Frequency conversion in a touch sensor
A touch sensitive device includes a touch interface having rows and columns, a signal generator for generating a plurality of unique orthogonal signals in a first range and having a first frequency spacing on at least a plurality of the rows, respectively, and a heterodyning frequency converter. The heterodyning frequency converter receives a signal present on a column, the signal comprising signals within the first range and having the first frequency spacing. The heterodyning frequency converter converts the signal to a heterodyned signal comprising signals in a second range and having the first frequency spacing, and outputs the heterodyned signal. A touch processor identifies touch on the touch interface at least in part by processing the heterodyned signal.
1. A touch sensitive device comprising:
touch interface comprising rows and a column;
signal generator for generating a plurality of unique orthogonal signals on at least a plurality of the rows, respectively, the plurality of unique orthogonal signals being in a first range of frequencies and having a first frequency spacing;
heterodyning frequency mixer configured:
to receive
a column signal present on the column, the column signal comprising signals within the first range of frequencies and having the first frequency spacing, and
an oscillating signal, and
to output
a heterodyned signal based upon the column signal and the oscillating signal, the heterodyned signal comprising signals within a second range of frequencies at the first frequency spacing;
touch processor configured to identify touch on the touch interface based at least in part on the heterodyned signal.
2. The touch sensitive device of claim 1 , wherein the heterodyning frequency mixer comprises a quadrature frequency mixer.
3. The touch sensitive device of claim 2 , further comprising:
local oscillator generating the oscillating signal; and
wherein the quadrature frequency mixer employs first and second frequency mixers driven from the local oscillator, one of the first and second frequency mixers being fed with a copy of a signal from the local oscillator that has been shifted by 90 degrees.
4. The touch sensitive device of claim 2 , wherein the column signal has a center frequency and the quadrature frequency mixer is configured to shift the center frequency of the column signal down to a position in baseband.
5. The touch sensitive device of claim 1 , wherein the touch processor is configured to perform spectral analysis using a Fourier transform.
6. The touch sensitive device of claim 5 , wherein an in-phase output of the frequency mixer is fed into a real input of the Fourier transform and a quadrature output of the frequency mixer is fed into an imaginary input of the Fourier transform.
7. The touch sensitive device of claim 1 , wherein the touch processor is configured to perform cross-correlation using a Fourier transform as a step of the cross-correlation.
8. The touch sensitive device of claim 7 , wherein an in-phase output of the frequency mixer is fed into a real input of the Fourier transform and a quadrature output of the frequency mixer is fed into an imaginary input of the Fourier transform.
9. The touch sensitive device of claim 1 , further comprising a bandpass filter configured to filter the column signal prior to mixing of the column signal with the oscillating signal.
10. The touch sensitive device of claim 1 , further comprising a bandpass filter configured to filter out a sum component of an output of the mixer and allow a difference component to pass.
11. The touch sensitive device of claim 1 , further comprising a bandpass filter configured to filter out a difference component of an output of the mixer and allow a sum component to pass.
12. The touch sensitive device of claim 1 , wherein the frequency generator is configured to allocate frequencies such that there is a gap in frequency coverage, the gap coinciding with a set of unusable frequencies near DC in the heterodyned signal.
13. The touch sensitive device of claim 1 , further comprising:
an analog-to-digital converter operatively connecting the output of the heterodyning frequency mixer to the touch processor, wherein the heterodyned signal is amplified before digitization by an analog-to-digital converter.
14. The touch sensitive device of claim 1 , further comprising:
an analog-to-digital converter operatively connecting the output of the heterodyning frequency mixer to the touch processor, wherein the analog-to-digital converter is a sigma-delta analog-to-digital converter.
15. The touch sensitive device of claim 1 , further comprising:
an analog-to-digital converter operatively connecting the output of the heterodyning frequency mixer to the touch processor, wherein the analog-to-digital converter is a continuous time sigma-delta analog-to-digital converter.
16. The touch sensitive device of claim 1 , configured to perform the spectral analysis digitally such that a spectrum of the column signal is analyzed to determine which and how much of a signal of interest from each row of the touch interface is present in the column signal.
17. The touch sensitive device of claim 1 , configured to perform a cross-correlation analysis digitally such that the column signal is analyzed to determine which and how much of a signal of interest from each row of the touch interface is present in the column signal.
18. The touch sensitive device of claim 1 , further comprising:
analog-to-digital converter operatively connecting a digital representation of the first column signal and the oscillating signal to the heterodyning frequency mixer, and wherein the heterodyning frequency mixer operates in the digital domain.
19. The touch sensitive device of claim 1 , further comprising:
amplifier operatively connecting the first column signal and the oscillating signal to the heterodyning frequency mixer, and wherein the heterodyning frequency mixer operates in the analog domain.
20. A touch sensitive device comprising:
touch interface comprising rows and columns;
signal generator for generating a plurality of unique orthogonal signals on at least a plurality of the rows, respectively, the plurality of unique orthogonal signals being in a first range of frequencies and having a first frequency spacing;
heterodyning frequency mixer configured:
to receive
a column signal present on each of the columns, the column signals comprising signals within the first range of frequencies and having the first frequency spacing, and
an oscillating signal, and
to output
a heterodyned signal for each column, each heterodyned signal based upon one of the column signals and the oscillating signal, the heterodyned signals each comprising signals within a second range of frequencies at the first frequency spacing;
touch processor configured to identify touch on the touch interface based at least in part on the heterodyned signals.