ADC/DAC & Control Logic for DC Offset Cancellation
HD155165BP (B6E)
The Receiver structure in HD155165BP is a zero-IF solution. That means RF signal is directly downconverted to the baseband signal. And by the way, all of the DC-offset canceling processes
are done within chip. We do not have to care about that.
The LNA amplifies the RF signal after passing the T/R switch and RF SAW filter and before it enters
the down-converter section. The RF signal is mixed with a local oscillator (LO) signal to generate the
baseband signal.
Three LPFs are used in the baseband signal processing for reducing blocking signals. The first LPF
employs two external capacitors, and we can check whether the front-end (LNA + Mixer) is
functionally well or not by probing these two capacitors to see if there is any baseband
signal(<200kHz).
After three stages of DC-offset cancelling, the signal (I+/I-/Q+/Q-) then output to the baseband IC for
further processing.
The HD155165BP receiver is based upon the HD155155NP direct conversion design. As
HD155165BP supports quad band, the front end incorporates four LNAs and mixers. The incoming
RF signals are mixed directly down to I/Q base-band by the front-end block. This incorporates four
LNAs / four buffers and Gilbert Cell mixer blocks optimized for operation at 850MHz, 900MHz,
1800MHz, and 1900MHz respectively.
The front-end block is followed by two closely matched base-band amplifier chains. These include
distributed low pass filter, three switched gain stages and one fixed gain stage.
Technical Documentation
06/2006
TTD_Repair_L3 _Theory of Operation_E61_R1.0.pdf Page 7 of 33
Release 1.0
In addition, the base-band section integrates A/D and D/A converters which provide automatic onchip correction of DC offsets. The three switched gain stages in each channel are DC coupled and
provide 90dB gain control range with 2dB step size. The first PGA has a voltage gain range (x8-x1)
with 6dB steps. The second PGA has a gain range (x8-x0.125) with 2dB steps. The third PGA has a
gain range (x8-x0.125) with 2dB steps. The final fixed gain amplifier provides a gain of x3 or x6. The
gain is set to match the on-chip levels to the input dynamic range of the base-band. The base-band
filtering in each channel comprises a single RC low pass filter at the input of the first switched gain
stage and three 2nd order Butterworth filters, one at the input of each of the other switched gain
stages. The R/C filter requires an off-chip capacitor for each channel. The Butterworth filters are fully
integrated on-chip.
The base-band PGA includes a DC offset cancellation system. The auto calibration system uses a
successive approximation technique and requires around 20us to perform a three stages calibration.
The system calibrates out the offsets arising in both I and Q receives channels.
The B6E generates a modulated signal at IF with a quadrature modulator and converts it to final
frequency with an Offset Phase Locked Loop (OPLL).
The Offset Phase Locked Loop is simply a PLL with a down conversion mixer in the feedback path.
Using a down converter in the feedback path acts as an up-converter in the forward path. This
allows the output frequency to be different from the comparison frequency without affecting the
normal operation of the loop. Phase/frequency changes in the reference signal are not scaled, as
they would be if a divider were used in the feedback path, hence the modulation is faithfully
reproduced at the final frequency.
The main advantage of the OPLL in this application is that it forms a tracking band pass filter around
the modulated signal. This is because the loop cannot respond to phase variations at the reference
that are outside its closed loop bandwidth. Thus the broad band phase noise from the quadrature
modulator is shaped by the frequency response of the closed loop allowing the TX noise
specification to be met without further filtering.
Technical Documentation
06/2006
TTD_Repair_L3 _Theory of Operation_E61_R1.0.pdf Page 8 of 33
Release 1.0
A secondary advantage of the OPLL is that the output signal, coming from a VCO, is truly constant
envelope. This removes the problem of spectral spreading caused by AM to AM and AM to PM
conversion in the power amplifier.
The OPLL is formed from an on chip Gilbert cell down converter, limiter and phase detector with on
chip passive loop filter. The phase detector is implemented as a Gilvert cell with current source
output stage. The current output allows an integrator to be included in the passive loop filter. This is
similar to the technique commonly used in PLL synthesizers. A digital phase detector is used to
speed OPLL locking. After locking, the digital phase detector is switched off and the analogue phase
detector becomes active.
5.3 VCXO Operation
+R
HD155165BP provides a DCXO function. With that function, we can build a reference clock
generation circuits as shown in the above graph. This means that the VCTCXO module is not
necessary for clock application, and only one crystal with 8ppm tolerance and one varactor are
enough.
The transistor in HD155165BP and two internal capacitors (C1, C2) provide a negative resistance,
and the crystal (X1) combined with some other passive components to provide a positive resistance.
When these two resistance values equal to each other at some frequency, the oscillation will happen
at that frequency. In our design target, the oscillation frequency should be within 26MHz +/-15 ppm
at least.
-R
Technical Documentation
TTD_Repair_L3 _Theory of Operation_E61_R1.0.pdf Page 9 of 33
06/2006
Release 1.0
6 Logic (Base-Band)
Introduction:
E61 utilizes TI’s chipsets (CALYPSO and IOTA) and RENESAS’s chipset (SHJ2SLSL) as baseband solution. Base-band is composed with three potions: Logic, Analog/Codec and MMP.
CALYPSO is a GSM/GPRS digital base-band logic solution included microprocessor, DSP, and
peripherals. IOTA is a combination of analog/codec solution and power management which contain
base-band codec, voice-band codec, several voltage regulators and SIM level shifter etc.
SHJ2SLSL is a multimedia solution included microprocessor, DSP, internal memory, and interrupt
controller. In addition, HerB1A integrates with other features such as LED backlight, TFT-LCD
display, CMOS DSC module, Mini-SD card, vibration, SW-midi, MP3 and charging etc. The following
sections will present the operation theory with circuitry and descriptions respectively.
Block Diagram CPU CALYPSO (HERCROM400)
Technical Documentation
TTD_Repair_L3 _Theory of Operation_E61_R1.0.pdf Page 10 of 33
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