Application Overview
Metrology laboratories require the highest accuracy frequency and time references for calibration services, maintaining national standards traceability, and conducting precision measurements. This reference design provides a complete primary standard system based on cesium atomic clock technology, meeting the requirements of ISO/IEC 17025 accredited calibration laboratories and national metrology institutes.
The system architecture addresses the full chain of traceability from primary cesium standards through working standards to the calibration of customer instruments. Key applications include calibration of rubidium oscillators, GPSDO modules, OCXOs, frequency counters, and time interval counters. The system also supports phase noise measurement services for high-performance oscillators.
Modern metrology demands both frequency accuracy (for calibration certificates) and phase noise characterization (for advanced oscillator development). This reference design provides comprehensive measurement capability covering both measurement domains with metrology-grade uncertainty specifications.
System Architecture
+---------------------------+
| BD1024-P |
| Cesium Atomic Clock |---------+
| (Primary Standard) | |
+---------------------------+ |
| |
10MHz | 5MHz |
| | | |
+----------v------+-----+----------+ |
| | |
v v |
+------------------------+ +------------------------+ |
| STT-PN | | STT-PC | |
| Phase Noise | | Multi-Channel |<--+
| Tester | | Phase Comparator | |
+------------------------+ +------------------------+ |
| | |
| | |
v v |
+------------------------+ +------------------------+ |
| Test Signal 1 | | UUT 1 UUT 2 UUT 3 | |
| (Reference) | | +---+ +---+ +---+ | |
+------------------------+ | | | | | | | | |
| | | | | | | | | |
+---------------------->| |<->| |<->| |<-+ |
| | | | | | | |
+---+---+---+---+---+-+ |
| |
+------------------------------------------------------+--+ |
| Calibration Management System | |
| (Data Collection & Certificate Generation) |<-+
+----------------------------------------------------------+
+--------------------------------------------------------------+
| Reference Standard Enclosure |
| (Temperature controlled ±0.1°C, Humidity ±5% RH) |
+--------------------------------------------------------------+Measurement System Architecture
The BD1024-P cesium atomic clock serves as the primary frequency standard, providing frequency accuracy of ≤±5×10⁻¹³ and long-term stability of ≤2.7×10⁻¹⁴ at 100,000 seconds. This performance level meets the requirements for primary calibration laboratory standards.
The STT-PN phase noise tester characterizes the spectral purity of oscillators under test, while the STT-PC multi-channel phase comparator measures frequency stability using Allan deviation methodology. Both instruments are traceable to national standards through the primary cesium reference.
Key Design Decisions
1. Primary Cesium Standard Selection
The BD1024-P provides frequency accuracy of ≤±5×10⁻¹³, exceeding the requirements for secondary laboratory standards and meeting CIPM recommendations for primary standards. The long-term stability of ≤2.7×10⁻¹⁴/100,000s ensures consistent calibration results.
Decision Rationale: Calibration uncertainty must be an order of magnitude better than the device under test. The BD1024-P provides the foundation for calibration services ranging from rubidium oscillators through high-performance OCXOs.
2. Multi-Channel Phase Comparator
The STT-PC supports 4/9/18 channel configurations, enabling simultaneous comparison of multiple oscillators. The stability floor of ≤2×10⁻¹⁴/s (typical ≤1.4×10⁻¹⁴/s) exceeds the performance of single-channel comparators.
Decision Rationale: Efficiency requirements for calibration laboratories demand parallel measurement capability. The STT-PC eliminates the need for sequential measurements, reducing calibration time by 4-18x.
3. Phase Noise Measurement Capability
The STT-PN provides phase noise floor of ≤-135dBc/Hz@1Hz and Allan deviation floor of ≤5×10⁻¹⁵/s. This enables characterization of high-performance oscillators used in radar, communication, and scientific applications.
Decision Rationale: Modern oscillators require both amplitude noise (stability) and phase noise characterization. The STT-PN provides comprehensive phase noise measurement without external reference sources.
4. No-Calibration-Required Operation
Both STT-PN and STT-PC are designed for no-calibration-required operation, with self-calibrating measurement techniques. This reduces operational overhead and maintains measurement integrity.
Decision Rationale: Calibration laboratories must demonstrate measurement traceability. Self-calibrating instruments simplify the uncertainty budget and reduce calibration frequency requirements.
