*2.2.1. Sensors of large instantaneous bandwidth*

There are several UWB approaches known exploiting signals of large instantaneous bandwidth. Usually, they are denoted according the sounding signal applied by the sensor. Typical representatives of this signal class are:


By assumption, these signals have a bandwidth in the GHz range requiring often Nyquist rates of the measurement receivers above 10 GHz. Disregarding the device costs, this is hardly to achieve with the limited power budget and the restricted means of data handling (see section 2.1 – *Figure of Merit* and *Data throughput*) which a sensor usually has at its disposal. Hence, all these devices must reduce their data rates at the expense of receiver efficiency, which is reflected by a reduced dynamic range *D*max (see(7)). The data rate reduction is either achieved by sub-sampling or by serializing the data recording.

HaLoS – Integrated RF-Hardware Components for Ultra-Wideband Localization and Sensing 377

ADC Sampling gate

ADC Sampling gate

Digitised stimulus **x***m*

ADC

**C***yx s m y t x t mt*

*yx n* **<sup>C</sup>** *m yn xn m*

Cross-correlation

Cross-correlation

Furthermore, the delay time depends on temperature, and the huge number of gates

System clock Stimulus signal System reaction Digitised system response

Sub-sampling receiver

*xt y t* **y***m*

*y t* DUT

Stimulus signal System reaction

ADC Sampling

ADC Sampling

Digitised stimulus **x***n m*

gate

gate

Stimulus signal System reaction Digitised system response

*x t y t* **y***n*

*<sup>P</sup> n t*

*<sup>P</sup> <sup>s</sup> nt mt*

Sub-sampling correlator

Signal DUT

Timing control

*x t*

*xt mt <sup>s</sup>*

*<sup>g</sup> t <sup>P</sup> n t*

Analog Correlator

Signal shaper

Signal shaper

*<sup>P</sup> n t <sup>P</sup> nt m t*

*g t*

shaper

*<sup>P</sup> <sup>s</sup> nt mt*

Timing control

Noise DUT

Timing control

*g t*

source

System clock

**Figure 2.** Three possible sensor structures exploiting signals of large instantaneous bandwidth.

consumes plenty of energy.

System clock

Fig. 2 refers to three possible device conceptions for illustration. The two upper approaches require periodic sounding signals. Here, the signal shaper may be a pulse generator, a binary PN-generator or an arbitrary waveform generator. The most often found device implementations apply sub-nanosecond pulse generators. Indeed, the concept allows the implementation of very cost-effective and power saving sensors. However, their system performance often suffers from reduced dynamic range due to the large crest factor of the sounding signal (compare (7)); they do not provide jitter suppression (see also sub-chapter 2.3) and they are not robust against jamming. Wideband PN-generators are an interesting alternative to pulse generators since they provide powerful signals of low magnitude (i.e. of low crest factor). Arbitrary waveform generators are able to provide signals which can flexibly be adapted to the measurement problem. However, they are quite expensive, power hungry and limited with respect to the bandwidth. Hence, they have not been found in practically applicable sensor concepts recently.

*Sub-sampling receiver*: It is the most often applied UWB concept. It supposes periodic sounding signals ( *Pt* - signal period). Typically, the measurement signals are captured by sequential sampling, providing one data sample per period whose time position is stepwise shifted over the whole signal. The actual sampling interval is *Pt t* , while the equivalent sampling interval which has to meet the Nyquist criteria is *t* . Newer concepts apply interleaved sampling permitting higher sampling rates since more than one point per period is taken. The classical concept of time shift control uses the fast ramp-slow ramp approach which, however, tends to non-linear time axis representation, sampling jitter and time drift. A second method deals with two stable sine wave generators (e.g. Direct Digital Synthesizers of slightly different frequency ( <sup>1</sup> <sup>1</sup> *<sup>P</sup> f t* ; <sup>1</sup> <sup>2</sup> *<sup>P</sup> <sup>f</sup> t t* ). This reduces time drift and avoids time axis non-linearity. However, it still keeps the sampling jitter quite high since the trigger events launching the sounding pulse and activating the sampling gates are based on relative flat edges of the two sine waves of (comparatively low) frequency 1*f* and 2*f* . Timing control based on digital counters for coarse timing exploits steep trigger edges improving the jitter performance. Then, the fine tuning is typically done by programmable delay chips which consist of hundreds of delay gates. As these gates are not absolutely identical, the delay line cannot ensure equidistant sampling. Furthermore, the delay time depends on temperature, and the huge number of gates consumes plenty of energy.
