Notes

Outline

Low Noise Oscillator Design and Performance Michael M. Driscoll Northrop Grumman Corporation Electronic Sensors and Systems Division Baltimore, Maryland 2000 IEEE Frequency Control Symposium Kansas City, MO - June 6, 2000

Contents 1. Short-Term Frequency/Phase/Amplitude Stability 2. Basic Oscillator Operation 3. Types of Resonators and Delay Lines 4. Useful Network/Impedance Transformations 5. Sustaining Stage Design and Performance 6. Oscillator Frequency Adjustment/Voltage Tuning 7. Environmental Stress Effects 8. Oscillator Circuit Simulation & Noise Modeling 9. Oscillator Noise De-correlation/Noise Reduction Techniques 10. Oscillator Test/Troubleshooting 11. Summary 12. List of References

1. Short-term Frequency/Phase/Amplitude Stability

Slide 4

Slide 5

Slide 6

Slide 7

Slide 8

Slide 9

Slide 10

Slide 11

Slide 12

Slide 13

Slide 14

Slide 15

Slide 16

Slide 17

Slide 18

Slide 19

Oscillator Viewed as a Two Terminal Negative Resistance Generator

Slide 21

In an Oscillator, Gain/Negative Resistance Reduction Must Occur to Achieve Steady-State Operation Types of automatic level control (ALC) and/or automatic gain control (AGC):        (1) Instantaneous signal amplitude limiting/waveform clipping via sustaining stage              amplifier gain compression or separate diode waveform clipping.*        (2) Gain reduction using a feedback control loop.  The oscillator RF signal is DC-              detected, and the amplified detector output fed to a variable gain control element              (i.e., PIN attenuator) in the oscillator. * Symmetrical diode waveform clipping provides better (harder) limiting, compared to single-ended clipping, and appears to provide more immunity from the effects of diode noise.  The least noisy form of transistor amplifier gain compression is single-ended current limiting, rather than voltage limiting (saturation).  Single-ended limiting is soft limiting.

Oscillator Turn-On Behavior Oscillation is initiated by spectral components of circuit noise and/or DC turn-on transients occurring at the frequency where the small signal conditions for oscillation are satisfied. Turn-on time is determined by the initial noise/transient spectral signal level, the steady-state signal level, the oscillator loop (resonator loaded Q) delay, and the small signal excess gain.

Slide 24

Slide 25

Frequency Control Element (i.e., Resonator) Performance Goals High group delay (high resonator loaded Q) High operating frequency Low Loss Moderate Drive Capability Low frequency sensitivity to environmental stress (vibration, temperature, etc.) Good short-term and long-term frequency stability Accurate frequency set-on capability External frequency tuning capability No undesired resonant modes or higher loss in undesired resonant modes or undesired resonant mode frequencies far from desired operating frequency High manufacturing yield of acceptable devices

Slide 27

Slide 28

Slide 29

Types of Resonators and Delay Lines (highlighted types used in lower noise oscillators) 4.  Dielectric 5.  Cavity, Waveguide 6.  Optical Fiber 7. Whispering Gallery Mode, Sapphire Dielectric

Quartz Acoustic Resonators Desirable Properties Very high Q. Controllable (selectable) frequency temperature coefficient. Excellent long-term and short-term frequency stability. Relatively low cost. Moderately small volume (especially SAW, STW). Well defined, mature technology. Undesirable Properties 1/f FM noise that often exceed effects of sustaining stage 1/f PM noise. Unit-to-unit 1/f FM noise level. variation; high cost associated with low yield of very low noise resonators. BAW resonator drive level limitations: 1-2mW for AT-cut, 5-7mW for SC-cut,  even lower drive for low drift/aging. Non-uniform vibration sensitivity. FOM (loaded Q) decreases with increasing frequency.

