CN-0248 Datasheet, PDF (2/6 Pages) Analog Devices – An IQ Demodulator-Based IF-to-Baseband Receiver with IF and Baseband Variable Gain and Programmable Baseband Filtering

CN-0248
Circuit Note
CIRCUIT DESCRIPTION
Receiver Architecture
A direct conversion (also known as a homodyne or zero IF)
architecture for a receiver is presented in this circuit note. Direct
conversion radios perform just one frequency translation
compared to a superheterodyne receiver that can perform
several frequency translations. One frequency translation is
advantageous because it
• Reduces receiver complexity and the number of stages needed;
increasing performance and reducing power consumption
• Avoids image rejection issues and unwanted mixing
products; one LPF at baseband is all that is needed
• Has high selectivity (adjacent-channel rejection ratio [ACRR])
Figure 1 shows the basic simplified schematic of the system that
consists of cascaded IF variable gain amplifiers (VGAs) with
integrated automatic gain control (AGC) loops, followed by a
quadrature demodulator and by programmable low-pass filters
with variable baseband gain. The grayed out components shown in
Figure 1 (ADF4350 and AD9248) are included for clarity but were
not included during system-level measurements (see the Common
Variations section for more information on these devices).
Ideally, the input of the first stage and the output of the last stage
should set the dynamic range (signal-to-noise ratio) of the
system. Practically, this may not be the case. Having a cascaded
VGA before the quadrature demodulator not only adds more
gain to the system, but it also helps with overall system noise
performance if the noise figure of the VGA is less than that of
the quadrature demodulator, and if the VGA still has gain, and
it is not attenuating. The noise figures of the subsequent stages
are divided by the gain of the initial VGA. Another benefit of
having a VGA (vs. just having a fixed gain amplifier) is that an
AGC loop can be designed to level the incoming signal to the
quadrature demodulator. It is important to have this ability to
limit the signal levels applied to the quadrature demodulator
and any subsequent stages.
IF VGAs and AGC Loops
The IF VGA and AGC loop functions are accomplished with
the ADL5336. It has two cascadable VGAs, each with 24 dB of
analog dynamic range and the ability to digitally change the
maximum gain on each VGA via a SPI port.
To achieve the signal leveling AGC function, each ADL5336
VGA has a square law detector connected to its output through
a programmable attenuator. The detector compares the output
of the attenuator to an internal reference of 63 mV rms. If there
is a difference between the output of the attenuator and the
63 mV rms reference, an error current is produced and is
integrated onto a CAGC capacitor. The AGC loop is closed by
connecting the DTO1/DTO2 pin to the GAIN1/GAIN2 pin. For
the AGC loop to function properly, pull the MODE pin low,
causing a negative VGA gain slope.
Each ADL5336 VGA has an allowable range of input power over
which the AGC will level to a particular setpoint. Outside that
range, the VGA output either increases or decreases dB-for-dB
with the input (assuming the VGA is not in compression or that
the signal is not in the noise floor).
IQ Demodulator
From the ADL5336, the signal is routed to the ADL5387, where
it is demodulated and the frequency is translated to a zero IF. The
ADF4350 synthesizer can provide the required 2×LO signal to the
ADL5387 (see the Common Variations section); however, a signal
generator was used instead of the ADF4350 for actual testing.
The ADL5387 uses two double-balanced mixers, one for the I
channel and one for the Q channel. The LO provided to the
mixers is generated using a divide-by-two quadrature phase
splitter. This provides the 0° and 90° signals for the I and Q
channels. There is about 4.5 dB of conversion gain provided by
the ADL5387 from the RF input to the baseband I and Q outputs.
Low-Pass Filter, Baseband VGA, and ADC Driver
The low-pass filtering, baseband gain, and ADC driver functions
are all achieved using the ADRF6510. The signal, now in its
separate I and Q paths, is applied to the ADRF6510 where the
signal is first amplified by the preamplifier, then low-pass
filtered to suppress any unwanted out-of-band signals and/or
noise, and finally amplified by the VGA.
Each channel of the ADRF6510 can be broken up into three stages:
• Preamplifier
• Programmable low-pass filter
• VGA and output driver
The preamplifier has a user-selectable gain, via the GNSW pin,
of either 6 dB or 12 dB. The low-pass filter can be programmed
for a corner frequency of 1 MHz to 30 MHz in 1 MHz steps via
the SPI port. The VGA has a 50 dB gain range with a gain slope
of 30 mV/dB. The gain of the VGA is controlled via the GAIN
pin, and it can range from −5 dB to +45 dB when the GNSW
pin is pulled low to +1 dB to +51 dB when the GNSW pin is
pulled high. The output driver has the ability to drive 1.5 V p-p
differential into a 1 kΩ load while maintaining a HD2 and a
HD3 of better than 60 dBc.
The maximum CW signal that can be applied to the low-pass
filters, while still maintaining acceptable HD levels in the
ADRF6510, is 2 V p-p. In applications where a large out-of-band
interferer is present that could overload the input of either the
ADL5387 and/or the ADRF6510, the out-of-band interferer
(and the in-band desired signal) can be attenuated by the
ADL5336 VGA. Once the out of-band interferer is rejected by
the low-pass filter of the ADRF6510, the wanted signal can then
be amplified with the X-AMP VGAs that follow the filters of the
ADRF6510.
From the ADRF6510, the IQ signal can be applied to an
appropriate analog-to-digital converter (ADC), such as the
AD9248.
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