Broadband digitizer with a low frequency bypass

Abstract

A broadband digitizer for an applied broadband analog input signal SA(t). The digitizer includes a low frequency analog-to-digital converter (LF ADC) channel and a high frequency analog-to-digital converter (HF ADC) channel, an input splitter coupled to respective inputs to the LF ADC channel HF ADC channels, a frequency divider, and a combining unit. Low frequency portions of SA(t) are digitized to digital signal SD.sub.LF[n] in the LF ADC channel and high frequency portions of SA(t) are digitized to digital signal SD.sub.HF[n] in the HF ADC channel. The combining unit combines the digital signals SD.sub.LF[n] and SD.sub.HF[n] to form distortion-reduced SD[n], corresponding to SA(t). Front ends of the LF ADC channel and HF ADC channel reduce level-caused distortions, and the combining unit reduces ADC frequency-caused, time-position-caused, and interpolation-caused distortions.

Claims

1. A method of digitizing a broadband analog signal SA(t), comprising the steps of: A. splitting the analog signal SA(t) into two signals: i. a high frequency (HF) analog signal SA.sub.HF(t) in a relatively high frequency (HF) spectral range, where t is representative of time, and ii. a low frequency (LF) analog signal SA.sub.LF(t), in a relatively low frequency (LF) spectral range, where t is representative of time, whereby SA.sub.HF(t)+SA.sub.LF(t)=SA(t), B. with processing components operative over the HF spectral range, i. adjusting the amplitude level of the HF analog signal SA.sub.HF(t), ii. digitizing the adjusted HF analog signal SA.sub.HF(t) to a high frequency (HF) digital signal SD.sub.HF[n] using a high frequency (HF) analog to digital converter characterized by a sampling rate SR.sub.HF, followed by an associated high frequency (HF) equalizer to generate SD.sub.HF[n], wherein the HF equalizer is adapted to reduce frequency dependent distortions caused by the level adjustment of the HF analog signal SA.sub.HF(t), and the digitizing of the adjusted HF analog signal SA.sub.HF(t), C. with processing components operative over the LF spectral range, i. adjusting the amplitude level of the LF analog signal SA.sub.LF(t), ii. digitizing the adjusted LF analog signal SA.sub.LF(t) to a low frequency (LF) digital signal SD.sub.LF[n] using a low frequency (LF) analog to digital converter characterized by a sampling rate SR.sub.LF, followed by an associated low frequency (LF) equalizer to generate SD.sub.LF[n], wherein the LF equalizer is adapted to reduce frequency dependent distortions caused by the level adjustment of LF analog signal SA.sub.LF(t), and the digitizing of the adjusted LF analog signal SA.sub.LF(t), and whereby the output digital signal SD[n] is characterized by reduced frequency dependent distortions compared to that which would be obtained without the frequency dependent distortion reductions of steps B.ii and C.ii, and D. combining the HF digital signal SD.sub.HF[n] with the LF digital signal SD.sub.LF[n] to obtain an output digital signal SD[n] corresponding to the analog signal SA(t).

2. The method of digitizing according to claim 1, comprising in step D, changing the sampling rate SR.sub.LF of the distortion-reduced LF digital signal by interpolation, whereby the changed sampling rate SR.sub.LF is equal to the sampling rate SR.sub.HF of the distortion-reduced HF digital signal, and whereby the output digital signal SD[n] is characterized by a sum of the distortion-reduced HF digital signal and the interpolated distortion-reduced LF digital signal.

3. The method of digitizing according to claim 2, comprising the further step of: aligning the level adjustments and relative time positions of the distortion-reduced LF digital signal whereby distortions in the output digital signal SD[n] are minimized.

4. The method of digitizing according to claim 3, comprising the further step of: subtracting an amount equal to a value V of a DC component from the LF analog signal SA.sub.LF(t), and adding an amount equal to a value V to the output digital signal SD[n], whereby, where the input analog signal includes a large DC component, high resolution of the low frequency analog to digital conversion is maintained.

