Analog-to-digital converters (ADCs) are required in almost all signal processing and communication
systems. They are often the most critical components, since they tend to determine the overall system
performance. Hence, it is important to determine their performance limitations and develop improved
realizations. One of the most challenging tasks for realizing software defined radio (SDR) is to move ND
conversion as close to the antenna as possible, this implies that the ADC has to sample the incoming
signal with a very high sample rate (over 100 MSample/s) and with a very high resolution (14 -to -16 bits).
To design and implement AID converters with such high performance, it is necessary to investigate new
designing techniques.
The focus in this work is on a particular type of potentially high-performance (high-resolution and highspeed)
analog-to-digital conversion technique, utilizing filter banks, where two or more ADCs are used in
the converter array in parallel together with asymmetric filter banks. The hybrid filter bank analog-todigital
converter (HFB ADC) utilizes analog filters (analysis filters) to allocate a frequency band to each
ADC in a converter array and digital synthesis filters to reconstruct the digitized signal. The HFB
improves the speed and resolution of the conversion, in comparison to the standard time-interleaving
technique by attenuating the effect of gain and phase mismatches between the ADCs.
Many of the designs available in the literature are compromising between some metrics: design
complexity, order of the filter bank (computation time) and the sharpness of the frequency response rolloff
(the transition from the pass band to the stop band).
In this dissertation, five different classes of near perfect magnitude reconstruction (NPMR) continuoustime
hybrid filter banks (CT HFBs) are proposed. In each of the five cases, two filter banks are designed;
analysis filter bank and synthesis filter bank. Since the systems are hybrid, continuous time IlR filter are
used to implement the analysis filter bank and digital filters are used for the synthesis filter bank. To
optimize the system, we used a new technique, known in the literature as frequency response masking
(FRM), to design the synthesis filter bank. In this technique, the sharp roll-off characteristics can be
achieved while keeping the complexity of the filter within practical range, this is done by splitting the
filter into two filters in cascade; model filter with relaxed roll-off characteristics followed by a masking
filter.
One of the main factors controlling the overall complexity of the filter is the way of designing the model
filter and that of designing the masking filter.
The dissertation proposes three combinations: use of HR model filter and IlR masking filter, HR model
filter/FIR masking filter and FIR model filter/FIR masking filter. To show the advantages of our designs,
we considered the cases of designing the synthesis filter as one filter, either FIR or IlR. These two filters
are used as base for comparison with our proposed designs (the use of masking response filter). The results showed the following:
1. Asymmetric hybrid filter banks alone are not sufficient for filters with sharp frequency response
roll-off especially for HR/FIR class.
2. All classes that utilize FRM in their synthesis filter banks gave a good performance in general in
comparison to conventional classes, such as the reduction of the order of filters
3. HR/HR FRM gave better performance than HR/FIR FRM.
4. Comparing HR/HR FRM using FIR masking filters and HR/IIR FRM using IIR masking filters,
the latter gave better performance (the performance is generally measured in terms of reduced
filter order).
5. All classes that use the FRM approach have a very low complexity, in terms of reduced filter
order. Our target was to design a system with the following overall characteristics: pass band
ripple of -0.01 dB, stop band minimum attenuation of - 40 dB and of response roll-off of 0.002.
Our calculations showed that the order of the conventional IIR/FIR filter that achieves such
characteristics is aboutN =2000. Using the FRM technique, the order N reduced to
aboutN = 244, N = 179 for IIRJFIR and IIR/IIR classes, respectively. This shows that the
technique is very effective in reducing the filter complexity.
6. The magnitude distortion and the aliasing noise are calculated for each design proposal and
compared with the theoretical values. The comparisons show that all our proposals result in
approximately perfect magnitude reconstruction (NPMR).
In conclusion, our proposal of using frequency-response masking technique to design the synthesis filter
bank can, to large extent, reduce the complexity of the system. The design of the system as a whole is
simplified by designing the synthesis filter bank separately from the design of the analysis filter bank. In
this case each bank is optimized separately. This implies that for SDR applications we are proposing the
use of the continuous-time HFB ADC (CT HFB ADC) structure utilizing FRM for digital filters. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/2842 |
| Date | January 2005 |
| Contributors | Dawoud, David. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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