Improvements in Acoustic Resonator Performance - 1985 to 1999

Dielectric-Filled Coaxial Resonators Very popular in wireless hardware High drive capability One piece, plated construction results in low vibration sensitivity Unloaded Q is only moderate (proportional to volume)  L(100Hz)=-100dBc/Hz, with -178dBc/Hz noise floor achieved at 640MHz using large volume resonators as multi-pole filter oscillator stabilization elements Even though resonators are “passive”,  excess 1/f noise has been measured in large volume, high delay devices with variations in 1/f noise level of up to 20dB

Dielectric Resonators Advantages High Q at high (microwave) frequency No measurable resonator 1/f noise High drive capability Near-zero temperature coefficient for some ceramic dielectric materials Amenable to mechanical adjustment and electronic frequency tuning Disadvantages Substantial Q degradation unless cavity volume is large compared to that of dielectric (low order mode resonances) Highest Q with modest volume occurs above C-band where sustaining stage amplifiers are primarily GaAs sustaining stage amplifiers exhibiting relatively high 1/f AM and PM noise Resonator frequency sensitivity to vibration is typically 10 to 100 times higher, compared to BAW, SAW resonators

Slide 35

Slide 36

Slide 37

Spectral Tradeoff: Near-Carrier vs Noise Floor Performance

Whispering Gallery Mode, Sapphire Dielectric Resonators Dielectric loss in sapphire is low at room temperature and rapidly decreases with decreasing temperature. High-order “whispering gallery” mode ring and solid cylindrical resonators have been built that exhibit unloaded Q values, at X-band, of 200,000 at room temperature and 5 to 10million at 80K. The ultra-high Q exhibited by these resonators has enabled oscillators to be designed and constructed whose X-band output signal spectra are significantly superior, in both a near-carrier noise and noise floor basis, to that attainable using any other resonator technology.

Slide 40

Slide 41

Slide 42

Slide 43

Slide 44

Slide 45

Slide 46

Slide 47

Useful Aspects of Lumped or Distributed Element Transmission Lines Impedance inversion/transformation (can transform a resonator series-resonance impedance to a parallel resonance) Relatively broadband impedance transformation, compared to band-pass structures (lower sensitivity to element value tolerance, temperature coefficient, etc.) All or some of the line can be realized using actual transmission line (coaxial cable) - Thermal isolation of ovenized components - Vibration isolation of acceleration sensitive components At HF and Low VHF, transmission line transformers can be realized with values for characteristic impedance not obtainable using conventional coaxial or twin lead cable. Positive or negative phase shifts may be obtained using high-pass or low-pass lumped element approximations

Dipole Transformation

Slide 50

The Transistor Viewed as a Reactance-plus-Negative Resistance Generator Normally, capacitors are used for the reactances X1 and X2. At microwave frequencies, transistor junction capacitance may comprise a significant part or all of the reactance.

Slide 52

Slide 53

Slide 54

Methods for Reducing Discrete Transistor Sustaining Stage 1/f PM Noise Use un-bypassed emitter resistance (a resistor or the resonator itself connected in series with the emitter and operated at series-resonance Use high frequency transistors having small junction capacitance and operate at moderately high collector-base bias voltage to reduce signal phase modulation due to junction capacitance noise modulation* Use heavily bypassed DC bias circuitry and regulated DC supplies* Consider the use of a base-band noise reduction feedback loop* Extract the signal through the resonator to the load, thereby using the resonator transmission response selectivity to filter the carrier noise spectrum * From the NIST Tutorial on 1/f AM and PM Noise in Amplifiers

Slide 56

Discrete Transistor Oscillator Example: Low Noise, VHF Crystal Oscillator

Discrete Transistor Sustaining Stages Advantages Low Cost Pre-fabrication and post-fabrication design and design change flexibility Biasing flexibility Efficiency (DC power consumption) Disadvantages For low noise, transistors with high ft should be used; circuit is then susceptible to high frequency instability due to layout parasitics and loss-less resonator out-of-band impedance Difficulty in predicting or measuring 1/f AM and PM noise using 50 ohm test equipment since actual sustaining stage-to-resonator circuit interface impedances are not usually 50 ohms.

