Whitepaper
Local Oscillator feedthrough suppression using on-chip calibration techniques

Cormac O’Sullivan, RF Technology Lead, Industrial ASIC BU

ContentsIntroduction ............................................................................................................................................................ 1
Feed Through Contributors .................................................................................................................................. 1
System Overview ................................................................................................................................................... 2
Results .................................................................................................................................................................... 3
Conclusion ............................................................................................................................................................. 4
Appendix................................................................................................................................................................. 4
Revision History .................................................................................................................................................... 5

Introduction
The high level of integration allowed by a CMOS RF SoC allows digitally assisted analog circuitry to achieve
improved performance through tightly coupled calibration loops. This white paper serves as a case study of one
such calibration loop as used on an RFIC developed by Renesas, formerly Dialog’s ASIC & IP division. This RFIC
is a highly integrated solution for Mobile Satellite Service (MSS) which allows data connectivity globally in an
increased number of applications and enables remote devices and sensors to be connected via the Internet ofThings (IoT).

Feed Through Contributors
In wireless systems, feed-through from the Transmitter (TX) Local Oscillator (LO) is an important specification. In
direct up-conversion transmitters, the feed-through can weaken modulation quality (EVM). For single side-band
intermediate frequency (IF) up-converters where the IF is too low to allow practical RF filtering, the feed-throughwill result in a failure to meet the transmitted spectral mask specifications. In this case feed-through acts as ablocker for channels adjacent to the intentional transmission. These strict specifications often introduce the needfor a calibration scheme to improve on the native performance that is achievable.
Current available solutions involve large device sizes to minimize offset contributors, careful symmetric layout andfinally calibration schemes that may require RF feedback with an envelope detector or full loop back of the transmit
signal through a receive path. The calibration scheme implemented in this paper only requires base-bandmeasurements.
LO feed-through consists of many components. Electromagnetically radiated LO feed-through can be mitigatedby using an off frequency VCO either oscillating at twice the required LO used in conjunction with a divider, orother 2/3 or 3/2 schemes. Coupling due to parasitic capacitances can be minimized to a certain extent by carefullayout. Base-band filter contributors can be minimized by DC offset correction.

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Local Oscillator feedthrough suppression using on-chip calibration techniques

System Overview
Renesas implements a calibration scheme that requires base-band measurements only. The calibration scheme
discussed here was implemented on a 0.18um CMOS RFIC. The IC utilized a low IF TX frequency plan with anactive RC polyphase filter to serve as a reconstruction filter and to generate quadrature signals for the side-band
suppression mixer as shown in the figure below.

Monte Carlo mismatch simulations showed offsets in the mixer and baseband poly-phase filter, with the mixer
dominant and indicated the need for a calibration scheme. The mixer was dominant due to the low gain of thepolyphase filter at DC which negated the need for base-band DC offset correction. As the mixer was dominant AC
coupling to the mixer would be of marginal benefit to the LO leakage performance. Gaussian distribution of offsets
in I and Q mixer branches will add to give a Rayleigh distribution of LO feed-through power as seen at the outputof the transmitter as shown in the following figure. The results showed the LO feedthrough passing specification
for less than 99.7% of devices even for this mechanism alone mandating a calibration scheme.

Two five-bit current DACs are used to compensate for offsets in the quadrature Gilbert cell mixers. Measurementof the DC offset is made using a 10-bit auxiliary ADC IP block with zero incremental complexity/cost as this isbeing utilised already on the custom RFIC for TX power control and sensor measurements.

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Local Oscillator feedthrough suppression using on-chip calibration techniquesADC offsets and load resistor mismatches, which do not significantly contribute to LO feed-through and would add
errors to the calibration, are cancelled by taking two measurements with the mixer switches in either direction.
(Double-sampling.) The ADC and resistor components cancel when the measurements are subtracted leaving ameasurement of the current mismatch which is the parameter of interest.
Offsets are reduced by minimizing the difference between ADC samples first for the I mixer, then for the Q mixer,
through adjustment of the corresponding DAC. While the I mixer is being calibrated the Q mixer commutation
switches are turned off to remove any effect from the Q mixer transconductor offsets on the I mixer and vice-versa.
The low frequency switching is controlled through set and re-set operation of the RF divider and quadrature
generation block needed to divide the X2 VCO. The calibration also compensates for TX base-band filter offsets.
A digital state machine could be used to close the loop on chip or alternatively an external microcontroller could
be used using the integrated SPI interface. As the ADC and DACs sample at 500kHz, the calibration can becomplete using a SAR algorithm in <50us.

Results
The figure below shows the measured LO feedthrough versus DAC codes for sample number #1 where thecontours of the feedthrough can be observed, and the optimum code is DAC I:15 DAC Q:13.

The effect of the DAC on the feed-through can be clearly seen. The feedthrough can be minimized to -65 dBmwith the VGA gain set to give an output power of -9dBm giving LO feed-through of -56dBc.
The following figure shows ADC measurements for both I and Q mixers for the same part as for the figure above.
A subsequent table shows measurements for 5 samples and compares to the actual LO feedthrough. The ADC
measurements are in fact the subtraction of two ADC measurements taken to cancel R and ADC offsets asoutlined previously. Over-laid on the plot is measured LO feed-through versus DAC code sweeps corresponding
to the I DAC swept with Q DAC at optimum code and Q DAC swept with I DAC at optimum code.
Ideally, we would expect to see the minimum LO feed through at the DAC codes where the ADC measurements
approach zero.

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Local Oscillator feedthrough suppression using on-chip calibration techniques

The LO feed through plots correspond to two cross sections through the contour plot at the optimum DAC code.
A strong correlation between the ADC measurements approaching zero and the corresponding LO feed throughplot can be seen. Other parts measured show similar characteristics. In our measurements, for all samples, theADC measurements serve as a practical predictor for optimum LO feed through DAC code, to within 1 DAC code,
as shown in the table. Any errors could be due to second order coupling effects not measured by the base-band
calibration, or noise in the ADC measurement which could be mitigated by filtering. These do not significantly
degrade the practical performance achieved.

Further details on the results with corresponding graphs can be viewed in the ISSC paper on this subject [1].

Conclusion
The calibration scheme is shown to be effective and a reliable method to reduce LO feed-through due to baseband components. The algorithm is easily implemented in a low-cost fashion using existing circuits and avoids theneed for RF feedback.

Appendix[1] O’Sullivan et al, “Carrier Leak Calibration Scheme on a 0.18m Transmitter”, in ISSC, 2002.

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Local Oscillator feedthrough suppression using on-chip calibration techniques

Revision History
Revision

Date

1.0

Mar 01, 2018

Initial release

1.1

Dec 22, 2021

Re-brand

WP_1 Rev.1.1
Dec 23, 2021

Description

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