amp into 2 parts and going in and out for filtering in

between.

In the AMPS mode the amplifier acts as a classic limiter with a fairly

constant output voltage for a very large input voltage range. The

RSSI (Received Signal Strength Indicator) is a temperature

compensated high impedance output signal which acts as a voltage

proportional to Logarithm of the input power and has a 90dB

dynamic range. In the Digital mode the IF amp acts as a linear AGC

(Automatic Gain Controlled) amplifier with maximum gain of 95dB

and an AGC range of 85dB. The control for the AGC can be derived

internally or supplied externally. If not supplied externally the

internal AGC detector will regulate the gain to set the IQ baseband

outputs to a pre–determined level (e.g., 250mVPEAK). The RSSI

output which is high impedance will then be available at Pin 6 after

external filtering. On the other hand an external AGC input can

force the voltage at this pin to regulate the gain externally based on

external measurement of say the IQ baseband output voltages. In

such a case the RSSI function is NOT provided by this pin. (This

has been done mainly to conserve pins. Depending on customer

feedback we can provide for separate RSSI output and AGC input

pins if necessary.)

The IQ Demodulator Section

The IQ Demodulator section takes the signal after external filtering

fronl the chip’s IF anlp output. The I/Q LO is supplied to the chip

externally as a x4 of the required correct LO signal such that the

internal divider can generate quadrature LO signals of the correct

frequency. After conversion to baseband the signals are filtered for

suppressing LO and its by–products. (We have to decide if this filter

will also provide for accurate baseband filtering. For example TI’s

ARCTIC baseband processor chip provides for such filtering in

which case we do not have to provide for it.) After buffering, the

single ended baseband IQ outputs are available for processing

externally. The signals are referenced to a fixed DC level generated

internally and available througn Pin 15. We can save a pin if the

Vccmid (Pin 21) can be used for this purpose. Single-ended outputs

as described above are used rather than differential outputs in order

to save pins.

The FM Demodulator section

The FM Demodulator section will incorporate a new technique wnich

will not require a quad tank. However, a backup technique of the

quad discriminator will be available if the new riskier approach fails.

Because of this, two pins have been allocated for the FM

demodulator in addition to the audio output (similar to our existing

FM–IF products).

In this pinout there is provision for 2 VCC pins and 2 ground pins in

addition to a VCC/2 bypass pin. This seems necessary in light of the

hign gain values involved in the IF amp block.

There are at least three pins which have dual functionality:

RSSI/AGC; Audio/LPF; Demod2/ IQ Offsets. We have to carefully

look at this to see if this is feasible. If not, the pinout has to be

changed appropriately.

Fallback Positions:

1. We have a fallback for the FM demodulator, the classic FM

discriminator.

2. If the “one IF amp will work for both modes’’ approach fails, we

can provide 2 separate IF amp paths and switch between them

for the different modes. The penalty is increased chip area.

3. The proposed solution for the lF amp is to have all its gain in one

section with no external filtering in between. This is aggressive

since our existing chips have 2 IF sections with separate DC

feedback for each section and external filtering in between. If the

proposed 1 section solution fails, going back to our old technique

will require more pins than we perhaps have!

1995 Feb 16

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