ML13146
LANSDALE Semiconductor, Inc.
CIRCUIT DESCRIPTION
General
The ML13146 consists of a low power amplifier, a 50
Ω
lin-
ear mixer with linearity control, divide by 64/65 dual modulus
prescaler and LPA. This device is designated for use as the low
power transmitter in analog and digital FM systems such as
UHF and 800 MHz Special Mobile Radio (SMR), UHF Family
Radio Services, PCS and 902 to 928MHz cordless telephones.
It features a mixer linearity control to preset or auto program
the mixer dynamic range, an enable function and a wideband
mixer output so the IC may be used either as an up converter
or for a direct conversion source. Additional details are covered
in the Pin by Pin Description which shows the equivalent inter-
nal circuit and external circuit requirements.
Current Regulation/Enable
The device features temperature compensating, voltage inde-
pendent current regulators which are controlled by the enable
function in which “high” powers up the IC.
Mixer: General
The mixer is a double–balanced four quadrant multiplier
biased class AB allowing for programmable linearity control
via an external current source. An input third order intercept
point of 20 dBm has been achieved. The mixer has a 50
Ω
sin-
gle–ended RF input and open collector differential outputs. An
onboard Local Oscillator transistor has the emitter, base and
collector pinned out to implement a low phase noise VCO in
various configurations. Additionally, a buffered prescaler out-
put is provided for operation with a low frequency synthesizer.
For direct conversion applications the input of the mixer may
be terminated to ground through a 120 to 330
Ω
resistor.
Local Oscillator/Voltage Control Oscillator
The on–chip transistor operates with coaxial transmission
line or LC resonant elements to over 1.8 GHz. Biasing is done
with a temperature/voltage compensated current source in the
emitter. A RFC from VCC to the base is recommended.The
transistor can be operated in the classic Colpitts, Clapp, or
Hartley configuration. The application circuit (Figure 8)
depicts a parallel resonant VCO which can cover the entire 902
to 928 MHz frequency band with phase noise of approximately
–80 dBc/Hz at a 10 kHz offset (see Figure 2). For this configu-
ration, the LO will be driven with approximately 100 mVrms,
and the frequency of oscillation can be approximated by:
where Cv is the equivalent capacitance of the varactor at the
control voltage.
For higher frequency operation, a series tuned oscillator con-
figuration is recommended. Table 1 contains the S–parameters
for the VCO transistor in a common collector configuration.
This information is useful for designing a VCO at other operat-
ing frequencies or for various other oscillator topologies.
The output power (at Mix/Buf Out) can be varied by adjust-
ing the value of R5 as illustrated in Figures 3 and 4. Figure 5
shows the typical operating window for the prescaler.
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