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SA624

High performance low power FM IF system with high-speed RSSI

Philips Electronics 

Philips Semiconductors

High performance low power FM IF system with

high-speed RSSI

Product specification

SA624

ω1

(2)

φ = ∠VO - ∠VIN = tg-1

Q1ω

1

–

(

ω1

ω

)2

( ) Figure 12 is the plot of φ vs. ω

ω1

It is notable that at ω = ω1, the phase shift is

π and the response is close to a straight

2

line with a slope of

∆φ

∆ω

=

2Q1

ω1

The signal VO would have a phase shift of

π

2

–

2Q1

ω1

ω

with respect to the VIN.

If VIN = A Sin ωt ⇒ VO = A

(3)

Sin

ωt +

π

2

–

2Q1

ω1

ω

Multiplying the two signals in the mixer, and

low pass filtering yields:

VIN • VO = A2 Sin ωt

(4)

Sin

ωt +

π

2

–

2Q1

ω1

ω

after low pass filtering

⇒ VOUT = 1 A2 Cos

2

π

2

–

2Q1

ω1

ω

(5)

( ) =

1 A2

2

Sin

2Q1

ω1

ω

( ) VOUT

∝

2Q1

ω1

ω

=

2Q1

ω1 + ∆ω

ω1

(6)

For

2Q1ω

ω1

<<

π

2

Which is discriminated FM output. (Note that ∆ω is the deviation

frequency from the carrier ω1.

Ref. Krauss, Raab, Bastian; Solid State Radio Eng.; Wiley, 1980, p.

311. Example: At 455kHz IF, with +5kHz FM deviation. The

maximum normalized frequency will be

455 +5kHz

= 1.010 or 0.990

455

Go to the f vs. normalized frequency curves (Figure 12) and draw a

vertical straight line at

ω

ω1

=

1.01.

The curves with Q = 100, Q = 40 are not linear, but Q = 20 and less

shows better linearity for this application. Too small Q decreases

the amplitude of the discriminated FM signal. (Eq. 6) ⇒ Choose a

Q = 20

The internal R of the 624 is 40k. From Eq. 1c, and then 1b, it results

that

CP + CS = 174pF and L = 0.7mH.

A more exact analysis including the source resistance of the

previous stage shows that there is a series and a parallel resonance

in the phase detector tank. To make the parallel and series

resonances close, and to get maximum attenuation of higher

harmonics at 455kHz IF, we have found that a CS = 10pF and CP =

164pF (commercial values of 150pF or 180pF may be practical), will

give the best results. A variable inductor which can be adjusted

around 0.7mH should be chosen and optimized for minimum

distortion. (For 10.7MHz, a value of CS = 1pF is recommended.)

Audio Outputs

Two audio outputs are provided. Both are PNP current-to-voltage

converters with 55kΩ nominal internal loads. The unmuted output

is always active to permit the use of signaling tones in systems such

as cellular radio. The other output can be muted with 70dB typical

attenuation. The two outputs have an internal 180° phase

difference.

The nominal frequency response of the audio outputs is 300kHz.

this response can be increased with the addition of external

resistors from the output pins to ground in parallel with the internal

55k resistors, thus lowering the output time constant. Singe the

output structure is a current-to-voltage converter (current is driven

into the resistance, creating a voltage drop), adding external parallel

resistance also has the effect of lowering the output audio amplitude

and DC level.

This technique of audio bandwidth expansion can be effective in

many applications such as SCA receivers and data transceivers.

Because the two outputs have a 180° phase relationship, FSK

demodulation can be accomplished by applying the two output

differentially across the inputs of an op amp or comparator. Once

the threshold of the reference frequency (or “no-signal” condition)

has been established, the two outputs will shift in opposite directions

(higher or lower output voltage) as the input frequency shifts. The

output of the comparator will be logic output. The choice of op amp

or comparator will depend on the data rate. With high IF frequency

(10MHz and above), and wide IF bandwidth (L/C filters) data rates in

excess of 4Mbaud are possible.

RSSI

The “received signal strength indicator”, or RSSI, of the SA624

demonstrates monotonic logarithmic output over a range of 90dB.

The signal strength output is derived from the summed stage

currents in the limiting amplifiers. It is essentially independent of the

IF frequency. Thus, unfiltered signals at the limiter inputs, spurious

products, or regenerated signals will manifest themselves as RSSI

outputs. An RSSI output of greater than 250mV with no signal (or a

very small signal) applied, is an indication of possible regeneration

or oscillation.

In order to achieve optimum RSSI linearity, there must be a 12dB

insertion loss between the first and second limiting amplifiers. With

a typical 455kHz ceramic filter, there is a nominal 4dB insertion loss

in the filter. An additional 6dB is lost in the interface between the

filter and the input of the second limiter. A small amount of

additional loss must be introduced with a typical ceramic filter. In the

test circuit used for cellular radio applications (Figure 5) the optimum

linearity was achieved with a 5.1kΩ resistor from the output of the

first limiter (Pin 14) to the input of the interstage filter. With this

resistor from Pin 14 to the filter, sensitivity of 0.25µV for 12dB

SINAD was achieved. With the 3.6kΩ resistor, sensitivity was

optimized at 0.22µV for 12dB SINAD with minor change in the RSSI

linearity.

Any application which requires optimized RSSI linearity, such as

spectrum analyzers, cellular radio, and certain types of telemetry,

1997 Nov 07

10

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