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ML13156

Wideband FM IF System

LANSDALE Semiconductor Inc. 

LANSDALE Semiconductor, Inc.

ML13156

Legacy Applications Information

COMPONENT SELECTION

The evaluation PC board is designed to accommodate specific

components, while also being versatile enough to use components

from various manufacturers and coil types. Figures 18 and 19 show

the placement for the components specified in the application cir-

cuit (Figure 17). The application circuit schematic specifies particu-

lar components that were used to achieve the results shown in the

typical curves and tables but equivalent components should give

similar results.

INPUT MATCHING NETWORKS.COMPONENTS

The input matching circuit shown in the application circuit

schematic is passive high pass network which offers effective image

rejection when the local oscillator is below the RF input frequency.

Silver mica capacitors are used for their high Q and tight tolerance.

The PC board is not dedicated to any particular input matching net-

work topology; space is provided for the designer to breadboard as

desired.

Alternate matching networks using 4:1 surface mount transformers

or BALUNs provide satisfactory performance. The 12 dB SINAD

sensitivity using the above matching networks is typically –100

dBm for fmod = 1.0 kHz and fdev = ±75 kHz at fIN = 144.45

MHz and fOSC = 133.75 MHz (see Figure 23).

It is desirable to use a SAW filter before the mixer to provide addi-

tional selectivity and adjacent channel rejection and improved sen-

sitivity. The SAW filter should be designed to interface with the

mixer input impedance of approximately 1.0 kΩ. Table 1 displays

the series equivalent single–ended mixer input impedance.

LOCAL OSCILLATORS

VHF APPLICATIONS – The local oscillator circuit shown in the

application schematic utilizes a third overtone crystal and an RF

transistor. Selecting a transistor having good phase noise perform-

ance is important; a mandatory criteria is for the device to have

good linearity of beta over several decades of collector current. In

other words, if the low current beta is suppressed, it will not offer

good 1/f noise performance. A third overtone series resonant crystal

having at least 25 ppm tolerance over the operating temperature is

recommended. The local oscillator is an impedance inversion third

overtone Colpitts network and harmonic generator. In this circuit a

560 to 1.0 kΩ resistor shunts the crystal to ensure that it operates in

its overtone mode; thus, a blocking capacitor is needed to eliminate

the dc path to ground. The resulting parallel LC network should

“free–run” near the crystal frequency if a short to ground is placed

across the crystal. To provide sufficient output loading at the collec-

tor, a high Q variable inductor is used that is tuned to self resonate

at the 3rd harmonic of the overtone crystal frequency.

The on–chip grounded collector transistor may be used for HF and

VHF local oscillator with higher order overtone crystals. Figure 18

shows a 5th overtone oscillator at 93.3 MHz and Figure 19 shows a

7th overtone oscillator at 148.3 MHz. Both circuits use a Butler

overtone oscillator configuration. The amplifier is an emitter fol-

lower. The crystal is driven from the emitter and is coupled to the

high impedance base through a capacitive tap network. Operation at

the desired overtone frequency is ensured by the parallel resonant

circuit formed by the variable inductor and the tap transistor and

PC board. The variable inductor specified in the schematic could be

replaced with a high tolerance, high Q ceramic or air wound sur-

face mount component. if the other component have good toler-

ance. A variable inductor provides an adjustment for gain and fre-

quency of the resonant tank ensuring lock up and start up of the

crystal oscillator. The overtone crystal is chosen with ESR of typi-

cally 80 Ω and 120 Ω maximum; if the resistive loss in the crystal

is too high, the performance of the oscillator may be impacted by

lower gain margins.

Table 1. Mixer Input Impedance Data

(Single–ended configuration, VCC = 3.0 Vdc, local oscillator drive = 100 mVrms)

Frequency

(MHz)

Series Equivalent

Complex Impedance

(R + jX)

(Ω)

Parallel

Resistance

Rp

(Ω)

Parallel

Capacitance

Cp

(pF)

90

190 – j380

950

4.7

100

160 – j360

970

4.4

110

130 – j340

1020

4.2

120

110 – j320

1040

4.2

130

97 – j300

1030

4.0

140

82 – j280

1040

4.0

150

71 – j270

1100

4.0

160

59 – j260

1200

3.9

170

52 – j240

1160

3.9

180

44 – j230

1250

3.8

190

38 – j220

1300

3.8

Page 12 of 21

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