APPENDIX A to VOLUME A1 TIMS FILTER RESPONSES
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TABLE OF CONTENTS... 5 Filter Specifications... 7 3 khz LPF (within the HEADPHONE AMPLIFIER)... 8 TUNEABLE LPF... 9 BASEBAND CHANNEL FILTERS - #2 Butterworth 7th order lowpass... 10 BASEBAND CHANNEL FILTERS - #3 Bessel 7th order lowpass... 11 BASEBAND CHANNEL FILTERS - #4 flat group delay 7th order lowpass... 12 60 khz LOWPASS FILTER... 13 100 khz CHANNEL FILTERS - #2 7th order lowpass... 14 100 khz CHANNEL FILTERS - #3 6th order bandpass (type - 1)... 15 100 khz CHANNEL FILTERS - #3 8th order bandpass (type - 2)... 16 A-3
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There are several filters in the TIMS system. In this appendix will be found the theoretical responses on which these filters are based. Except in the most critical of applications - and the TIMS philosophy is to avoid such situations - these responses can be taken as representative of the particular filter you are using. A-5
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Filter Specifications A knowledge of filter terminology is essential for the telecommunications engineer. Here are some useful definitions. approximation: a formula, or transfer function, which attempts to match a desired filter response in mathematical form. order: the size of the filter, in terms of the number of poles in the transfer function. passband: a frequency range in which signal energy should be passed. passband ripple: the peak-to-peak gain variation within a passband. Usually expressed in decibels (db). realization: a physical circuit whose response matches as closely as possible that of the approximation. slotband: regulatory organizations such as CCITT, Austel, FCC, etc, provide their clients with spectrum slots. The regulatory definition of a slot may be fairly involved, but, in simple terms, it is equivalent to specifying an allowed band for transmission, within which the user is free to exploit the resource as s/he wishes, and to ensure extremely low levels of leakage outside the limits. In terms of specifying a filter characteristic it means the band limit is determined by the stop frequencies for a bandpass filter, or from DC to the start of the stopband for a lowpass filter. Thus it is the sum of the passband plus transition band (or bands). stopband: a frequency range in which signal energy should be strongly attenuated. stopband attenuation: the minimum attenuation of signal energy in the stopband, relative to that in the passband. Usually expressed in decibels (db). transition band: a frequency region between a passband and a stopband. transition band ratio: the ratio of frequencies at either end of the transition band; generally expressed as a number greater than unity. Specification mask Filters are often specified in terms of a specification mask. Any filter whose response will fit within the mask is deemed to meet the specification. Typical specification masks are shown in the Figures below. a lowpass specification mask a bandpass specification mask A-7
3 khz LPF (within the HEADPHONE AMPLIFIER) This is an elliptic lowpass, of order 5. passband ripple 0.2 db passband edge 3.0 khz stopband attenuation 50 db slotband DC to 4.78 khz transition band ratio 1.59 A8
TUNEABLE LPF This is an elliptic lowpass, of order 7. It is shown plotted with a slotband of 4.0 khz passband ripple 0.5 db passband edge 3.55 khz stopband attenuation 50 db slotband DC to 4.0 khz transition band ratio 1.127 Filter cutoff frequency is given by: NORM range: clk / 880 WIDE range: clk / 360 For more detail see the TIMS User Manual. A-9
BASEBAND CHANNEL FILTERS - #2 Butterworth 7th order lowpass This filter is selected with the front panel switch in position 2 response passband stopband monotonic falling -1 db at 1.88 khz -40 db at 4.0 khz A10
BASEBAND CHANNEL FILTERS - #3 Bessel 7th order lowpass This filter is selected with the front panel switch in position 3 response passband edge stopband monotonic falling -1 db at 620 Hz -40 db at 4.0 khz A-11
BASEBAND CHANNEL FILTERS - #4 flat group delay 7th order lowpass This filter is selected with the front panel switch in position 4 It exhibits an equiripple ( flat ) group delay response over the complete passband and into the transition band. passband ripple passband edge stopband attenuation slotband delay ripple delay bandwidth 0.1 db 1.75 khz 40 db DC to 4 khz 10 µs peak-to-peak DC to 1.92 khz A12
60 khz LOWPASS FILTER This is an elliptic lowpass, of order 7. passband ripple 0.1 db passband edge 60 khz stopband attenuation 50 db slotband DC to 71.4 khz. transition band ratio 1.19 A-13
100 khz CHANNEL FILTERS - #2 7th order lowpass This filter is selected with the front panel switch in position 2 An inverse-chebyshev lowpass filter, of order 7. passband ripple passband edge stopband attenuation slotband 0.1 db 120 khz 40 db DC to 190 khz. A14
100 khz CHANNEL FILTERS - #3 6th order bandpass (type - 1) This filter is selected with the front panel switch in position 3 There are two version of this filter, type 1 and type 2. The characteristic below is that of type 1. This filter was delivered before mid-1993. The board bears no indication of type. Type 1 is an inverse Chebyshev bandpass filter, of order 6. passband ripple lower passband edge upper passband edge stopband attenuation slotband 1.0 db 85 khz 115 khz 45 db 52 khz to 187 khz A-15
100 khz CHANNEL FILTERS - #3 8th order bandpass (type - 2) This filter is selected with the front panel switch in position 3 There are two version of this filter, type 1 and type 2. The characteristic below is that of type 2. This filter was not delivered before mid-1993. The inscription type 2 will be found on the circuit board. Type 2 is an inverse Chebyshev bandpass filter, of order 8. 100 khz, order_8, BPF passband ripple lower passband edge upper passband edge stopband attenuation slotband 1 db 90 khz 110 khz 45 db 76 khz to 130 khz A16