5. A broadband digitizer comprising: A. a splitter adapted to receive an applied input analog signal SA(t) at a splitter input and split the received input analog signal SA(t) into a high frequency (HF) analog signal SA.sub.HF(t) in a relatively high frequency (HF) spectral range, and a low frequency (LF) analog signal SA.sub.LF(t) in a relatively low frequency (LF) spectral range, B. a high frequency (HF) analog to digital converter (ADC) channel, including a high frequency (HF) processor operative over the HF spectral range to: a. receive the HF analog signal SA.sub.HF(t) from the splitter at an HF ADC channel input and adjust the level of the received HF analog signal SA.sub.HF(t), and b. in a high frequency analog to digital converter (HF-ADC) characterized by a relatively high frequency sampling rate SR.sub.HF, digitize the amplitude-adjusted HF analog signal SA.sub.HF(t) to a high frequency (HF) digital signal SD.sub.HF[n], C. a low frequency (LF) analog to digital converter (ADC) channel, including a low frequency (LF) processor operative over the LF spectral range, to: a. receive the LF analog signal SA.sub.LF(t) from the splitter at an LF ADC channel input and adjust the level of the received LF analog signal SA.sub.LF(t), and b. in a low frequency analog to digital converter (LF-ADC) characterized by a relatively low frequency sampling rate SR.sub.LF, digitize the amplitude-adjusted LF analog signal SA.sub.LF(t) to a low frequency (LF) digital signal SD.sub.LF[n], D. a frequency distortion processor including: a. an HF frequency distortion-reducing processor comprising a high frequency (HF) digital equalizer including: i. a high frequency (HF) equalizer input adapted to receive SD.sub.HF[n], and ii. a high frequency (HF) equalizer output, wherein the HF equalizer is adapted to reduce frequency dependent distortions in the signal SD.sub.HF[n] caused by the level adjustment and digitizing processing in the high frequency analog to digital converter channel, whereby a frequency distortion-reduced form of high frequency digital signal SD.sub.HF[n] is presented at the HF equalizer output, b. an LF frequency distortion-reducing processor comprising a low frequency (LF) digital equalizer including: i. a low frequency (LF) equalizer input adapted to receive SD.sub.LF[n], and ii. a low frequency (LF) equalizer output, wherein the LF equalizer is adapted to reduce frequency dependent distortions in the signal SD.sub.LF[n] caused by the level adjustment and digitizing processing in the low frequency analog to digital converter channel, whereby a frequency distortion-reduced form of low frequency digital signal SD.sub.LF[n] is presented at the LF equalizer output, and E. a combiner processor including: a combiner adapted to combine a distortion-reduced form of HF digital signal SD.sub.HF[n] received from the HF equalizer output with a distortion-reduced form of LF digital signal SD.sub.LF[n] received from the HF equalizer output, thereby providing an output signal SD[n] corresponding to the input analog signal SA(t).

6. A broadband digitizer according to claim 5, wherein the combiner processor further comprises: a low frequency (LF) level distortion reducer including: a. a LF level distortion-reducing input adapted to receive the frequency distortion-reduced form of low frequency digital signal SD.sub.LF[n] presented at the LF equalizer output, and b. a LF level distortion-reduced output, wherein the LF level distortion reducer is adapted to reduce level distortions in the signal SD.sub.LF[n] caused by processing in the low frequency analog to digital converter channel, whereby a frequency and level distortion-reduced form of the low frequency digital signal SD.sub.LF[n] is presented at the LF level distortion-reduced output.

7. A broadband digitizer according to claim 6, wherein the combiner processor further comprises: a low frequency (LF) time distortion reducer including: a. a low frequency (LF) time distortion reducer input adapted to receive the frequency and level distortion-reduced form of low frequency digital signal SD.sub.LF[n] presented at the LF level distortion-reduced output, and b. a LF time delay distortion reduced output, wherein the LF time distortion reducer includes a controlled delay line adapted to reduce time delay distortions in the signal SD.sub.LF[n] caused by processing in the low frequency analog to digital converter channel, by adjusting the time position of the signal SD.sub.LF[n] received at the LF time distortion reducer input to match the time-position of the signal SD.sub.HF[n] at the HF equalizer output, whereby a frequency, level and time delay distortion-reduced form of the low frequency digital signal SD.sub.LF[n] is presented at the LF time delay distortion reduced output.