Bill of Materials (BOM)
| Item | BRIDZA Model | Function | Qty | Notes |
|---|---|---|---|---|
| ------ | ------------- | ---------- | ----- | ------- |
| Cesium Atomic Clock (Professional) | BD1024-P | Primary frequency standard | 1 | ≤±5×10⁻¹³ accuracy |
| Phase Noise Tester | STT-PN | Phase noise & stability measurement | 1 | ≤-135dBc/Hz@1Hz floor |
| Multi-Channel Phase Comparator | STT-PC | Frequency stability comparison | 1 | 4/9/18 channel options |
| Frequency Distributor | STZ-PF | Multi-output distribution | 1 | ≤3fs jitter, reference distribution |
| Frequency Divider | STD-FP | Signal adaptation | 1 | 100MHz to 10MHz conversion |
| Phase Micro-Stepper | STZ-MS | Phase adjustment | 1 | Optional, for synthesis |
| Temperature Chamber | - | Environmental testing (not supplied) | 1 | ±0.1°C control |
| Vibration Isolation Table | - | Mechanical isolation (not supplied) | 1 | Active or passive |
| UPS System | - | Power protection (not supplied) | 1 | Online double-conversion |
Performance Targets
| Parameter | Requirement | Achieved | Notes |
|---|---|---|---|
| ----------- | ------------ | ---------- | ------- |
| Frequency Accuracy (BD1024-P) | ≤±5×10⁻¹³ | ±5×10⁻¹³ | Primary standard |
| Frequency Stability (1s) | ≤5×10⁻¹³ | ≤5×10⁻¹³ | Allan deviation |
| Frequency Stability (100ks) | ≤3×10⁻¹⁴ | ≤2.7×10⁻¹⁴ | Allan deviation |
| Phase Noise Floor | ≤-130dBc/Hz | ≤-135dBc/Hz | STT-PN @ 1Hz offset |
| Stability Floor | ≤1×10⁻¹⁴ | ≤5×10⁻¹⁵ | STT-PN @ 1s |
| Comparator Channels | 4 minimum | 4/9/18 | STT-PC options |
| Comparator Stability | ≤2×10⁻¹⁴ | ≤1.4×10⁻¹⁴ typical | STT-PC @ 1s |
| Reference Distribution Jitter | <10fs | <3fs | STZ-PF channels |
Performance Specifications
| Measurement Capability | Uncertainty | Coverage Factor (k=2) |
|---|---|---|
| ------------------------ | ------------- | ------------------------ |
| 10MHz Calibration | U = 5×10⁻¹³ | k=2, 95% confidence |
| 1PPS Time Interval | U = 10ns | k=2, 95% confidence |
| Phase Noise @ 1kHz | U = 2dB | k=2, 95% confidence |
| Allan Deviation (τ=1s) | U = 10% | k=2, 95% confidence |
Implementation Notes
Laboratory Environment
The reference standard system requires controlled environmental conditions:
- Temperature: 20°C ± 1°C (preferably ±0.1°C for critical measurements)
- Relative humidity: 40-60% RH
- Vibration: <100μm/s (0.1g) above 5Hz
- EMI: Compliant with CISPR 11 Class A
The BD1024-P cesium clock should be located in a dedicated enclosure or shielded room to minimize environmental influences.
Power System
Install high-quality power conditioning:
- Online UPS with sine wave output
- Line voltage regulation ±5%
- Harmonic distortion <5% THD
- Grounding per IEC 60364
The BD1024-P operates from 220VAC or 48VDC. UPS backup should provide minimum 2 hours operation for orderly shutdown if main power fails.
Measurement Procedures
Establish documented procedures per ISO/IEC 17025:
- Warm-up period: Minimum 24 hours for BD1024-P to reach specified accuracy
- Measurement sequence: Reference characterization before UUT measurement
- Data collection: Minimum 3 runs per UUT, 10,000 samples per run
- Uncertainty budget: Include all contributors per JCGM 100:2008 (GUM)
Traceability Chain
Maintain documented traceability to national standards:
- Primary: BD1024-P frequency accuracy (direct realization of the second)
- Secondary: Calibration against national metrology institute (annual)
- Working: STT-PC/STT-PN calibration (biennial)
- Transfer: Calibration certificates for UUT (as required)
Test & Verification Approach
System Performance Verification
- Self-Comparison Test: Compare BD1024-P against itself using two STT-PC channels to verify noise floor
- Stability Analysis: Calculate Allan deviation over τ = 1s to τ = 100,000s
- Phase Noise Verification: Measure noise floor with terminated inputs
Calibration Validation
- Reference Transfer Test: Calibrate transfer oscillator, compare results to another laboratory
- Blind Test: Measure artifacts with known values to verify accuracy
- Proficiency Testing: Participate in interlaboratory comparisons
Uncertainty Budget
Document all uncertainty contributors:
- Reference standard uncertainty (u₁)
- Measurement repeatability (u₂)
- Environmental influences (u₃)
- Resolution/bucket size (u₄)
- Drift during measurement (u₅)
Alternative Configurations
High-Volume Calibration Laboratory
For laboratories processing >100 oscillators per week, add additional STT-PC units to parallelize measurements. The STT-PC network management supports up to 255 devices for centralized control.
Additional Components: STT-PC (additional units), calibration management software
Phase Noise Specialized Laboratory
For facilities specializing in ultra-low phase noise measurement, add specialized phase noise measurement capability with cross-correlation techniques for extending dynamic range.
Additional Components: Second STT-PN for cross-correlation, low-noise amplifiers
Mobile Calibration Service
For on-site calibration services, configure portable system with:
- BD1024-C (transportable cesium)
- Laptop-based data collection
- Portable environmental chamber
Component Changes: Replace BD1024-P with BD1024-C, add portable accessories
GPS-Disciplined Option (Secondary Laboratory)
For secondary calibration laboratories with relaxed traceability requirements, the STM-Rb-H rubidium clock provides hydrogen maser-level performance suitable for routine calibration work.
Component Changes: Replace BD1024-P with STM-Rb-H, adjust uncertainty budget
Related Resources
- BD1024-P Cesium Atomic Clock (Professional)
- STT-PN Phase Noise Tester
- STT-PC Multi-Channel Phase Comparator
- STZ-MS Phase Micro-Stepper
- ISO/IEC 17025:2017 General Requirements for the Competence of Testing and Calibration Laboratories
- JCGM 100:2008 (GUM) Evaluation of Measurement Data - Guide to the Expression of Uncertainty in Measurement