Frequency References

Novus Frequency Reference Considerations

Posted on by aharvey@novuspower.com

There are several attributes regarding a 10 MHz reference that significantly impact system performance. (Novus also offers references up to 100 MHz.)

A Rubidium reference offers outstanding long-term stability (~<1 ppb/year) compared to an OCXO (~50 ppb/year). However, for demanding data capture or recovery systems, long-term stability is only one of several important attributes.

Clock Noise and Its Impact

Clock noise introduces clocking errors that degrade system performance. It can be characterized in several ways:

Long-Term Accuracy

This is the most straightforward aspect. Rubidium provides excellent stability and is ideal when GNSS locking is not possible, with drift of approximately 1 ppb/year. Adding a GNSS-locking system can improve long-term stability by roughly three orders of magnitude.

Stability – Allan Deviation

Allan Deviation measures stability over intervals (1, 10, 100, 1000 seconds, etc.). Even with strong long-term performance, short-term stability may suffer. Stability is affected by many design factors.

Below are Allan Deviation curves comparing a standard and a high-performance GNSS-locked system:

The high-performance unit outperforms by more than an order of magnitude.

The Basic Timing Source

Low-frequency noise begins with the master source. Oscillator design and power supply quality are crucial. A SC-cut crystal provides superior performance but is more expensive. Rubidium sources, while stable, are noisier.

Advanced systems combine a Rubidium source with a low-noise OCXO. The OCXO is oven-controlled for thermal consistency, combining Rubidium’s long-term stability with the OCXO’s low phase noise.

Below is a phase noise plot comparing Rubidium alone vs. Rubidium/OCXO pair.

The Receiver

GNSS receivers vary widely. Novus selects receivers optimized for both reference and PPS (Pulse Per Second) performance.

PPS performance depends on each radio’s algorithm. Two radios may output vastly different PPS patterns—one Gaussian, the other not. Spectral differences, especially low-frequency components, contribute to phase noise.

Novus algorithms process radio outputs to generate a more stable reference. Below is a jitter plot after processing:

Standard deviation improved from 17 ns to approximately 400 ps.

Timing Source Control

Control mechanisms add noise. Any adjustment introduces timing changes, affecting stability. Novus algorithms statistically evaluate both receiver and timing source characteristics.

The goal: extract useful timing signals while ignoring noise, then correct the master source accordingly.

The following plot demonstrates noisy input vs. a clean locked reference.

Close-in phase noise is especially difficult. In PLL loop designs, handling low-frequency noise often introduces limitations. A digital loop overcomes these, managing long time constants efficiently.

See the phase noise comparison between analog PLL and digital loop systems below.

Environmental and Mechanical Design

High-end references isolate the OCXO to shield against vibration and mechanical shock. Housing the control loop in a temperature-controlled environment minimizes temperature-related drift.

Amplifiers and Distribution

Most systems require multiple reference or PPS outputs. Novus ten-channel distribution amplifiers are robust, noise-minimizing, and fault-tolerant.

  • Latency: ~20 ns
  • Skew: typically <5 ns

Novus supports both 10 MHz square wave and LVDS formats in various form factors.

PPS Distribution

PPS voltage levels (CMOS 3.3V or 5V) and load impedance (often 50 Ohms) are critical. Standard CMOS drivers cannot drive 50 Ohm loads. Novus PPS amplifiers are built for this and are factory-configurable.

In a ten-channel PPS amplifier, skew between outputs can cause timing errors. Novus systems achieve channel skew under 200 ps.

Graph: PPS skew performance

Other Features

  • Remote Status & Control: RS232 or internet, with SNMP options. Includes output monitoring, GNSS lock, and local displays.
  • Dual-Time Base: Optional redundant GNSS receiver enables internal frequency counter for alerting when out-of-tolerance.
  • Statistical Analysis: Continuous Allan Deviation reporting.
  • Redundancy: Ten-channel redundancy switch and dual-input amplifiers with auto/manual failover.
  • AC/DC Power: Diode-OR configurations supported.
  • Mechanical Isolation: Prevents vibration-induced noise or lock loss.
  • Extended Temperature Range: -40°C to 70°C operation.
  • External Locking: Supports locking to external frequency, GNSS, or PPS.
  • Lock-In Amplifiers: Clean up degraded references by locking low-phase-noise OCXO to noisy input.
  • Portable References: Battery-powered units available.

Novus Power Products
Independence, Missouri
816.836.7446

Leave a Reply