FMIN, FMAGC, FMRF
The FM local oscillator consists of a transistor in
grounded collector configuration. The negative
resistance at the base of the transistor is generated by an
external capacitor connected between emitter FMOSCE
(Pin 5) and GNDRF (Pin 4) An other external capacitor
is connected between base FMOSCB (Pin 6) and emitter
FMOSCE (Pin 5) which increases the resistance at the
emitter and leads to a higher oscillator swing. The
Wnegative resistance at FMOSCB is approximately 250 .
WTherefore, the resonant LO tank resistance of
approximately 5 k (depends on Q-factor of the coil) is
transformed to this magnitude via a capacitor.
The FM pre-amplifier input FMIN (Pin 28) consists of a
transistor grounded base circuit (T2) which provides
excellent noise performance and large signal behavior. It
Wis recommended to connect a source impedance of 100
in order to achieve optimal performance. The dc current
through the amplifying transitor is reduced by the internal
AGC. This means in the case of large input signals, the
input ac current is bypassed via the wideband AGC
transistor T1. A capacitor is connected between FMAGC
(Pin 2) and GNDRF (Pin 4). It shortens the transistor base
to GNDRF and it also serves for smoothing of the AGC
voltage. A tuned RF circuit is connected between FMRF
(Pin 3) and VS (Pin 27). The amplified RF signal is fed
internally to the mixer input.
A resistor is connected between OSCOUT (Pin 8) and
VREF (Pin 14). It determines the amplitude of oscillator
voltage which is fed to the PLL circuit. The TEMIC PLL
familiy U428xBM is recommended because it offers high
signal to noise ratio and low current consumption.
Rev. A2, 04-Nov-96