8. A broadband digitizer according to claim 7, wherein the combiner processor further comprises: an interpolator including: a. an interpolator input connected to the LF time delay distortion reducer output and adapted to receive a reduced distortion form of the digital signal SD.sub.LF[n] at a clock rate corresponding to the low frequency input signal sampling rate SR.sub.LF associated with the LF-ADC, and b. an interpolator output, wherein the interpolator is adapted to raise the clock rate of the received reduced distortion form of SD.sub.LF[n] from SR.sub.LF associated with the LF-ADC, to the high frequency input signal sampling rate SR.sub.HF, associated with the HF ADC, whereby the received reduced distortion form of SD.sub.LF[n] with an increased sampling rate SR.sub.HF is presented at the interpolator output.

9. A broadband digitizer according to claim 8, wherein the combiner processor further comprises: a DC adder including: a. a first DC adder input coupled to the interpolator output, and adapted to receive the synchronized received distortion-reduced form of SD.sub.LF[n] at the interpolator output, b. a second DC adder input adapted to receive an applied DC signal, and c. a DC adder output, wherein the DC adder sums the signal at the first DC adder input with the signal at the second DC adder input, and presents the sum corresponding to a DC-augmented form of SD.sub.HF[n] at the DC adder output.

10. A broadband digitizer according to claim 9, wherein the combiner processor further comprises: an output adder including: a. a first output adder input connected to the DC adder output and adapted to receive the DC-augmented form of SD.sub.HF[n], b. a second output adder input connected to the HF equalized output and adapted to receive the frequency distortion-reduced form of high frequency digital signal SD.sub.HF[n], and c. an adder output adapted to form a sum of the DC-augmented form of SD.sub.HF[n] and the distortion-reduced form of LF digital signal SD.sub.LF[n], thereby providing an output signal SD[n] corresponding to the input analog signal SA(t).

11. A broadband digitizer comprising: A. a splitter adapted to receive an applied input analog signal SA(t) at a splitter input and split the received input analog signal SA(t) into a high frequency (HF) analog signal SA.sub.HF(t) in a relatively high frequency (HF) spectral range, and a low frequency (LF) analog signal SA.sub.LF(t) in a relatively low frequency (LF) spectral range, B. a high frequency (HF) analog to digital converter (ADC) channel, including a high frequency (HF) processor operative over the HF spectral range to: a. receive the HF analog signal SA.sub.HF(t) from the splitter at an HF ADC channel input and adjust the level of the received HF analog signal SA.sub.HF(t), and b. in a high frequency analog to digital converter (HF-ADC) characterized by a relatively high frequency sampling rate SR.sub.HF, digitize the amplitude-adjusted HF analog signal SA.sub.HF(t) to a high frequency (HF) digital signal SD.sub.HF[n], C. a low frequency (LF) analog to digital converter (ADC) channel, including a low frequency (LF) processor operative over the LF spectral range, to: a. receive the LF analog signal SA.sub.LF(t) from the splitter at an LF ADC channel input and adjust the level of the received LF analog signal SA.sub.LF(t), and b. in a low frequency analog to digital converter (LF-ADC) characterized by a relatively low frequency sampling rate SR.sub.LF, digitize the amplitude-adjusted LF analog signal SA.sub.LF(t) to a low frequency (LF) digital signal SD.sub.LF[n], D. a frequency distortion processor including: a. an HF frequency distortion-reducing processor comprising a high frequency (HF) digital equalizer including: i. a high frequency (HF) equalizer input adapted to receive SD.sub.HF[n], and ii. a high frequency (HF) equalizer output, wherein the HF equalizer is adapted to reduce frequency dependent distortions in the signal SD.sub.HF[n] caused by the level adjustment and digitizing processing in the high frequency analog to digital converter channel, whereby a frequency distortion-reduced form of high frequency digital signal SD.sub.HF[n] is presented at the HF equalizer output, b. an LF frequency distortion-reducing processor comprising a low frequency (LF) digital equalizer including: i. a low frequency (LF) equalizer input adapted to receive SD.sub.LF[n], and ii. a low frequency (LF) equalizer output, wherein the LF equalizer is adapted to reduce frequency dependent distortions in the signal SD.sub.LF[n] caused by the level adjustment and digitizing processing in the low frequency analog to digital converter channel, whereby a frequency distortion-reduced form of low frequency digital signal SD.sub.LF[n] is presented at the LF equalizer output, and E. a combiner processor including: a combiner adapted to combine a distortion-reduced form of HF digital signal SD.sub.HF[n] received from the HF equalizer output with a distortion-reduced form of LF digital signal SD.sub.LF[n] received from the HF equalizer output, thereby providing an output signal SD[n] corresponding to the input analog signal SA(t), wherein the splitter comprises: a capacitor having a first capacitor terminal and a second capacitor terminal, wherein: AA. the first capacitor terminal is connected to the splitter input, and the second capacitor terminal is connected to the HF ADC channel input, wherein a portion of the input signal SA(t) at the second capacitor terminal corresponds to SA.sub.HF(t) referenced to ground, and is in the high frequency spectral range, and BB. a signal across the first capacitor terminal and the second capacitor terminal corresponds to SA(t)-SA.sub.HF(t), and is coupled to the LF ADC channel input, wherein SA(t)-SA.sub.HF(t) is in the low frequency spectral range.