Modular Amplifier Sustaining Stages Advantages Easily characterized using 50 ohm test equipment, including measurement of amplifier s-parameters, 1/f AM , 1/f PM, and KTBF noise both for linear and at gain-compression operating points. Availability of unconditionally stable amplifiers eliminates possibility of parasitic oscillations. Amplifiers available (especially silicon bipolar and GaAs HBT types) exhibiting low 1/f AM and PM noise. Certain models maintain low noise performance when operated in gain compression thereby eliminating a requirement for separate ALC/AGC circuitry in the oscillator. Amplifier use allows a building block approach to be used for all of the oscillator functional sub-circuits: amplifier, resonator, resonator tuning, resonator mode selection filter, etc. Relatively low cost amplifiers (plastic, COTS, HBT darlington pair configuration) are now available with multi-decade bandwidths operating from HF to microwave frequencies.  Some types include means for setting bias current (therefore compression point).

Slide 60

Slide 61

Slide 62

Modular Amplifiers: General Comments In general, amplifier vendors do not design for, specify, or measure device 1/f AM and PM noise. It is usually necessary to evaluate candidate sustaining stage amplifiers in terms of measured 1/f AM and PM noise at intended drive level (i.e., in gain compression when the oscillator will not employ separate ALC/AGC). Amplifier S21 phase angle sensitivity to gain compression, as well as gain magnitude and phase sensitivity to DC supply variation (noise) must be considered. Silicon bipolar amplifiers and HBT amplifiers operating below L-band normally exhibit lower levels of 1/f AM and PM noise, compared to microwave amplifiers.

Slide 64

Slide 65

Oscillator Frequency Tuning Reactance tuning Carrier signal is maintained at center of the transmission response of the resonator-reactance combination Impedance transformation is often required between the resonator and the tuning circuit Phase Shift Tuning Carrier signal moves within the resonator transmission response pass-band; tuning range is restricted to less than the passband width Phase shift circuit can be implemented as a 50 ohm device For electronic (voltage) tuning, the placement of the phase shift tuning circuit in the oscillator effects the sideband response of the oscillator, and must be taken into account in phase-locked oscillator applications.

Slide 67

Methodology of Linear Frequency Tuning Using Abrupt Junction Varactor Diodes A resonator operated at/near series resonance exhibits a near-linear reactance vs frequency characteristic. Connection of a linear reactance vs voltage network in series with the resonator will then result in a circuit whose overall resonant frequency vs voltage characteristic is near          -linear. The same holds true for a parallel connection of a parallel resonant resonator and a linear susceptance vs voltage circuit. Impedance transformation between the resonator anf the tuning circuit is often required to increase tuning range using practical value components in the tuning circuit. Use of back-to-back varactor diodes in the tuning circuits has been found to eliminate effects of tuning circuit diode noise n oscillator signal spectral performance.  The effects of Johnson noise and 1/f noise in large value resistors in the tuning circuit can result in oscillator signal spectral degradation.

Obtaining Linear Reactance vs Voltage For near-linear reactance vs voltage using abrupt junction varactor diodes [Cv = K/(v+f)1/2], 1/(LpCvo) = wo2/3 where Cvo is the varactor diode capacitance at the band center voltage = Vo For zero reactance at the band center tuning voltage, Ls=Lp/2

Slide 70

Linear Tunable Low Noise Oscillators: Typical Results

Slide 72

Environmentally-Induced Oscillator Signal Frequency Change Resonator/Oscillator signal frequency change can be induced by changes in: Temperature Pressure Acceleration (vibration) Other (radiation, etc) Vibration constitutes the primary environmental stress affecting oscillator signal short    -term frequency stability (phase noise). Although resonator sensitivity to vibration is often the primary contributor, vibration      -induced changes in the non-resonator portion of the oscillator circuit can be significant. High Q mechanical resonances in the resonator and/or non-resonator oscillator circuitry and enclosure can cause severe signal spectral degradation under vibration.

