Sensor Electronics Update

DC Amplifiers Mixer Offsets Mixer Noise Mixer + RF Noise Mixer Options RF Source Module

Richard Partridge

May 6, 2003

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 = 10s

Predicted Filter ResponseLow Frequency Amplifier Response

0.01

0.1

1

100 1000 10000 100000

Frequency (Hz)

Mag

nit

ud

e

0

20

40

60

80

100

120

140

160

180

Magnitude

Phase

Amplifier Noise Noise floor of 2.2 V/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 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

Mixer Offset @ 450 MHzMixer Offset vs LO DriveOffset(V) ~ 0.00545*LO - 0.0026

-1

0

1

2

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9

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

LO Amplitude (Vrms)

Mix

er

Off

se

t (m

V)

MeasuredFit

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 Noise floor of ~2.2 V/Hz½ is not affected by mixer Substantial 1/f noise component below ~10 Hz Noise is ~30 V/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 Noise floor increased to ~7.2 V/Hz½ due to RF amplifiers

» Expected ~10 V/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

1. 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

2. 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

3. 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 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 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