12. A broadband digitizer, comprising: A. a splitter adapted to receive an applied input analog signal SA(t) at a splitter input and split the received input analog signal SA(t) into a high frequency (HF) analog signal SA.sub.HF(t) in a relatively high frequency (HF) spectral range, and a low frequency (LF) analog signal SA.sub.LF(t) in a relatively low frequency (LF) spectral range, B. a high frequency (HF) analog to digital converter (ADC) channel, including a high frequency (HF) processor operative over the HF spectral range to: a. receive the HF analog signal SA.sub.HF(t) from the splitter at an HF ADC channel input and adjust the level of the received HF analog signal SA.sub.HF(t), and b. in a high frequency analog to digital converter (HF-ADC) characterized by a relatively high frequency sampling rate SR.sub.HF, digitize the amplitude-adjusted HF analog signal SA.sub.HF(t) to a high frequency (HF) digital signal SD.sub.HF[n], C. a low frequency (LF) analog to digital converter (ADC) channel, including a low frequency (LF) processor operative over the LF spectral range to: a. receive the LF analog signal SA.sub.LF(t) from the splitter at an LF ADC channel input, and adjust the level of the received LF analog signal SA.sub.LF(t), and b. in a low frequency analog to digital converter (LF-ADC) characterized by a relatively low frequency sampling rate SR.sub.LF, digitize the amplitude-adjusted LF analog signal SA.sub.LF(t) to a low frequency (LF) digital signal SD.sub.LF[n], and D. a frequency distortion processor including: a. a HF frequency distortion-reducing processor comprising a high frequency (HF) digital equalizer including: i. a high frequency (HF) equalizer input adapted to receive SD.sub.HF[n], and ii. a high frequency (HF) equalizer output, wherein the HF equalizer is adapted to reduce frequency dependent distortions in the signal SD.sub.HF[n] caused by the level adjustment and digitizing processing in the high frequency analog to digital converter channel, whereby a frequency distortion-reduced form of high frequency digital signal SD.sub.HF[n] is presented at the HF equalizer output, and b. an LF frequency distortion-reducing processor comprising a low frequency (LF) digital equalizer including: i. a low frequency (LF) equalizer input adapted to receive SD.sub.LF[n], and ii. a low frequency (LF) equalizer output, wherein the LF equalizer is adapted to reduce frequency dependent distortions in the signal SD.sub.LF[n] caused by the level adjustment and digitizing in the low frequency analog to digital converter channel, whereby a frequency distortion-reduced form of low frequency digital signal SD.sub.LF[n] is presented at the LF equalizer output, E. a combiner processor including: a combiner adapted to combine a distortion-reduced form of HF digital signal SD.sub.HF[n] received from the HF equalizer output with a distortion-reduced form of LF digital signal SD.sub.LF[n] received from the HF equalizer output, thereby providing an output signal SD[n] corresponding to the input analog signal SA(t), and wherein: AA. the high frequency analog to digital converter channel includes a high frequency (HF) front end connected in cascade with the HF ADC, a. wherein the HF front end is coupled between the second terminal of the capacitor and an input of the high frequency ADC, and b. wherein the high frequency front end includes a cascade-coupled amplifier and a high frequency digitally controlled attenuator enabling level adjustment of the input signal SA.sub.HF(t), and is adapted to receive ground-referenced SA.sub.HF(t) and perform level adjustment of the signal SA.sub.HF(t), and apply the level-adjusted SA.sub.HF(t) to the input of the HF ADC, which is adapted to digitize the level-adjusted SA.sub.HF(t) to corresponding SD.sub.HF[n], and BB. the low frequency analog to digital converter channel includes a low frequency (LF) front end connected in cascade with the LF ADC, a. wherein the LF front end is coupled between both the first and second terminals of the capacitor and an input of the LF ADC, and wherein the low frequency front end includes: a differential amplifier having an inverting input terminal and a non-inverting input terminal adapted to receive ground-referenced SA(t) and ground-referenced SA.sub.HF(t) from respective ones of the first and second terminals of the capacitor, and a differential amplifier output terminal presenting SA(t)-SA.sub.HF(t), corresponding to SA.sub.LF(t), and b. an amplifier and a digitally controlled attenuator cascade-coupled from the differential amplifier output terminal, whereby the differential amplifier and attenuator are adapted to perform level adjustment of the input signal SA.sub.LF(t), and apply the level-adjusted SA.sub.LF(t) to the input of the LF ADC, which is adapted to digitize the level-adjusted SA.sub.LF(t) to a corresponding SD.sub.LF[n].

