High bandwidth oscilloscope
A method for improving bandwidth of an oscilloscope involves, in preferred embodiments, the use of frequency up-conversion and down-conversion techniques. In an illustrative embodiment the technique involves separating an input signal into a high frequency content and a low frequency content, down-converting the high frequency content in the analog domain so that it may be processed by the oscilloscope's analog front end, digitizing the low frequency content and the down-converted high frequency content, and forming a digital representation of the received analog signal from the digitized low frequency content and high frequency content.
1. A method for digitizing a signal, comprising:
receiving an input analog signal spanning an original frequency range that exceeds a bandwidth of a first digitizer and that exceeds a bandwidth of a second digitizer;
splitting the received input analog signal into at least a first split signal and a second split signal;
digitizing the first split signal using the first digitizer;
converting a frequency range of the second split signal in a first conversion operation;
digitizing the second split signal using the second digitizer;
converting the frequency range of the second split signal in a second conversion operation; and
combining the first split signal and the second split signal mathematically to form a single output data stream that provides a substantially correct representation of the received input analog signal,
wherein the single output data stream has a bandwidth of frequency content that has been digitized using at least the first digitizer and the second digitizer, and
wherein the bandwidth of the frequency content exceeds a bandwidth of the first digitizer and exceeds a bandwidth of the second digitizer.
2. The method of claim 1 , wherein splitting the received input analog signal includes splitting the input analog signal with a 50 Ohm splitter.
3. The method of claim 1 , wherein converting the frequency range of the second split signal in the first conversion operation includes mixing the second split signal with a predetermined periodic function with a predetermined frequency.
4. The method of claim 3 , wherein the predetermined periodic function is a low-distortion sinusoid.
5. The method of claim 1 , further comprising band limiting the first split signal to a first predetermined band range, wherein the first predetermined band range spans a frequency range smaller than the original frequency range.
6. The method of claim 5 , further comprising band limiting the second split signal to a second predetermined band range, wherein the second predetermined band range spans a frequency range smaller than the original frequency range.
7. The method of claim 6 , wherein:
converting the frequency range of the second split signal in the first conversion operation includes mixing the second split signal with a predetermined periodic function with a predetermined frequency; and
a frequency of the predetermined periodic function is at a low side of the second predetermined band range.
8. The method of claim 6 , wherein:
converting the frequency range of the second split signal in the first conversion operation includes mixing the second split signal with a predetermined periodic function with a predetermined frequency; and
a frequency of the predetermined periodic function is at a high side of the second predetermined band range.
9. The method of claim 1 , further comprising, in processing of the second split signal that follows conversion of the frequency range of the second split signal in the first conversion operation, passing the second split signal through an image reject filter.
10. The method of claim 9 , wherein the image reject filter comprises an intrinsic bandwidth of a digitized channel used for digitizing the second split signal with the second digitizer.
11. The method of claim 1 , wherein:
the first split signal spans a frequency range smaller than the original frequency range; and
the second split signal spans a frequency range smaller than the original frequency range.
12. The method of claim 1 , wherein:
converting the frequency range of the second split signal in the first conversion operation includes converting the frequency range of the second split signal down in frequency; and
converting the frequency range of the second split signal in the second conversion operation includes converting the frequency range of the second split signal up in frequency.
13. The method of claim 1 , wherein:
converting the frequency range of the second split signal in the first conversion operation includes converting the frequency range of the second split signal away from an initial frequency range of the second split signal; and
converting the frequency range of the second split signal in the second conversion operation includes converting the frequency range of the second split signal back in frequency to the initial frequency range of the second split signal.
14. The method of claim 1 , wherein:
converting the frequency range of the second split signal in the first conversion operation occurs in processing of the second split signal that is before digitizing using the second digitizer; and
converting the frequency range of the second split signal in the second conversion operation occurs in processing of the second split signal that follows digitizing using the second digitizer.
15. The method of claim 1 , wherein combining the first split signal and the second split signal mathematically occurs in processing of the first split signal and processing of the second split signal that follows digitizing of the first split signal and digitizing of the second split signal.