SUMMER CIRCUITSCOLLECTION

75

7-8/2001

Elektor Electronics

G.Baars

This 455-kHz quadrature detector for narrow-band FM sig-
nals boasts two important advantages: it is pleasantly sim-
ple and it does not require any alignment.
The heart of the circuit is formed by the well-known NE612
IC, which is a double-balanced mixer cum oscillator in an
8-pin DIL package.
The signal is first buffered by T1 and then fed to the input
of the NE612. At the same time, a small portion of the sig-
nal is passed to the mixer via a low-value capacitor (C4).
The operation of the circuit is such that when the input fre-
quency matches the resonant frequency of the parallel L-
C network, the signal on pin 7 has a phase lead of 90
degrees with respect to the signal on pin 2. The phase
angle increases when the input frequency rises and
decreases when the input frequency drops. Since the sig-
nals on pins 2 and 7 are multiplied together, the average
output level is maximum when the signals are in phase and
zero when they are anti-phase. This is the operating point of
the detector. Consequently, an input signal with a varying
frequency produces an output signal with a varying level.
The operating range of the detector is inversely propor-
tional to the Q factor of the parallel resonant network.
This circuit works best with an input signal level of
0.5–2 V

pp

. Since it is linear over a very wide range

(420–500 kHz), it does not need alignment, and normal tol-
erance variations in the values of the inductance and
capacitance in the resonant circuit have little effect. The
output level varies by approximately 1 V over the working
range, so the detection sensitivity is around 13 mV/kHz.
This is adequate for most narrow-band FM applications

with an intermediate frequency of 455 kHz.
The supply voltage may lie between 4.5 and 8 V. The cur-
rent consumption is limited to approximately 2.5 mA. Using
the small printed circuit board shown here, you should
have no difficulty assembling this FM detector in less than
half an hour.

(014002-1)

040

Alignment-free FM Detector

T1

BC547

NE612AN

OUT A

OUT B

IC1

IN A

IN B

OSC

1

2

5

4

8

3

6

7

R1

560k

R2

560k

C5

10n

C6

100p

C7

82n

C8

100n

C1

22n

C3

10n

C2

10n

C4

33p

L1

820µH

JP1

R3

47

Ω

K1

4V5...8V

4mA

014002 - 11

3V5

3V

1V4

6V

4V9

FM

5V3

014002-1

(C) ELEKTOR

C1

C2

C3

C4

C5

C6

C7

C8

H1

H2

H3

H4

H6

IC1

K1

L1

R1

R2

R3

T1

014002-1

+

0

014002-1

(C) ELEKTOR

COMPONENTS LIST

Resistors:

R1,R2 = 560k

Ω

R3 = 47

Ω

Capacitors:

C1 = 22nF
C2,C3,C5 = 10nF
C4 = 33pF

C6 = 100pF
C7 = 82nF
C8 = 100nF

Inductors:

L1 = 820µH

Semiconductors:

T1 = BC547
IC1 = NE612AN