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a block diagram of a broadband RF amplifier component disclosed in prior art U.S. Pat. No. 7,511,577.

(2) FIG. 2 shows a block diagram of a broadband RF amplifier component disclosed in prior art U.S. Pat. No. 7,221,220.

(3) FIG. 3 shows a block diagram of an exemplary broadband digitizer in accord with the current disclosure.

(4) FIG. 4 shows a block diagram of an exemplary front end for the low frequency ADC, incorporated in an exemplary broadband digitizer according to the current disclosure.

(5) FIG. 5 shows a block diagram of a front end for the high frequency ADC, incorporated in an exemplary broadband digitizer according to the current disclosure.

(6) FIG. 6 shows a block diagram of a combining unit, incorporated in an exemplary broadband digitizer according to the current disclosure.

DETAILED DESCRIPTION

(7) As it was noted above, the technologies and device components which are typically used to build a prior art broadband digitizer, are different for signals of the various frequencies over the broad band of intended operation. For this reason, it is difficult to create a digitizer operative with a market-demanded high precision in a broad frequency range. In the present disclosure, an input analog signal-to-be-processed SA(t) is split into two signals: a first analog signal SA.sub.LF(t), characterized by a relatively low frequency spectral portion of SA(t), and a second analog signal SA.sub.HF(t), characterized by a relatively high frequency spectral portion of SA(t). Each of these two analog signals, SA.sub.LF(t) and SA.sub.HF(t), is processed separately and is separately converted from analog to digital form, producing a first digital signal SD.sub.LF[n] (the low frequency digital signal) and a second digital signal SD.sub.HF[n] (the high frequency digital signal). The preparation for analog to digital conversion, and the conversion itself, would normally, in accord with the prior art, cause distortions of the resultant processed signals (mostly in the form of frequency dependent amplitude and phase distortions) with different gains and delays for both the low frequency digital signal and the high frequency digital signal. Correction/reduction of the distortions, together with alignment of the gains and delays of the digital signals SD.sub.LF[n] and SD.sub.HF[n], collectively, the distortions, are provided by the systems and methods of this disclosure. After equalization and correction, the resultant the low frequency digital signal and the high frequency digital signal are added up to form an output distortion-reduced digital signal SD[n] corresponding to SA(t).

