A simple analog clock used for reducing pedestal noise

Experimental facilities are inherently noisy places. Try as one might to eliminate it, there always seems to be some residual low frequency electronic hum that works its way into low level analog signals, such as calorimetry into analog-to-digital converters (ADC). At the Fermilab Wide Band Lab, the predominant noise (between experimental hall and counting room, and between electronics racks within experimental hall and within counting room) consists of a combination of 60 Hz plus 180 Hz adding together with equal amplitudes and identical phase. The noise contributions at these low frequencies are easily removed by capacitor-coupling or isolation transformers. This approach works well for most detector systems. However, for the highest rate analog detector systems, e.g. the calorimeters within the beam stops for the electron (RESH-0), positron (POSH-O), and photon (BGM) beams for E-831 lFOCUS, these AC couplers introduce unacceptable signal distortion and rate-dependent analog baseline shifts. The residual 60/180 Hz hum, between these detectors and their ADCS, is typically on the order of 10 revolt peak-to-peak at the Wide Band Laboratory, which drastically smears the pedestal spectra and constitutes the major contribution to the resolution for these detector systems. The author, rather than continue to beat his head against the wall in trying to eliminate all this noise, implemented a simple 60 Hz analog clock ramp which was read into an ADC channel simultaneously as part of the data event record and which indicated the instantaneous phase relative to the 60 Hz AC power at Wide Band Laboratory. The pedestal was then monitored and parameterized as a function of this phase. During analysis, the digitized analog system had its phase-dependent pedestal subtracted, thereby substantially reducing the low-frequency pedestal noise. The 60 Hz analog clock circuit is a simple saw-tooth ramp generator. It is packaged in two NIM modules to provide additional isolation between the 120 VAC and 6 VAC and the output ramp. The 74121 Schmitt-trigger one-shot multi-vibrator fires at approximately 1/3 of the (positive) amplitude of the clipped 6 VAC, providing a 4 y-see TTL reset pulse at this specific phase of the AC line voltage. No additional phase-locking or missing cycle compensation was included. The 4 p-see pulse was used to reset the charging capacitor via the 75452 switch. The 741 op amp provided buffering and offset adjustment, producing the desired sawtooth ramp from {minus}30 revolt to {minus}90 revolt into the 50 ohm input of the ADC. The analog ramp, along with the half-rectified 6 VAC phase is shown in Figure 2. Although the analog ramp is fairly linear, for off-line correction of pedestal noise, the only requirement is that there be a one-to-one correspondence between the analog ramp and the phase of the line power