MC13156

6

MOTOROLA WIRELESS SEMICONDUCTOR

SOLUTIONS – RF AND IF DEVICE DATA

CIRCUIT DESCRIPTION

General

The MC13156 is a low power single conversion wideband

FM receiver incorporating a split IF. This device is designated

for use as the backend in digital FM systems such as CT–2

and wideband data links with data rates up to 500 kbaud. It

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

amplifier, 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

regulators 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 approximately 22 dB. Figure 9 shows the mixer gain

versus the IF output frequency with the local oscillator of

150 MHz at 100 mVrms LO drive level. The RF frequency is

swept. The sensitivity 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.

The single–ended parallel equivalent input impedance of

the mixer is Rp ~ 1.0 k

Ω 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.

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 section:

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

2) A 307.8 to 309.3 MHz manually tuned, varactor controlled

RSSI

The Received Signal Strength Indicator (RSSI) output is a

current proportional to the log of the received signal

amplitude. The RSSI current output is derived by summing

the currents from 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

an 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 small (see

Figure 11).

Carrier Detect

When the meter current flowing through the meter load

resistance reaches 1.2 Vdc above ground, the comparator

flips, causing the carrier detect output to go high. Hysteresis

can be accomplished by adding a very large resistor for

positive feedback 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 decoupling 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 amplifier over

temperature, while Figure 12 shows the IF amplifier gain as a

function of the IF frequency.

The fixed internal input impedance is 1.4 k

Ω. It is designed

for applications where a 455 kHz ceramic filter is used and no

external output matching is necessary since the filter requires

a 1.4 k

Ω source and load impedance.

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 output matches the 330

Ω source impedance

of the filter. For 455 kHz applications, an external 1.1 k

Ω

resistor must be added in series with the mixer output to

obtain the required matching impedance of 1.4 k

Ω 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 k

Ω. The total gain of the limiting amplifier section is

approximately 55 dB. This IF limiting amplifier section

internally drives the quadrature detector section.