(8) A block diagram of an exemplary broadband digitizer 290 of the disclosure is shown in FIG. 3. As illustrated, broadband digitizer 290 principally includes a low frequency analog-to-digital converter (LF ADC) channel 309 and a high frequency analog-to-digital converter (HF ADC) channel 359, an input splitter 390 coupled to inputs of LF ADC channel 309 and HF ADC channel 359, respectively, a frequency divider 380, and a combining unit 330.

(9) Splitter 390 is adapted to receive a broadband analog input signal SA(t) on an input line 390A, split that broadband analog input signal into a low frequency analog input signal SA.sub.LF(t) characterized by a relatively low frequency spectrum, and a high frequency analog input signal SA.sub.HF(t) characterized by a relatively high frequency spectrum.

(10) LF ADC channel 309 includes a low frequency (LF) front end 310 coupled in series to an input 320A of a low frequency analog to digital converter LF ADC 320. LF front end 310 includes differential inputs 310A and 310B adapted to receive SA.sub.LF(t) across those inputs, process that signal and apply it to an input 320A of LF ADC 320.

(11) HF ADC channel 359 includes a high frequency (HF) front end 360 coupled in series to an input 370A of a high frequency analog to digital converter (HF ADC) 320. HF front end 360 includes an input 360A adapted to receive SA.sub.HF(t) referenced to ground potential, process that signal and apply it to an input 370A of HF ADC 370.

(12) Frequency divider 380 is adapted to receive a system clock and provide a divided-down low frequency (LF) ADC sampling clock to a LF clock input 320B of LF ADC 320. In response to the LF ADC sampling clock, LF ADC 320 is operative to digitize the analog signal at input 320A and apply that digitized signal SD.sub.LF[n] via output 320C to an input 330B of combining unit 330.

(13) Similarly, Frequency divider 380 is further adapted to provide a high frequency (HF) ADC sampling clock to a HF clock input 370B of HF ADC 370. in response to the HF ADC sampling clock, HF ADC 370 is operative to digitize the analog signal at input 370A and apply that digitized signal SD.sub.HF[n] via output 370C to an input 330C of combining unit 330.

(14) In response to SD.sub.LF[n] and SD.sub.HF[n], combining unit 330 processes those applied digital signals and provides a distortion-reduced digital signal SD[n] at a system output.

(15) With this configuration, in operation, a broadband analog signal SA(t) applied to the signal input 390A of the digitizer 290 follows to the left (as shown in FIG. 3) terminal of the capacitor 350. The low frequency components of the signal SA(t) are isolated at the capacitor 350, forming the signal SA.sub.LF(t), which is applied across to the differential inputs 310A and 310B of the LF front end 310. The high frequency components of the input signal pass through the capacitor 350 without difficulty and are applied to the input of the HF front end 360. According to the Second Kirchhoff's low, the voltage between the left (as shown in FIG. 3) terminal of the capacitor 350 and ground potential (the signal SA(t)) equals the sum of the voltage over the capacitor 350 (the signal SA.sub.LF(t)) and the voltage between the right (as shown in FIG. 3) terminal of the capacitor 350 and ground potential (the signal SA.sub.HF(t)):
SA(t)=SA.sub.LF(t)+SA.sub.HF(t).(1)

(16) Thus, if the input signals of differential inputs 310A and 310B of LF front end 310 and the input of HF front end 360 are added up, the resulting sum equals the signal at the device 290 input exactly.

(17) The processing of signals in the LF front end 310 and HF front end 360 inevitably distorts them to some extent, as well as disrupts the relationship between their levels and their relative position in time. For these reasons, the adding up the output signals of the LF front end 310 and HF front end 360 in general does not result in an accurate reproduction of the initial signal SA(t).

