1. Simple Technique for Suppression of Line Frequency Noise in IR Array Systems Bruce Atwood, Jerry A Mason, and Daniel Pappalardo The Ohio State University Both Mux-Readout IR arrays and CCD type detectors are now routinely operated in system that can produce images with only a few electrons RMS noise. Obtaining a low noise image from either type detector requires subtracting a baseline image at some point in the readout process. In CCD detectors the baseline information is obtained on a pixel by pixel basis, usually by measuring the level of the output node after resetting it, just before the signal charge packet is transferred to the output node. In IR Mux-Readout type arrays a baseline image is normally acquired prior to the exposure to the signal of interest. In either case the baseline sample (or image) is subtracted from the signal sample (or image) to form the final image. This subtraction process creates a high pass filter which rejects signals with periods long compared to the time between the two samples. Since the time between signal and baseline in CCD detectors is typically a few to a few tens of microseconds, disturbances at the power line frequency (hereinafter PLF) are attenuated by a factor of order 100. Mux readout type IR arrays, on the other hand, require that all pixels be reset prior to exposure. Thus the time between the baseline sample and the signal sample is longer than the exposure time, typically a few to many seconds and PLF disturbances are in the resulting bandpass. Even when the wide dynamic range signal chains used for theses systems are engineered (or trimmed) to be insensitive to PLF electric or magnetic fields, small changes in the environment can cause these disturbance to reappear. We describe here in a simple way to greatly reduce the sensitivity of such systems to PLF disturbance. The typical IR array read sequence begins by reading a frame with nearly zero exposure, in the case of the Ohio State systems with Hawaii arrays, by resetting a full line, reading that line, and advancing to the next line. This “pre-read” is stored digitally. The exposure timer is started at the beginning of this pre-read process. When the desired exposure time is complete the process is repeated, with the exception of using dummy delays instead of the line resets, to form the post-read frame. The pre-read frame is then subtracted from the post-read frame. Structure in the pre-read frame is dominated by the offsets from the individual preamplifiers associated with each pixel. There is, inevitably, also a small amount of coherent noise a the PLF and harmonics. The image in Figure 1 was obtained by subtracting two frames taken as outlined above. The diagonal banding is the result of the random phase of the line frequency disturbance with respect to the read process. The bands point toward the center because this type of device reads out in quadrants arranged so that the read process proceeds from the corners toward the center. Figure 2 was taken in an identical fashion with the exception that the start of both pre-reads and post reads are synchronized to the PLF phase. The RMS in Figure 1 is 2.74 data numbers while the RMS in Figure 2 is 2.67 data numbers. Thus the improvement in noise level is largely cosmetic. Figure 3 shows the circuit used to generate the synchronizing signal. The output of a small 12 volt transformer is clipped with d1 and d2. A low pass filter formed form R1 and C1attenuate any spikes. Comparator U1 then detects the sign of the resulting signal. This square wave is applied to the RI input of a UART, though any available digital input that can be poled could be used. A hardware interrupt could also be used. In our case the RI (ring input) input is polled at ~ 50 kHz and the read process is started when a zero is detected on sample n and a one on sample n+1. Note that the scheme fails if the line frequency is changing rapidly enough that the total number of power line cycles in a pre-read and a post-read are different. The effect would be good cancellation of the PLF disturbances in the pixels read first, where the phases match due to the active synchronization, and decreasing cancellation as the read process continues. If the pre- read PLF were dramatically different for the pre and post reads there would be bands of good and bad cancellation as the pre and post PLF beat. Figure 1, No synchronization to line frequency, RMS=2.74 Figure 1, Synchronzed to line frequency, RMS=2.67 Figure 3, Schematic of circuit used to synchronize readout to line frequency