В статье описаны методы снижения помех в конструировании спутниковых приемников, в частности применение высокоскоростных аналого-цифровых преобразователей с уменьшенным выходным размахом напряжения и специальными схемами кодирования выходного цифрового сигнала [англ.].
Wireless receiver design requires extreme care in dealing with noise sources that affect the analogue-to-digital converter (ADC).
High sensitivity receivers, such as those in satellite or basestation applications demand the highest dynamic range and therefore need to minimise the noise contribution from every possible source. These include nonlinearities (INL and DNL errors) in the ADC and digital feedback from the data output bus.
Interference from the high speed ADC digital outputs is sometimes unavoidable. This interference may be caused by capacitive or inductive coupling or even coupling through the ground plane, and is known commonly as digital feedback. Even a tiny coupling factor can result in discernible unwanted tones in the ADC output spectrum.
While good layout can help reduce the effects of digital coupling, it may not be enough to eliminate the problem. One approach is to reduce the digital output voltage swing that will correspondingly reduce digital noise coupled back into the analogue circuitry.
One approach is to choose high speed ADCs which incorporate CMOS output drivers that can operate on digital supply voltages as low as 0.5V without any speed penalty. Higher speed devices also include LVDS outputs where each output bit is a differential pair with 0.35Vp-p swing.
If this is not enough, the LTC2209 16-bit 160Msample/s ADC, for example also provides an optional digital output randomiser to encode the data using the least significant bit (LSB) of the 16-bit digital output word.
Each of the data bits is exclusive-ORed with the LSB for randomisation before transmission. This minimises coupling of the data lines due to large voltage transitions and effectively spreads the spurious energy that was concentrated at certain harmonic frequencies down into the noise floor. The trade-off is a slight increase in the noise floor for a large reduction in unwanted tone amplitude. The received digital output bus is easily decoded by performing the same operation in the FPGA. This data encode scheme can reduce the residual tones caused by digital feedback by 10-15dB.
Part of the solution to avoiding digital feedback is also a good layout. It is important to ensure that the digital ground currents don’t flow in the analogue section of the ground plane. High frequency currents like to return on the plane directly under the signal trace as this is the path that provides the lowest impedance (inductance). For designs using a single ADC, it is not recommended to split the analogue and digital ground planes as there is risk of routing traces across the split plane, causing additional radiation and crosstalk. Instead, use a single ground plane and partition the analog and digital sections so that the digital return currents do not interfere with the analogue signals.
Another issue that receiver designers face is increased distortion at the ADC output when sampling low-level signals. Low-level input signals utilise only a small region of the ADC transfer function, exercising only a handful of digital output codes. Even slight linearity imperfections will generate unwanted harmonics at the output that reduce the ADC’s spurious free dynamic range (SFDR) performance.
To maximise the SFDR performance for low level signals, the ADC provides an on-chip dither feature to decorrelate (randomise) the effects of linearity errors at certain locations along the ADC transfer curve.
The dithered signal randomises the location of the sampled signal on the transfer curve so that the energy in the unwanted spurious frequencies is spread into the noise floor. By dithering the input using a pseudo-random number generator driving an internal dither DAC as shown in Figure 2, the ADC is forced to operate over a wider range of the transfer curve. The pseudo-random number is then subtracted from the ADC result with only a small amount of dither leak-through. The correlated spurious tones are thus converted to random noise that can be reduced by processing gain.