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Sensor Electronics Update. Richard Partridge May 6, 2003. DC Amplifiers Mixer Offsets Mixer Noise Mixer + RF Noise Mixer Options RF Source Module. DC Amplifiers. 4 Channels of x100 low-frequency amplifier built 1 st stage: AD8628 auto-zero amplifier Extremely low offset voltage

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sensor electronics update

Sensor Electronics Update

Richard Partridge

May 6, 2003

DC Amplifiers

Mixer Offsets

Mixer Noise

Mixer + RF Noise

Mixer Options

RF Source Module

dc amplifiers
DC Amplifiers
  • 4 Channels of x100 low-frequency amplifier built
  • 1st stage: AD8628 auto-zero amplifier
    • Extremely low offset voltage
    • Excellent low frequency performance
    • Gain set to 10
  • 2nd stage: OP27 low noise amplifier
    • Able to drive ±10V output
    • Low frequency performance not as good as AD8628, but meets requirements
    • Gain set to 10
  • RC filters on inputs to both gain stages
    • Roll off high-frequency noise
    • RC = 10ms
amplifier noise
Amplifier Noise
  • Noise floor of 2.2 mV/Hz½ is below RF amplifier noise
  • Almost no 1/f low frequency noise
  • Offset measured to be 0.44 mV on output
  • Conclusion: amplifier meets all design goals
mixer performance
Mixer Performance
  • Low-noise amplifier, SRS SR785 low-frequency spectrum analyzer allows mixer performance to be studied in more detail
  • Double-balanced mixer is designed to have small LO feedthrough and low DC offset
  • After x100 amplifier, mixer offset is clearly seen
  • Diodes in mixer also have a small effective resistance, giving rise to “flicker noise” at low frequencies
  • Mixer performance studied using Rhode & Schwartz RF source to drive mixer LO input
  • Will first show measurements and then discuss options for dealing with these problems
why is mixer offset a problem
Why is Mixer Offset a Problem?
  • Double balanced mixer does reduce offset to “only” ~0.5% of LO amplitude
  • Unfortunately, this is large compared to desired sensor sensitivity
    • When amplified by 104, offset will be 10’s of volts
    • Exceeds ADC dynamic range
  • Offset doesn’t appear to be stable
    • Drifts by ~1% seen over ~1 hour period
    • Offset sensitive to RF input
      • Increase by x3 when RF amplifiers are connected to mixer RF input
    • May be sensitive to other factors as well
mixer noise no rf input
Mixer Noise – no RF Input
  • Noise floor of ~2.2 mV/Hz½ is not affected by mixer
  • Substantial 1/f noise component below ~10 Hz
  • Noise is ~30 mV/Hz½ at 0.1 Hz
    • 0.1 Hz noise is a factor of 4-5 above the RF amplifier noise
    • Most of the electronic contribution to the position noise is from the very low frequency sources
mixer noise with rf amplifier
Mixer Noise with RF Amplifier
  • Noise floor increased to ~7.2 mV/Hz½ due to RF amplifiers
    • Expected ~10 mV/Hz½ for loss-less mixer
  • Offset increased to 348 mV after IF amplifier
  • Increased 60 Hz +harmonics noise
    • Appears to be due to ground loop formed by RF and IF amplifier power
    • Will likely need to isolate RF grounds
mixer offset options
Mixer Offset Options
  • Move sensor position to point where mixer output is 0
    • Drifts in LO amplitude look like a position change
    • Drifts in Sensor drive amplitude look like a position change
    • Drifts in Sensor transfer function look like a position change
    • Drifts in RF amplifier gain look like a position change
  • Add electrical offset to mixer output
    • Drifts in LO amplitude look like a position change
    • Drifts in electrical offset look like a position change
    • Drifts in mixer balance look like a position change
  • Increase RF gain, decrease IF gain
    • Offset becomes manageable
    • Set position to null sensor RF output so position measurement is largely insensitive to RF gain and transfer function
    • Mixer flicker noise becomes negligible, 60 Hz harmonic noise reduced
    • Requires attenuator or variable gain RF amplifier to provide large motion dynamic range
rf source module
RF Source Module
  • Michael Irwin (controls) actively working on the design
  • Verified that PLL chip can be frequency modulated
    • PLL chip tested with 100 kHz frequency modulation amplitude with 100 Hz modulation frequency
    • Spectrum analyzer showed expected frequency spectrum
  • Mixer noise measured with PLL evaluation board to drive LO signal
    • Similar results as for Rhode & Schwartz generator at low frequencies
    • Increased 60 Hz harmonics due to grounding issues
conclusions
Conclusions
  • DC amplifiers perform well
  • Mixer offset needs to be addressed
    • Best option appears to be to increase RF gain, decrease IF gain
  • Mixer introduces non-negligible flicker noise
    • Could probably live with it, but problem solved by increasing RF gain
  • Grounding needs to be done carefully
    • Isolate RF grounds from mixer/IF amplifier grounds
    • VME amplifier appears to have floating inputs
    • Single low-impedance ground established at amplifier power supply
  • ZComm RF source appears to work well
    • Can be frequency modulated by varying reference frequency
    • Michael Irwin is now working on the project
    • Mixer noise similar to Rhode and Schwartz RF source
    • Need to test phase noise of both RF sources at some point
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