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Comprehensive Review of DC Amplifiers and Mixer Performance in RF Applications

This article discusses the design and performance of DC amplifiers and mixers used in RF applications. It highlights a four-channel low-frequency amplifier with exceptional low-frequency performance, featuring an AD8628 auto-zero amplifier with low offset voltage, followed by an OP27 low noise amplifier. The study examines mixer offset issues at 450 MHz, the impact of flicker noise, and strategies for managing mixer performance. Grounding techniques and RF gain adjustments are suggested to optimize signal integrity and minimize noise contributions.

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Comprehensive Review of DC Amplifiers and Mixer Performance in RF Applications

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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

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