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Sensor Electronics Update

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  1. Sensor Electronics Update Richard Partridge May 6, 2003 DC Amplifiers Mixer Offsets Mixer Noise Mixer + RF Noise Mixer Options RF Source Module

  2. 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

  3. Predicted Filter Response

  4. 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

  5. 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

  6. Mixer Offset @ 450 MHz

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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