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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


  • 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