Noise Studies April 4-8 M. Johnson April 2016 1
Testing Crew • L. Bagby • S. Chappa • A. Hahn • M. Johnson • B. Kirby • L. Scott • T. Shaw • M. Stancari • T. K. Warburton 2
Outline • The presentation is split into 2 parts • Brian will discuss the following topics • Adding noise to the low voltage line • Reducing regulator input voltage to eliminate 11 KHz noise • Turning off ADC voltages • Comparison of FFT frequencies to those measured with a spectrum analyzer • Methods of inducing the high noise state • I will cover the following topics • 11 KHz noise based on a hardware modification at Microboone that we did last week • Short review of previous data on the high noise state • Spectrum analyzer measurements of the high noise state 3
11 KHz Noise • We observe an 11 KHz signal that is present across all channels in all APA’s • This noise events are correlated in groups that have a common voltage regulator chip • FFT’s also show the same phase for wires in a voltage regulator group • Microboone sees a very similar effect but in a 20 to 30 KHz band • Last week we installed a filter down stream of the warm voltage regulator which eliminated the microboone noise
Microboone Noise • Microboone has noise in the 20 to 30 KHz band that is also associated with voltage regulator groups • They use the same front end ASIC • Mother board layout is similar • Noise is present on the wires • Frequency width is about 6 times that of the 35 ton • Microboone has 6 times as many chips per regulator • All evidence supports the idea that the noise source is identical 5
Microboone Data • Spectrum analyzer plot looking at the output of the intermediate amplifier at feed through 5 at microboone • Flat noise peak between 20 to 30 KHz • Flat response is unusual indicating many different frequencies at the same amplitude • Noise counts depend on wire length • U~first 2500,V~2500 to 5000, Y~5000 on • Noise counts show a marked increase (spikes in histogram) at the last channel in a MB pair and the starting channel of the next MB pair. • This corresponds to a group of ASICs Histogram of noise counts versus wire powered by a common voltage regulator channel number for µboone 6
Effect of Wire Capacitance on µboone Noise Signals • If the wires for MB pair A or B are at the same potential, then the inter wire capacitance will have no effect • However, if the potential for A is different from B (because of a different phase) then there will be a larger charge stored on A384 than on A383 resulting in a larger noise signal on A384 • Get same effect for B1 so last channel of first MB pair and first channel of second MB pair Simplified schematic showing the last channels should have more noise of mother board pair A and the leading channels of mother board B. All capacitor value are assumed • If this interpretation is correct, there must be to be equal. noise signal on the wires • Noise on the wires is likely from a voltage since noise depends on capacitance (wire length) 7
Microboone Modification • Made a proposal to µboone last December to modify a service board which holds the regulators • The collaboration agreed to this so I designed a Chebyshev filter for feed through 5 • Two service boards were modified ao an entire feed through could be changed • This was installed last Wed and tests were run over night • Since this was a kludge, the modified boards were removed on Thursday
Chebyshev Filter • Chebyshev filter installed downstream of the regulator Chebyshev Filter chip (U1 below) • Cut off is 5 KHz Microboone Regulator board SPICE model from D. Huffman 9
Filter Results • Noise reduction before setting ASIC gain and shaping was ~25db • After setting gain, the reduction was 12 db or a factor of 4 • Hardware filter and software filter now give same results • The hardware filter removes all the coherent noise • Hardware filter has no impact on the signal • Each ASIC on the mother board a has three 33 µF and two 0.1 µF bypass capacitors • While it is possible that it is voltage feed back, current feed back seems much more likely 10
No Hardware Filter Hardware Filter on Central Region 11
What About the Edge Effect? • Noise at the edges of the mother boards appears to actually increase • Preliminary analysis • If MB A and B are both oscillating, sometimes they will have the same voltage and thus no additional noise • If A is 0, then there will be added noise whenever B is away from 0 volts so the noise could increase
High Noise Levels • Lots of information from observations over the last few months • Ran for 24 hours with cathode off and bias on with no high noise states • Ran 24 hours with cathode at 60 KV and bias off with no high noise states • High noise state is associated with a current drop on the low voltage supply for the front end • Noise has the same frequency over all front end channels even if only a few show the current drop
High Frequency • 2 GHz band width so values below 1 MHz were measured on the turn on slope • Peaks were still obvious and matched the ones found in the FFT • This is conclusive evidence that the noise Grid signal on APA3 signals are present on the High noise state wires 14
From Nuno’s Talk Last Week • There is circumstantial evidence that noise increased with HV drift • At 60 KV: No noise transitions (according to Alex) • At 90 KV: Short lived high noise and would recover on its own • At120 KV: Often in high noise state and only recovered when power cycling ASICs • FEMB 9,10, 12-15 were not being used • Power cycling FEMB-00 was enough to recover low noise • When started running with only RCE00, 04 and 08 high noise state became less common
Low Voltage Current Plots From 120 KV Running • The plot shows that one can have a high noise state with only 2 FEMBs showing a voltage drop and also the direct opposite • This data is also from Nuno.
Spectrum Analyzer Measurements • Need a method of looking at signals inside the cryostat • Each APA has a grid plane in front of the wires which is only connected to a bias line • Nearly ideal antenna into the cryostat • Only drawback is that it is not a 50 ohm system • Used 2 spectrum analyzers: 0 to 110 KHz (high resolution), ~200 KHz to 2 GHz (medium resolution) 17
RCE00 Spectrum • Low noise state spectrum taken from APA 7-1 grid • Two peaks; one at 100 KHz and one at 109 KHz • Spectrum analyzer resolution is 125 Hz so width of both peaks is less than 200 Hz! • Consistent with feed back saturation • Signal does not change between high and low noise state 18
FEMB 00 Noise • 100 KHz noise has been present since ����������������������������������� cool down ������������������ • It is also present in the high noise state • Since the bandwidth is so narrow, it is likely that it has harmonics � • Start up operations such as adding more ASICs to the readout change the frequencies- often by a large amount • Last weeks data shows a 60 KHz line and harmonics. • Many runs had a 75 KHz or higher frequency (next slide) Data taken last week 19
High Nose State FFT Note the large number of evenly spaced lines starting at 75 KHz 20
High Noise Boards • Not all boards enter the high noise state • Data below from N Barros shows that different combinations of boards go into the high noise state • Spectrum analyzer shows the same frequencies on the grid for all APA’s as seen in the data 21
External Noise Sources • Steve Chappa again looked for external noise sources at the 35 ton. • We found only the 54 KHz from the lights. • This signal is also seen in the data and in the spectrum analyzer looking at the grids • We observed the decay of the spectrum analyzer signal in real time when the lights were turned off • Spectrum analyzer signal disappeared when ASIC power was recycled to start or stop noise (not well documented) • The variable frequencies observed in the high noise state also indicate that it is not from an external source 22
Model • Microboone tests show that there is signal on the wires • Signal source could be from the supply current or perhaps something with the ASIC • It is likely that there can also be feed back that is local to a chip. • This is similar to a microphone placed too close to a speaker which results in a high pitched squeal • This would require a very narrow spectral line but this is what we observe in RCE00 • The high noise signal is spread to other boards via capacitive coupling to the close in cathode • All grids show the high frequency signals so there has to be some coupling to the cathode • Simplest model that has uniform coupling between low regulator current and high regulator current boards 23
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