LANSDALE Semiconductor, Inc.

ML13150

Figure 12. BWadj Current

versus BWadj Voltage

BWadj VOLTAGE (Vdc)

Figure 13. Demodulator Output

versus Frequency

2.3

2.5

2.7

f, FREQUENCY (kHz)

0.1

1.0

10

100

10–3

10–4

10–5

10–6

10–7

VCC = 3.0 Vdc

TA = 25 C

10

0

–10

–20

–30

–50

–40

VCC = 3.0 Vdc

TA = 25 C

fRF = 50 MHz

fLO = 50.455 MHz

LO Level =–10 dBm

No IF Bandpass Filters

fdev = ±4.0 kHz

RB = 560 k

RB = 1.0 M

COILLESS DETECTOR

The quadrature detector is similar to a PLL. There is an inter-

nal oscillator running at the IF frequency and two detector

outputs. One is used to deliver the audio signal and the other

one is filtered and used to tune the oscillator.

The oscillator frequency is set by and external resistor at the

Fadj pin. Figure 9 shows the control current required for a

particular frequency; Figure 10 shows the pin voltage at that

current. From this the value of RF is chosen. For example,

455 kHz would require a current of around 50

µA. The pin

voltage (Pin 16 in the 32 pin QFP package) is around 655mV

giving a resistor of 13.1 k

Ω. Choosing 12 kΩ as the nearest

standard value gives a current of approximately 55

µA. The

5.0

µA difference can be taken up by the tuning resistor, RT.

The best nominal frequency for the AFTout pin (Pin 17)

would be half supply. A supply voltage of 3.0 Vdc suggests a

resistor value of (1.5 – 0.655) V/5.0

µA = 169 kΩ. Choosing

150 k

Ω would give a tuning current of 3/150 kΩ = 20 µA.

From Figure 9 this would give a tuning range of roughly 10

kHz/

µA or ± 100 kHz which should be adequate.

The bandwidth can be adjusted with the help of Figure 11.

For example, 1.0

µA would give a band width of ± 13 kHz.

The voltage across the bandwidth resistor, RB from Figure 12

is VCC – 2.44 Vdc = 0.56 Vdc for VCC = 3.0 Vdc, so RB =

0.56V/1.0

µA = 560 kΩ. Actually the locking range will be

±13 kHz while the audio bandwidth wil be approximately

±8.4 kHz due to an internal filter capacitor. This is verified in

Figure 13. For some applications it may be desireable that the

audio bandwidth is increased; this is done by reducing RB.

Reducing RB widens the detector bandwidth and improves

the distortion at high input levels at the expense of 12 dB

SINAD sensitivity. The low frequency 3.0dB point is set by

the tuning circuit such that the product

RTCT = 0.68/f3dB.

So, for example, 150 k

Ω and 1.0 µF give a 3.0 dB point of

4.5 kHz. The recovered audio is set by RL to give roughly

50mV per kHz deviation per 100 k of resistance. The dc

level can be shifted by RS from the nominal 0.68 V by the

following equation:

Detector DC Output = ((RL + RS)/RS) 0.68 Vdc

Thus RS = RL sets the output at 2 x 0.68 = 1.36 V; RL =

2RS sets the output at 3 x .068 = 2.0V.

www.lansdale.com

Page 7 of 20

Issue A