atr2730 ATMEL Corporation, atr2730 Datasheet - Page 5

• Manufacturer Part Number: atr2730
• Description: L-band Down-converter For Dab Receivers
• Manufacturer: ATMEL Corporation

3. Functional Description
3.1
3.2
3.3
4903C–DAB–03/07
Gain-controlled Amplifier
Gain-controlled Mixer and Output Buffer
Gain-control Circuitry
The ATR2730 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 fre-
quency synthesizer block. Designed for applications in a DAB receiver, the circuit down-converts
incoming L-band signals in the frequency range of 1452 MHz to 1492 MHz to an IF frequency in
the 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
RF signals applied to the RF input pin are amplified by a gain-controlled amplifier. The comple-
mentary 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 internal gain-control circuitry. The
output signal of this amplifier is fed to a gain-controlled mixer.
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.
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
In order to meet this functionality, the output signal of the buffer amplifier is weakly band-pass fil-
tered (transition range of about 60 MHz to 550 MHz), rectified, low-pass filtered, 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 recommended to keep the power threshold
below –25 dBm. An appropriate application is shown in
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 5.75V for maxi-
mum gain and 0.3V for minimum gain. This voltage can be use to control a dual-gate GaAs-FET
in front of the ATR2730 to achieve an extended AGC range. By applying an external voltage to
the AGC pin, the internal AGC loop can be overdriven.
Figure 10-1 on page
Figure 1-1 on page
ATR2730 [Preliminary]
14.
2.
Figure 8-1 on page
12. Depending on
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