Car Antenna Amplifier Concepts

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					50   Atmel Automotive Compilation, Vol. 6



     Car Antenna Amplifier Concepts
     Stephan Gerlach

     With the antenna driver IC ATR4251,          It is no surprise that the collected     cially in a car with its inherent long-
     Atmel provides a very successful             field strength results in a very low     lasting temperature, humidity and
     solution for AM and FM antenna               antenna voltage, which is even           vibration test cycles.
     amplifiers. The device is mainly             worse if loaded with the high capac-
     used in the cars radio’s reception           itance of a long antenna cable. To       However, only combined integrated
     path facing challenging antenna              avoid this, it is recommended to         technologies – e.g., BiCMOS as
     types     and     reception      positions   provide, for example in AM band,         used in Atmel’s integrated antenna
     within the car body. The ATR4251’s           not only an LNA with an adjustable       amplifiers – can fully play its advan-
     advantages comprise high reliability         gain but also a buffer to drive long     tages over both discrete and simple
     and high functional density result-          50- or 75-Ohm cables.                    integrated CMOS solutions. An AM
     ing in small design, maximum AC/                                                      antenna has a very high impedance
     DC parameter repeatability during                                                     and a fairly low capacitance, result-
     production and long-time stability.          Advantages of IC                         ing in low antenna voltage. For such
     The IC’s high flexibility enables to         over Discrete Solu-                      an antenna, an FET transistor with
     meet the individual customer needs           tions                                    a low input capacitance is the pre-
     – especially in terms of cost and                                                     ferred choice as first stage amplifier
     performance ratio.                           There are various advantages of          to match the antenna. It also needs
                                                  antenna solutions using integrated       to be taken into account that the
     As car design requirements more              circuits over discrete solutions,        high impedance and the far better
     and     more      dominate       technical   amongst others, stable operating         noise/ linearity behaviour (com-
     aspects, however, the technical              point against temperature variation,     pared to a bipolar transistor within
     requirements for car antenna design          component tolerances and aging,          the AM frequency band) allow to
     also change. Since the amount of             as well as a smaller design form         fully use the small signal from the
     antennas in a medium- or luxury-             factor which allows adding features      antenna, and to handle large-signal
     class car easily reaches a number            that improve the technical perfor-       reception conditions that might
     of 20 to 30, current antennas need           mance but are too cost-expensive         occur during driving.
     to be almost invisible, or have at           when realized as a discrete solu-
     least a decent appearance. Con-              tion. Even if the amount of passive      The receiver probably needs a
     sequently, antennas – especially             and active components within the         long coaxial cable that may cause
     those for radio and TV reception –           integrated circuit exceeds those of      a substantial loss due to its high
     tend to be integrated into the cars’         a discrete solution, the stability and   inherent capacitance. The best way
     windows. This is critical due to the         reliability of an IC-based solution      to overcome such a challenge is
     limited window area, and also due            is far beyond the discrete solution.     to attach a driver or buffer ampli-
     to the fact that different antenna           This is due to the fact that each        fier right before the cable. A smart
     compete for the best window area             additional soldering connection is a     biasing technique plus a push-pull
     position.                                    potential failure root cause – espe-     circuit, comprising complementary

bipolar NPN and PNP transistors,                A combination of different transistor        Most advanced CMOS technolo-
perfectly matches the requirements              types is useful for all other antenna        gies restrict the voltage swing more
and will maintain the high linearity            amplifier features such as auto-             than necessary due to their inher-
performance of the entire amplifier             matic gain control (AGC), voltage            ent low breakthrough voltages. By
chain.                                          stabilization, plug detection, over-         using bipolar transistors, however,
                                                voltage and ESD protection.                  a maximum output voltage swing of
To achieve best FM amplifier low-                                                            up to 12 Vpp can be achieved at the
noise quality and easy input-output             For maximum linearity performance            output of the FM stage, resulting
matching, a bipolar transistor is the           and best large-signal behaviour it is        in excellent OIP3 (145 dBuV) and
best choice as GaAs or compara-                 advantageous to use the entire bat-          OIP2 (170 dBuV) values.
ble solutions are too expensive in              tery voltage range, provided that a
most cases.                                     well-stabilized regulation is in place.

                                         Voltage                        ATR4252

  8V to 16V DC              VS
                                         Voltage         Reference
                                        Regulator         Voltage

                                                                     FM Amp

                                             Pin Diode
                                                                                  Detector                AGC Threshold

                                                                                  FM AGC                  AGC Time Constant
         AM/FM Antenna

                                             Antenna                                          AM/FM                       Antenna
                                             Detection                                       Combiner                      Cable

                                                                                  AM AGC                  AGC Time Constant

                                             Pin Diode
                                                                                  Detector                AGC Threshold