(18) In the configuration illustrated in FIG. 3, the output signal of the LF front end 310 is converted into digital form by the LF ADC 320, while the output signal of the HF front end 360 is converted into digital form by the HF ADC 370 respectively. In other words, the LF front end 310, connected in cascade with the LF ADC 320, form a low frequency analog to digital conversion channel 309, while the HF front end 360, connected in cascade with the HF ADC 370, form a high frequency analog to digital conversion channel 359.

(19) The sequences of digital samples SD.sub.LF[n] and SD.sub.HF[n], produced by the conversion channels, are applied to associated inputs of combining unit 330. In combining unit 330, frequency distortions of both signals are corrected and the relationship between their levels and their relative position in time are restored. The summing up the corrected digital replicas of the analog signals SA.sub.LF(t) and SA.sub.HF(t) produces a distortion-reduced, accurate digital reproduction SD[n] of the initial input analog signal SA(t).

(20) A DC control input of the analog to digital conversion system 290 of FIG. 3, carries information about any direct current (DC) component of an input analog signal SA(t). This information is applied in a digital form and is converted into analog signal by a digital to analog converter (DAC) 300. The output of the DAC 300 is connected to an associated input 310D of LF front end 310 to be subtracted from the signal SA.sub.LF(t). In the combining unit 330, the DC control signal is added to the output signal SD[n]. This makes it possible for LF front end 310 and LF ADC 320 to operate without DC offset.

(21) An exemplary embodiment of the LF front end 310 is shown in FIG. 4. In that figure, the signal SA.sub.LF(t) is connected across terminals 310A and 310B of a differential amplifier 44 through the resistors 42 and 43. The output of the differential amplifier 44 is connected to an input of a digitally controlled attenuator 45. An output signal of digitally controlled attenuator 45 is amplified by an ADC driver 46 and transmitted through a low pass filter 47 to an output 48 of LF front end 310.

(22) DC input 40 of LF front end 310 is connected to a non-inverting terminal of a differential amplifier 44 through a resistor 41. That configuration enables subtraction of a direct current component of the initial analog signal SA(t) from the voltage SA.sub.LF(t) coming to the signal input across terminals 310A and 310B of the LF front end 310.

(23) An exemplary embodiment of HF front end 360 is shown in FIG. 5. In that figure, the input signal SA.sub.HF(t) passes to an input 360A of a digitally controlled attenuator 51. An output signal of digitally controlled attenuator 51 is amplified by an ADC driver 52 and is transmitted through a low pass filter 53 to an output 360B of HF front end 360.

(24) A block diagram of an exemplary form of the combining unit 330 is shown in FIG. 6. Combining unit 330 corrects frequency distortions present in signals SD.sub.LF[n] and SD.sub.HF[n] produced by LF ADC 320 and HF ADC 370, removing discrepancies between those signals which occurred because of different gains and delays in the low frequency analog to digital converter channel 309 and high frequency analog to digital converter channel 359, as well as adds up the corrected and aligned signals thereby producing the output signal SD[n] of the digitizer 290.

(25) As shown in FIG. 6, the combining unit 330 has two inputs: In LF input 330B which receives digital signal SD.sub.LF(n) from the output 320C of LF ADC 320 and In HF input 330C which receives the digital signal SD.sub.HF(n) from the output 370C of HF ADC 370. The signal coming to the input In LF input 330B, is processed in a LF equalizer 60 where frequency distortions introduced in LF front end 310 and LF ADC 320, are corrected. The signal coming through the input In HF input 330C is processed in a HF equalizer 66, where frequency distortions introduced in the HF front end 360 and HF ADC 370, are corrected.

(26) An output of LF equalizer 60 passes to a first input of a multiplier 61. A second input of multiplier 61 receives a gain control signal, which is set up by an associated program. Multiplier 61 multiplies the signal which combining unit 330 receives from LF ADC 320 by a gain control voltage so that the level of a resulting signal corresponds to the level of the signal received from HF ADC 370. In this way, any discrepancies which may have occurred because of different gains in the low frequency analog to digital converter channel 309 and high frequency analog to digital converter channel 359, are reduced or eliminated.

