TBA120 Series FM Demodulator ICs.
Version 1.00. 29th Sept. 2006.
The TBA120 Series ICs provide a high-gain limiting IF amplifier
and a quadrature coincidence detector in one package. These ICs
are primarily intended for extraction of TV intercarrier sound,
which (in Europe) is frequency modulated onto a 5.5MHz or 6MHz
subcarrier. The TV waveform constitutes an extreme interfering
signal, and the TBA120 provides superior performance in comparison
to early Foster-Seeley and Ratio Detector systems, with a considerable
reduction in component count.
The equivalent circuit of the original TBA120 (TBA120A, SN76660)
is given below. Input to the IF amplifier is at pin 14, with DC
bias provided at pin 13. The original intention was that an IF
transformer should be connected between pins 13 and 14, but a
capacitive or ceramic filter input can be implemented simply by
connecting a resistor (Rin
in place of
the coil to provide the DC feedback. Typical values for Rin
lie between 330Ω and
in the case of ceramic filter input, according to the filter maker's
recommendations. The capacitor network between pins 2 and 13 provides
AC feedback to stabilise the amplifier.
The FM demodulator simply requires an LC resonator and a pair
of feedback capacitors (Cx
For 5.5 or
6MHz FM systems, the recommended value for Cx
is 50pF, but for lower frequencies a larger value is required.
A value of 1nF gives satisfactory NBFM performance at both 455KHz
and 100KHz. Manufacturers tend not to quote maximum operating
speed, but there seems to be no reason why the IC should not also
work (as a wideband FM detector) at 10.7MHz.
The output at pin 8 follows the familiar S-curve characteristic
of FM discriminators. The figure on the right shows a typical S-curve,
plotted against a notional IF bandpass characteristic. The actual
shape of the IF bandpass plateau is of little importance, since
limiting of the IF amplifier occurs with about 30μV input and
the system is effectively immune to amplitude fluctuations. The
DC component swings about a nominal 7V (for the recommended Vcc
= 12V), and can be used, in conjunction with a bridge circuit,
to provide a centre-zero tuning meter. Note however, that the
DC level drifts with chip temperature, and temperature compensation
of the bridge is required if the tuning meter needs to be calibrated
on zero within 15 minutes of switch-on.
Television and Broadcast FM systems use HF pre-emphasis in transmission
and subsequent de-emphasis in the receiver. The de-emphasis capacitor
is placed across the nominal 2.6K output load resistor; ie., between
pins 8 and 11, or between pin 8 and ground. For a TV system, the
required time constant (t=CR) is 50μS, giving a de-emphasis
capacitor value of 19.2nF, or 22nF in practice. Note however,
that in subsequent revisions of the TBA120, the audio output impedance
has been changed, and the capacitor must be changed accordingly.
For 4KHz bandwidth communication systems, the de-emphasis capacitor
is irrelevant, and is left in situ to facilitate removal of IF
components from the audio output.
The slope of the S-curve depends on the LC ratio and Q of the
quadrature circuit resonator. Manufacturers tend not to publish
data on this subject, but for NBFM systems the following have
been found to work well: From ref. 1; at 455KHz, C = 10nF, L =
50 turns on a 3/16" dia former with slug, gives 200mV audio
for 3KHz deviation. From the author's work; at 100KHz, C = 18nF,
L = 130 turns pile-wound 32swg on a 3/16" former with slug,
gives approx. 1.9V rms for 3KHz deviation. In the example circuits
to follow, note that some designers have placed a resistor across
the LC circuit to moderate the Q.
The equivalent circuit of the TBA120S, with suitable external
components for 5.5MHz use, is shown below. The TBA120S provides
additional functionality over the TBA120, in the form of a 12V
(nominal) zenner diode at pin 12, an audio amplifier at pins 4
and 3, a remote AF gain control input at pin 5, and internal 50pF
quadrature feedback capacitors across pins 9 and 10 and pins 6
and 7. External capacitors can, of course, be added to make the
demodulator work at low frequencies as before.
Supply Voltage Range: 6 - 18V.
Input Limiting Voltage (f=5.5MHz, Δf=±50KHz, fmod
=1KHz, Q=45): <60μV,
AF Gain control range (pin 5): >70dB. 75dB typ.
Some TBA120S circuit examples are shown below:
The TBA120U is the currently available version. In this IC, the
remote gain control facility has been considerably improved, voltage
regulation has been incorporated into the main circuit and pin
12 has become a fixed level audio output, and pin 3 has become
an auxiliary AF input (for VCR sound). Its IF and demodulator
functionality, and adaptability for LF use, is otherwise unchanged
voltage (pin 11)
|| 10 - 18V.
|| 13.5mA typ.
| IF Voltage
|| 68dB typ.
<60μV. 30μV typ.
| IF input
40KΩ // 4.5pF typ.
resistor (pin 13 - 14)
Suppression (50KHz deviation):
>50dB. 60dB typ.
| AF output
for 50KHz deviation:
|| 1.2V rms
1.0V rms (pin 12)
1KΩ nom. (pins 8 & 12)
|| 30dB (pin
35dB (pin 8) typ.
Adjustment resistor (pins 4 - 5)
|| 1 to 10
>70dB, typ. 85dB.
(50KHz deviation, Q=20)
|| 1% typ.
gain, pin 3 - pin 8 (R5-1=20K)
Application circuitry for the TBA120U is given below. Both pins
12 and 8 require de-emphasis capacitors, the values shown being
for 50μS. Pins 4 and 5 must be joined together when the remote
volume adjustment facility is not used.
The TBA120T is a version optimised for ceramic input filter and
ceramic resonator in the quadrature circuit. The Differences between
the TBA120T and TBA120U are shown in the figure below. Note that
the TBA120T has an internal 800Ω bypass resistor between
pins 13 and 14, and a 500Ω resistor between pins 7 and 9;
the latter Q reducing resistor making this IC a poor (but not
unworkable) choice if an LC resonator is to be used.
1) J.R.Hey (G3TDZ), "Consumer ICs in Amateur Design",
RAD. COM., Sept. 1972.
1a) NBFM Manual, R.S.Hewes (G3TDR) and G.R.Jessop (G6JP), RSGB
2) Various TV service manuals.
3) A. Franzen, "6MHz TV Sound Demodulator", Elektor,
4) Philips Components Technical Handbook, Book 4, Part IC02a,
"TV, Video and assoc. systems, Bipolar, MOS", 1989.
5) Telefunken electronic, Selection Guide, Integrated Circuits,
"Using the low-cost TBA120 ic device" Tech Topics, Rad
Com, Nov 1985, pp860-861.
Subject: Use of the TBA120 as an oscillator + balanced mixer in
the LA8AK audio up-converter for tape-recording of high-speed
morse. Circuit can also be used as a balanced mixer for SSB generation,
or as a product detector.
(electronic parts, including TBA120B,C,S,T,U,UB)