A Perspective on Dimensional Change Resulting in Low Noise Oscillator Signal Spectral Degradation A signal can be generated using low noise, 100MHz crystal oscillator technology exhibiting a phase noise sideband level at 1KHz carrier offset frequency of -163dBc/Hz. The corresponding fractional frequency instability is Sy(f=100Hz) = 1X10-26/Hz. The corresponding phase instability is 1X10-16 rad2/Hz. The corresponding crystal vibration level that would degrade the at-rest oscillator signal spectrum, based on 5X10-10/g crystal vibration sensitivity is: 4X10-8 g2/Hz. The corresponding allowable signal path dimensional change, based on a wavelength of 300cm is: 4.8X10-7 cm or 48 angstroms/Hz1/2. In the oscillator circuit, where the nodal impedance level is 25ohms (RG=RL=50 ohms), a capacitance variation (due to vibration-induced printed board or enclosure cover movement) of: 6X10-7 pF/Hz1/2 would degrade the at-rest signal spectrum.

Slide 75

“Poor Mans” Method for Reducing Quartz Crystal Vibration Sensitivity

Measurement of Oscillator/Resonator Vibration Sensitivity Entire oscillator or resonator alone can be mounted on a shaker for determination of vibration sensitivity. Resonator vibration sensitivity measurements can be made with the resonator connected to the oscillator sustaining stage or connected in a passive phase bridge. For passive bridge measurements, the resonator can be connected as a one port device and connected to the bridge via a single (odd number of quarter-wavelengths) piece of coaxial cable. In either measurement method, the effects of coaxial cable vibration must be taken into account, especially for measurement of devices having very small values of vibration sensitivity. The effects of cable vibration can be determined by re-orienting the DUT on the shake table 180 degrees while not re-orienting the connecting coaxial cable and measuring the relative change in the magnitude and phase of the recovered, vibration-induced carrier signal sideband, relative to that of the shake table accelerometer.

Slide 78

"Test Results for 40MHz Oscillator..." Test Results for 40MHz Oscillator Sustaining Stage and Coaxial Cables (vibration-induced phase shift increases with carrier frequency)

Slide 80

CAD Small Signal Analysis/Simulation of Oscillator Circuits Small signal analysis is useful for simulating linear (start-up) conditions. Simulation of steady-state condition s possible if/when large signal (i.e., in-compression) device s-parameters or ALC diode steady-state impedance values are known. Two port analysis is most appropriate for oscillator circuits employing modular amplifier sustaining stages.  Open loop simulation in a 50 ohm system is valid for simulation of closed loop performance only when the loop is “broken” at a point where either the generator or load impedance is 50 ohms (i.e., at the amplifier input or output if the amplifier has good input or output VSWR). One port (negative resistance generator) analysis is useful when simulating discrete oscillators employing transistor sustaining stage circuitry. Circuit analysis/simulation should include component parasitic reactance (inductor distributed capacitance and loss, component lead inductance, etc).  For circuits operating at and above VHF, printed board/substrate artwork (printed tracks, etc) should also be included in the circuit model.

Slide 82

Circuit Simulation of the Negative Resistance (Sustaining Stage) Portion of a 100MHz Crystal Oscillator Cx and Cy values optimized to provide Zin = -70 + j0 at 100MHz Zin calculated from 50MHz to 1GHz to insure negative resistance is only generated over a small band centered at 100MHz (note use of  Rc) Large signal condition (where the negative resistance portion of Zin drops to 50 ohms = crystal resistance) simulated by reducing the ALC impedance value.

