U2730B-N Datasheet, PDF (5/17 Pages) ATMEL Corporation – L-band Down-converter for DAB Receivers

U2730B-N
Functional Description
The U2730B-N is an L-band down-converter circuit covering a gain-controlled amplifier,
a gain-controlled mixer, an output buffer, a gain control circuitry, an L-band oscillator
and a frequency synthesizer block. Designed for applications in a DAB receiver, the cir-
cuit down-converts incoming L-band signals in the frequency range of 1452 MHz to
1492 MHz to an IF frequency in a range of 190 MHz to 230 MHz which can be handled
by a subsequent DAB tuner. A block diagram of this circuit is shown in Figure 1.
Gain-controlled Amplifier
RF signals applied to the 'RF' input pin are amplified by a gain-controlled amplifier. The
complementary pin NRF is not internally blocked, it is recommended to block this pin
carefully by an external capacitor. The gain-control voltage is generated by an internal
gain-control circuitry. The output signal of this amplifier is fed to a gain-controlled mixer.
Gain-controlled Mixer
and Output Buffer
The purpose of this mixer is to down-convert the L-band signal in the frequency range of
1452 MHz to 1492 MHz to an IF frequency in the range of about 190 MHz to 230 MHz.
Like the amplifier, the gain of the mixer is controlled by the gain-control circuitry. The IF
signal is buffered and filtered by a one-pole low-pass filter at a 3 dB frequency of about
500 MHz, and then it is fed to the single-ended output pin IF.
Gain-control Circuitry
The gain-control circuitry measures the signal power, compares it with a certain power
level and generates control voltages for the gain-controlled amplifier and mixer. An
equivalent circuit of this functional block is shown in Figure 6.
In order to meet this functionality, the output signal of the buffer amplifier is weakly
band-pass filtered (transition range of about 60 MHz to 550 MHz), rectified, low-pass fil-
tered and fed to a comparator whose threshold can be defined by an external resistor,
RTH, at pin TH. By varying the value of this resistor, a power threshold of about -33 dBm
to -20 dBm can be selected. In order to achieve a good intermodulation ratio, it is recom-
mended to keep the power threshold below -25 dBm. An appropriate application is
shown in Figure 3. Depending on the selection made by the comparator, a charge pump
charges or discharges a capacitor which is applied to the AGC pin. By varying this
capacitor, different time constants of the AGC loop can be realized. The voltage arising
at the AGC pin is used to control the gain setting of the gain-controlled amplifier and
mixer. The voltage at pin AGC is in the range of 5.75 V for maximum gain and 0.3 V for
minimum gain. This voltage can be use to control a dual-gate GaAs-FET in front of the
U2730B-N to achieve an extended AGC range. By applying an external voltage to the
AGC pin, the internal AGC loop can be overdriven.
Voltage-controlled
Oscillator
A voltage-controlled oscillator supplies a LO signal to the mixer. An equivalent circuit of
this oscillator is shown in Figure 7. In the application circuits Figure 8 and Figure 9, a
ceramic coaxial resonator is applied to the oscillator's TANK and VREF pins. It should
be noted that V_ref has to be blocked carefully. Figure 9 shows a different application
where the oscillator is overdriven by an external oscillator. In any case, a DC path at a
low impedance must be established between the TANK and VREF pins. The output sig-
nal of the oscillator is fed to the LO divider block of the frequency synthesizer unit which
locks the VCO's frequency on the frequency of a reference oscillator. Figure 5 shows
the typical phase-noise performance of the oscillator in locked state.
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