(27) Differences in delays of signals processed in the low frequency converter channel 309 and high frequency converter channel 359, are compensated by a controlled delay line 62. The delay of the controlled delay line 62 is set up by a delay control signal in such a way that the signals at the output of HF equalizer 66 and the output of delay line 62 become aligned in time.

(28) Samples in digital signal SD.sub.LF(n) (and corresponding samples at the output of controlled delay line 62) follow with a frequency which equals the sampling frequency of LF ADC 320 and is different from the sampling rate of the signal SD.sub.HF(n). The output of controlled delay line 62 is connected to an input of an interpolator 63. The interpolator 63 performs a conversion of the signal sampling rate, making it equal to the sampling rate of the signal SD.sub.HF(n).

(29) The output of the interpolator 63 is applied to a first input of an adder 64. A second input of adder 64 receives from a DC entry input of the combining unit 330, a value of the DC component in the initial analog signal SA(t) (this value has been subtracted from the processed signal in the LF front end 310). By the operation of the adder 64, a correct DC component of the signal SD.sub.LF[n] is restored.

(30) Digitally controlled attenuators 45 and 51 of the LF front end 310 and HF front end 360, respectively, operated by an associated program, provide correct levels of signals at the inputs of LF ADC 320 and HF ADC 370. As a result, possible distortions caused by ADCs overload (in case of a too large input level) or by incomplete use of the ADC's resolution (in case of a too small input level), are eliminated. However, the use of different technology during manufacturing can cause different features of the low frequency and the high frequency digitally controlled attenuators 41 and 51 (for example, different values of attenuation which may be set up, different input and output resistance, different delays and so on). This fact may result in additional discrepancies between features of the low frequency analog to digital conversion channel 309 and high frequency analog to digital conversion channel 359.

(31) During a production stage, the gains and the delays of both analog to digital conversion channels 309 and 358 (from the input of the LF front end 310 up to a first input of the adder 65 for the low frequency channel and from the input of the HF front end 360 up to a second input of the adder 65 for the high frequency channel) are measured for each possible combination of the attenuations of the digitally controlled attenuators 45 and 51. The measurement results are used to determine the necessary values of the gain control to be applied to the multiplier 62, and the delay control to be applied to the delay line 63. The so-determined values are stored in a control values memory. At an operation stage, a controlling program sets up attenuations of the digitally controlled attenuators 45 and 51 in accordance with the input signal level. After that, the corresponding control values are read from a control values memory and are used to adjust proper gain control of the multiplier 62 and delay control of the controlled delay line 63. As a result, the accurate agreement of a digital signal SD[n] produced with the input analog signal SA(t), is achieved automatically.

(32) A DC component of an analog signal at the input of the digitizer 290 may be of a substantial size, as compared with the signal's peak-to-peak magnitude. In such a case, a DC component would likely take away a considerable part of the ADC's dynamic range, making difficult a comprehensive digital reconstruction of the signal's fine details. The current disclosure provides some steps for maintaining high resolution of the ADC even in the case of a large DC component.

(33) For example, a value for the estimation of an initial analog signal DC component (in a digital form) can be applied to the DC control input of the analog to digital conversion system (see FIG. 3). That value is transformed to an analog amount by DAC 300 and that analog amount enters the LF front end 310 through the DC input 310A. Then, that analog amount is subtracted from the input signal SA.sub.LF(t) of the LF front end 310. As a result, the DC component at the input of the LF ADC 320 is made equal to zero. In this way, the LF ADC 320 is relieved from the necessity of converting the DC component into digital form.

(34) The digital signal which carries information about the initial size of the DC component, also arrives at the DC entry input of the combining unit 330. That value is added to the low frequency corrected signal formed by interpolator 63.

(35) The operations described above lead to a situation, where LF ADC 320 converts a signal with an eliminated DC component to a digital form wherein the digital output signal of the conversion system contains a DC component of correct magnitude.

(36) The foregoing description of the embodiment of the present technology contains some details for purposes of clarity of understanding, the technology is not limited to the detail provided. There are many alternative ways of implementing the technology. The disclosed embodiment is illustrative and not restrictive.