                                              AM LNA                 AM Buffer

Figure 1. ATR4252 Block Diagram

52   Atmel Automotive Compilation, Vol. 6

     For the functional block “AGC” it         were carried out. The achieved                                        amplifier part. If a maximum band-
     is also advantageous to use mixed         results for the common base con-                                      width of 30 MHz (especially for DRM
     circuitry, e.g., bipolar transistors as   figuration are shown in figure 2,                                     reception) has to pass the low-pass
     current sources for the PIN diode         which illustrates the noise figure                                    filter of the combination point net-
     attenuators because bipolar tran-         comparison of final simulation run                                    work, the noise figure will be about
     sistors require less space than           and measurement within the 80- to                                     1.9 dB. For the most common AM
     comparable MOS transistors for            170-MHz FM frequency band.                                            bandwidth of 450 kHz to 1.7 MHz,
     the same function.                                                                                                                 however,        the
                                                                                                                                        increase is only
                                                              3.0                                                                       0.1    dB.     This
     Antenna Amplifier                                                                                                                  results in a final
     ATR4252                                                  2.5
                                                                                                                                        noise figure of
                                                   NF in dB

                                                                                                                                        about 1.6 dB,
     The ATR4252 is a further enhance-                                                                                                  which is still a
     ment: it integrates even more exter-                                                                                               very good value.
     nal components, functional blocks
     and features, and the technical                                                                                                    If the designer
     characteristics of the already exist-                    0.5
                                                                    80   90    100    110   120    130   140   150    160      170
                                                                                                                                        wants to focus
     ing blocks have been improved.                                                     Frequency in MHz                                mainly on maxi-

                                                                              NF / dB (measured)         NF / dB (simulated)
                                                                                                                                        mum        linearity
                                                                                                                                        (more than 145
     Performance Improvements                   Figure 2. FM Amplifier Noise Figure                                                     dBuV for OIP3)
                                                                                                                     and less on the NF performance,
     The ATR4252 provides outstand-            Thanks to proper current density,                                     this can easily be accomplished
     ing noise level and linearity perfor-     dedicated biasing techniques and                                      by adapting one external resistor
     mance in both separate as well as         further means, the achieved NF of                                     placed at the emitter of the FM tran-
     combined AM and FM band ampli-            1.5 dB in FM is even better than                                      sistor.
     fiers. The goal for the FM amplifier      results with dedicated discrete RF
     was to reduce the noise figure (NF)       transistors, and matches well with                                    A further reduction in BOM cost
     while maintaining the best possible       the pre-running simulations.                                          leads to the common emitter circuit
     intermodulation performance.              When combining the AM and FM                                          that can also be realized using the
                                               parts, however, it is inevitable to                                   ATR4252. The benefit of cost and
     To perform this task, extensive sim-      raise the FM noise figure at the                                      board space reduction is accompa-
     ulations of the circuit, bond wires,      combining point, depending on                                         nied with a slight reduction in noise
     package and external components           the required bandwidth of the AM                                      and intermodulation performance.

AM Amplifier                             Overvoltage Protection and               above the GND level) it can detect
                                         Antenna Detection                        a voltage window in the range of
One of the AM amplifier design                                                    0V to 3V, and 6V to 16V. The failure
goals was to provide a voltage gain      The antenna detection functional-        signalization to the car’s diagnos-
of about 9 dB in addition to the         ity is used to detect errors that may    tic system is performed by a pre-
insertion gain of the transimped-        occur due to the following three         defined DC current reduction of the
ance amplifier used in ATR4251,          main failure root causes:                complete antenna amplifier down
while keeping the noise and inter-                                                to 20 mA.
modulation level on the same level       1. A antenna plug that has not
as with ATR4251.                           been fit properly during car           Finally the overvoltage protection
This has been achieved by using a          production                             unit is able to reduce the overall
monolithic combination of MOS and        2. A broken cable or a damaged           current consumption of the com-
bipolar technology. The DC bias-           reception structure of a printed       plete IC to an innocuous value of
ing of the relevant RF transistors,        glass antenna                          12 mA in case the applied DC volt-
for example, has been optimized          3. A temporarily bad cable con-          age exceeds 16V.
for best noise and intermodulation         nection to antenna that is
performance. Since the ATR4252             caused by vibrations/ move-
offers maximum modularity, it is           ments of the car                       Summary
possible to insert an external filter
between LNA output and buffer            The ATR4252 enables to set-up            Thanks to the device's outstanding
input. Moreover, the voltage supply      diagnostic system, which can be          technical features, the new inte-
range has been extended to lower         realized as an additional plug con-      grated antenna amplifier ATR4252
values (8V) without noticeable per-      tact in the antenna-cable-amplifier      enables to meet the car industry's
formance reduction.                      chain or placed into the DC path of      highest performance and quality
                                         a window heating field in case the       requirements. Its inbuilt flexibility
In all other cases where this lower      antenna is part of this structure.       allows creating one standard plat-
value is not needed, the ATR4252                                                  form for the different car markets
provides a well stabilized and fil-      This is possible because the inte-       and antenna types, thus resulting
tered supply voltage of 10V due to       grated diagnostic system can both        in cost-efficient and durable prod-
the integrated voltage stabilization     drive a current as well as detect a      ucts.
circuit and one external PNP power       current that flows into the circuit.
transistor. This stabilization loop is   To realize one of the two possible
able to suppress ripple and noise        placements in the heating struc-
by more than 40 dB.                      ture (just below the 12V level or just