Method and apparatus for acquiring noise reduced high frequency signals
A system and method receive the output signal from a capacitance diaphragm gauge (CDG) and generate a noise reduced output signal. An input signal processing circuit receives an input signal from a signal source that drives the CDG. The input signal processing circuit generates a segment of N normalized digital samples of the input signal. An output signal processing circuit receives the output signal from the CDG and generates M segments of N digital samples of the CDG output signal and averages the corresponding samples in the M segments to generate a signal segment of N averaged samples. Each of the N averaged samples is multiplied by a corresponding one of the N normalized samples to generate N products. The N products are averaged to generate an average product, which is multiplied by a constant to generate a system output signal with reduced noise.
1. A method for reducing the noise in an output signal from a capacitance diaphragm gauge (CDG) that operates in response to a signal source that generates an input signal at a predetermined frequency and amplitude, the method comprising:
amplifying the output signal from the CDG;
converting the output signal to N digital samples during a duration of a first predetermined time segment;
converting the output signal to N digital samples during at least one additional predetermined time segment, each additional predetermined time segment having a duration corresponding to the duration of the first predetermined time segment;
averaging each digital sample in the first predetermined time segment with a corresponding digital sample in each additional predetermined time segment to generate N averaged samples;
converting the input signal to N digital samples during a time segment having a duration corresponding to the duration of the first predetermined time segment;
normalizing the N digital samples of the input signal with a maximum amplitude of the N digital samples to generate N normalized samples of the input signal;
multiplying each of the N normalized samples of the input signal with a corresponding one of the N averaged samples to generate N digital products;
averaging the N digital products to generate an average product; and
generating an output signal responsive to the average product, the output signal having a value representing a pressure applied to the CDG.
2. The method as defined in claim 1 , wherein:
the at least one additional predetermined time segment comprises M additional predetermined time segments and M is 10;
N is 200; and
the samples are converted at a rate of one sample per 20 microseconds.
3. A noise reduction system for a capacitance diaphragm gauge (CDG) that generates a CDG output signal in response to an input signal from a signal source operating at a predetermined frequency and amplitude, the output signal having an amplitude modulated by changes in pressure applied to the CDG and further modulated by noise, the noise reduction system processing the output signal from the CDG to reduce the noise and generate a system output signal, the noise reduction system comprising:
an input signal processing circuit that generates a segment of N normalized digital samples of the input signal;
an output signal processing circuit that generates M segments of N digital samples of the CDG output signal and that averages corresponding samples of the M segments to generate a single segment of N averaged digital samples;
a first multiplier that multiplies each of the N averaged digital samples by a corresponding one of the N normalized digital samples to generate N digital products;
a product averager that averages the N digital products to generate an average product; and
a second multiplier that multiplies the average product by a constant to generate a system output signal.
4. The system as defined in claim 3 , wherein the second multiplier multiplies the average product by two to generate the system output signal.
5. The system as defined in claim 3 , wherein:
the input signal processing circuit comprises:
a first analog-to-digital (A/D) converter that converts the time-varying amplitude of the input signal to a plurality of digital values;
a first sample and store circuit that samples the digital values from the A/D converter and that stores N digital values as a digitized input segment; and
a normalizer circuit that determines a maximum magnitude of the N digital values in the digitized input segment and that normalizes the N digital values to the maximum magnitude to generate the N normalized digital samples;
and
the output signal processing circuit comprises:
a second analog-to-digital (A/D) converter that converts the time-varying amplitude of the CDG output signal to a plurality of digital values;
a second sample and store circuit that samples the digital values from the A/D converter and that stores M segments, each of the M segments comprising N digital samples of the CDG output signal; and
a sample average circuit that averages corresponding samples of each of the M segments to generate the single segment of N averaged digital samples of the CDG output signal.
6. The system as defined in claim 3 , wherein:
M is 10;
N is 200; and
the samples are converted at a rate of one sample per 20 microseconds.
7. The system as defined in claim 3 , further comprising a third multiplier that multiplies each of the N normalized digital samples from the input signal processing circuit by the average product from the product averager to generate digitized reproduction of the input signal modulated by changes in amplitude caused by the pressure applied to the CDG.
8. A method for reducing the noise in an output signal from a transducer that operates in response to a signal source that generates an input signal at a predetermined frequency and amplitude, the method comprising:
amplifying the output signal from the transducer;
converting the output signal to N digital samples during a duration of a first predetermined time segment;
converting the output signal to N digital samples during at least one additional predetermined time segment, each additional predetermined time segment having a duration corresponding to the duration of the first predetermined time segment;
averaging each digital sample in the first predetermined time segment with a corresponding digital sample in each additional predetermined time segment to generate N averaged samples;
converting the input signal to N digital samples during a time segment having a duration corresponding to the duration of the first predetermined time segment;
normalizing the N digital samples of the input signal with a maximum amplitude of the N digital samples to generate N normalized samples of the input signal;
multiplying each of the N normalized samples of the input signal with a corresponding one of the N averaged samples to generate N digital products;
averaging the N digital products to generate an average product; and
generating an output signal responsive to the average product, the output signal having a value representing a parameter applied to the transducer.