U4065B Datasheet, PDF (6/23 Pages) TEMIC Semiconductors – FM Receiver

U4065B
Functional Description
The U4065B FM-frontend IC is the dedicated solution for
high end car radios. A new design philosophy enables to
build up tuners with superior behavior. This philosophy
is based on the fact that the sensitivity of state of the art
designs is at the physical border and cannot be enhanced
any more. On the other hand, the spectral power density
in the FM-band increases. An improvement of reception
can only be achieved by increasing the dynamic range of
the receiver. This description is to give the designer an
introduction to get familiar with this new product and its
philosophy.
1. The Signal Path
The U4065B offers the complete signal path of an FM-
frontend including a highly linear mixer and two IF
preamplifiers. The mixer is a double balanced high cur-
rent Gilbert Cell. A high transit frequency of the internal
transistors enables the use of the emitter grounded circuit
with its favorable noise behavior. The full balanced out-
put offers LO carrier reduction.
The following IF preamplifier has a dB-linear gain adjust-
ment by dc means. Thus different ceramic filter losses can
be compensated and the overall tuner gain can be adapted
to the individual requirements. The low noise design sup-
W ^{presses post stage noise in the signal path. Input- and}
output resistance is 330 to support standard ceramic fil-
ters. This was achieved without feedback, which would
cause different input impedances when varying the output
impedance.
W ^{The second IF preamplifier enables the use of three ce-}
ramic filters with real 330 input- and output
termination. Feedthrough of signals is kept low. The high
level of output compression is necessary to keep up a high
dynamic range.
Beneath the signal path the local oscillator part and the
AGC signal generation can be found on chip. The local
oscillator uses the collector grounded colpitts type. A low
phase noise is achieved with this access. A mutual cou-
pling in the oscillator coil is not necessary.
2. The AGC Concept
Special care was taken to design a unique AGC concept.
It offers 3 AGC loops for different kinds of reception
conditions. The most important loop is the interference
sensor part.
In today’s high end car radios, the FM AGC is state of the
art. It is necessary to reduce the influence of 3rd and
higher order intermodulation to sustain reception in the
presence of strong signals in the band. On one hand, it
makes a sense to reduce the desired signal level by AGC
as few as possible to keep up stereo reception, on the other
hand two or more strong out of channel signals may inter-
fere and generate an intermodulation signal on the desired
frequency. By introducing input attenuation, the level of
the intermod signal decreases by a higher order, whereas
the level of the desired signal shows only a linear depen-
dency on the input attenuation. Therefore input
attenuation by pin diodes may keep up reception in the
presence of strong signals.
The standard solution to generate the pin diode current is
to pick up the RF-signal in front of the mixer. Because the
bandwidth at that point is about 1.5 MHz, this is called
wideband AGC. The threshold of AGC start is a critical
parameter. A low threshold does not allow any intermo-
dulation but has the disadvantage of blocking if there is
only one strong station on the band or if the intermod sig-
nals do not cover the desired channel. A higher AGC
threshold may tolerate a certain ground floor of intermo-
dulation. This avoids blocking, but it has the
disadvantage, that no reception is possible, if the interfer-
ing signals do generate an intermod signal inside the
desired channel. This contradiction could not be over-
come in the past.
With the new U4065B IC, a unique access to this problem
appears. This product has an interference sensor on chip.
Thus an input signal attenuation is only performed, if the
interfering signals do generate an intermod signal inside
the desired channel. If they do not, the still existing wide-
band AGC is yet active but at up to 20 dB higher levels.
The optimum AGC state is always generated.
The figures 1 to 4 illustrate the situation. In figure 1 the
AGC threshold of a standard tuner is high to avoid block-
ing. But then the intermod signal suppresses the desired
signal. The interference sensor of the U4065B takes care
that in this case the AGC threshold is kept low as illus-
trated in figure 2.
In figure 3 the situation is vice versa. The AGC threshold
of a standard tuner is kept low to avoid intermod prob-
lems. But then blocking makes the desired signal level
drop below the necessary stereo level. In this case, the
higher wideband AGC level of the U4065B enables per-
fect stereo reception.
By principle, this interference sensor is an element with
a third order characteristic. For input levels of zero, the
output level is zero, too. With increasing input level, the
output level is increased with the power of three, thus pre-
ferring intermod signals compared to linear signals. At
the same time, a down conversion to the IF level of
10.7 MHz is performed. If a corresponding 10.7 MHz IF
filter selects the intermod signals, an output is only gener-
ated, if an intermod signal inside the 10.7 MHz channel
is present.
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Rev. A3, 15-Oct-98