Results of 100MHz Oscillator Sustaining Stage Circuit Simulation 80MHz to 120MHz Note that the sustaining stage input impedance optimized to exactly -70 + j0 ohms Real part of input impedance is negative from approx. 91MHz 108MHz

Slide 85

Slide 86

80MHz Crystal Oscillator Using Modular Amplifier Sustaining Stage and Diode ALC Output signal near-carrier (1/f FM) noise primarily determined by crystal self noise TP1-to-TP2 voltage is maximized via trimmer capacitor adjustment.  The voltage level (at maximum) is a measure (verification) of requisite loop excess gain.

Slide 88

Slide 89

Slide 90

Slide 91

Slide 92

Slide 93

Slide 94

Slide 95

Oscillator Noise Reduction Within the Oscillator Circuit Use the resonator impedance or transmission response selectivity to reduce noise (i.e., extract the signal though the resonator to the load). Use multiple, parallel sustaining stage amplifiers (amplifier 1/1 PM noise de-correlation) Use multiple, series connected resonators (resonator 1/f FM noise de-correlation) Use multiple resonators in an isolated cascade or multi-pole filter configuration (increased loop group delay) Consider sustaining stage amplifier noise reduction via: (1) noise detection and base-band noise feedback (to phase and amplitude modulators) or (2) feed-forward noise cancellation External to the Oscillator Circuit External active (phase-locked VCO) or passive, narrow-band spectral cleanup filters Overall subsystem noise reduction via feedback or feed-forward noise reduction techniques

Use of Resonator Impedance/Transmission Response Selectivity

Multiple Amplifiers and or Resonators to Lower Noise

Wide Bandwidth Oscillator Noise Reduction Schemes

Slide 100

Slide 101

Slide 102

Slide 103

Slide 104

Slide 105

Oscillator Test/Troubleshooting: Discrete Transistor Sustaining Stage Measure one-port negative resistance vs frequency using ANA s11 measurements (may need to use a series build-out resistor to keep the sustaining stage from oscillating). For the closed loop (oscillating circuit), measure the circuit nodal voltage amplitude and relative phase and view the amplitude waveforms to estimate the degree of limiting (excess gain) using a vector voltmeter or similar test equipment. If the circuit does not oscillate, break open the oscillator loop where accurate duplication of source and load impedances is not critical (i.e., where ZS is much smaller than ZL and drive the circuit with an external generator to determine ‘faulty’ portion of the circuit from phase and amplitude measurements made along the signal path. As necessary, make circuit modifications to achieve desired circuit open loop phase and gain characteristics. In-circuit resonator effective Q can be determined by intentionally altering the circuit phase shift by a known amount and measuring the resultant oscillator signal frequency shift.

Discrete Transistor Oscillator Test (cont.)

Slide 108

Slide 109

Slide 110

Slide 111

Methods for Achieving (Specified) Low Phase Noise Oscillator Signal Spectral Performance Determine the oscillator/resonator technology best suited for the application - Operating frequency - Unloaded Q - Drive level - Short-term stability - Environmental stress sensitivity Determine the optimum sustaining stage design to be used - Discrete transistor - Modular amplifier -  Silicon bipolar, GaAs, HBT, etc - ALC, AGC, or amplifier                                                                                             gain compression Determine if use of noise reduction techniques, including multiple device use, noise feedback, feed-forward noise cancellation, vibration isolation, etc is needed

Slide 113

Further Improvement ‘Wish List’ Improvements in resonator performance - New resonator types having higher Q, higher drive capability, higher frequency,          smaller volume, better short-term stability, and lower vibration sensitivity Microwave (sustaining stage) transistors/amplifiers with lower levels of 1/f AM and PM noise - New semiconductor designs, materials, processing - Circuit noise reduction schemes (feedback, etc) Improved vibration sensitivity reduction schemes - Cancellation, feedback control, mechanical isolation, etc

Slide 115

Slide 116

Slide 117

Slide 118

Slide 119

Slide 120

Slide 121

Slide 122

Slide 123

Slide 124