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Fabricante

ML13156

Wideband FM IF System

LANSDALE Semiconductor Inc. 

ML13156

LANSDALE Semiconductor, Inc.

CIRCUIT DESCRIPTION

GENERAL

The ML13156 is a low power single conversion wideband FM

receiver incorporating a split IF. This device can be used as a single

conversion receiver or as the backend in digital FM systems such as

CT–2 and wide band data links with data rates up to 500 kbaud. It

contains a mixer, oscillator, signal strength meter drive, IF amplifi-

er, limiting IF, quadrature detector and a data slicer with a hold

function (refer to Figure 8, Simplified Internal Circuit Schematic).

CURRENT REGULATION

Temperature compensating voltage independent current regula-

tors are used throughout.

MIXER

The mixer is a double–balanced four quadrant multiplier and is

designed to work up to 500 MHz. It can be used in differential

or in single–ended mode by connecting the other input to the

positive supply rail.

Figure 4 shows the mixer gain and saturated output response as

a function of input signal drive. The circuit used to measure this

is shown in Figure 1. The linear gain of the mixer is approxi-

mately 22 dB. Figure 9 shows the mixer gain versus the IF out-

put frequency with the local oscillator of 150 MHz at 100

mVms LO drive level. The RF frequency is swept. The sensitivi-

ty of the IF output of the mixer is shown in Figure 10 for an RF

input drive of 10 mVrms at 140 MHz and IF at 10 MHz.

RSSI current output is derived by summing the currents for the

IF and limiting amplifier stages. An external resistor at Pin 20

sets the voltage range or swing of the RSSI output voltage.

Linearity of the RSSI is optimized by using external ceramic or

crystal bandpass filters which have and insertion loss of 8.0 dB.

The RSSI circuit is designed to provide 70+ dB of dynamic

range with temperature compensation (see Figures 6 and 7

which show RSSI responses of the IF and Limiter amplifiers).

Variation in the RSSI output current with supply voltage is 5 ma

total delta (see Figure 11).

CARRIER DETECT

When the meter current flowing through the meter load resist-

ance reaches 1.2 Vdc above ground, the comparator flips, caus-

ing the carrier detect output to go high. Hysteresis can be

accomplished by adding a very large resistor for positive feed-

back between the output and the input of the comparator.

IF AMPLIFIER

The first IF amplifier section is composed of three differential

stages with the second and third stages contributing to the RSSI.

This section has internal dc feedback and external input decou-

pling for improved symmetry and stability. The total gain of the

IF amplifier block is approximately 39 dB at 10.7 MHz. Figure

5 shows the gain and saturated output response of the IF ampli-

fier over temperature, while Figure 12 shows the IF amplifier

gain as a function of the IF frequency.

The single–ended parallel equivalent input impedance of the

mixer is Rp ~ 1.0 kΩ and Cp ~ 4.0 pF (see Table 1 for details).

The buffered output of the mixer is internally loaded resulting in

an output impedance of 330 Ω.

The fixed internal input impedance is 1.4kΩ. It is designed for

application where a 455 kHz ceramic filter is used and no exter-

nal output matching is necessary since the filter requires a 1.4

kΩ source and load impedance.

LOCAL OSCILLATOR

The on–chip transistor operates with crystal and LC resonant

elements up to 220 MHz. Series resonant, overtone crystals are

used to achieve excellent local oscillator stability. 3rd overtone

crystals are used through about 65 to 70 MHz. Operation from

70 MHz up to 180 MHz is feasible using the on–chip transistor

with a 5th or 7th overtone crystal. To enhance operation using

an overtone crystal, the internal transistor’s bias is increased by

adding an external resistor from Pin 23 to VEE. –10 dBm of

local oscillator drive is needed to adequately drive the mixer

(Figure 10).

The oscillator configurations specified above, and two others

using an external transistor, are described in the application sec-

tion:

1) A 133 MHz oscillator multiplier using a 3rd overtone

crystal, and

2) A 307.8 to 309.3 MHz manually tuned, varactor

controlled local oscillator.

RSSI

The Received Signal Strength Indicator (RSSI) output is a cur-

rent proportional to the log of the received signal amplitude. The

For 10.7 Mhz ceramic filter applications, an external 430 Ω

resistor must be added in parallel to provide the equivalent load

impedance of 330 Ω that is required by the filter; however, no

external matching is necessary at the input since the mixer out-

put matches the 330 Ω source impedance of the filter. For 455

kHz applications, an external 1.1 kΩ resistor must be added in

series with the mixer output to obtain the required matching

impedance of 1.4 kΩ of the filter input resistance. Overall RSSI

linearity is dependent on having total midband attenuation of 12

dB (6.0 dB insertion loss plus 6.0 dB impedance matching loss)

for the filter. The output of the IF amplifier is buffered and the

impedance is 290 Ω.

LIMITER

The limiter section is similar to the IF amplifier section except

that four stages are used with the last three contributing to the

RSSI. The fixed internal input impedance is 1.4 kΩ. The total

gain of the limiting amplifier sections is approximately 55 dB.

This IF limiting amplifier section internally drives the quadra-

ture detector section.

Page 6 of 21

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