Precision Frequency Reference


Operating Frequency
2 - 10 MHz, any single frequency that is a multiple of 2kHz (other steps such as 1kHz and 2.5kHz possible with minor code changes).
Frequency Control Technique
Phase Locked Loop operating at 2kHz, based on counters in micro, locked by undersampling to reference at 50 Hz or any integer fraction of 1khz using proportional-only 16 bit digital feedback firmware algorithm with decimation filtering. Control is by 8-bit PWM, with PWM frequency improved by oversample 8-bit PWM dithering. The control algorithm rejects noisy samples (those with large errors) from feedback solution. Single pole 0.5Hz low pass filter in hardware. Lock time < 2 minutes from cold.
Frequency Accuracy
1 part in 107 over a temperature range 0C to 40C, when locked to a Rubidium (TV) or GPS source. This is equivalent to an accuracy of 0.5 Hz at 5MHz. Typical performance is closer to 2 parts in 108 This performance is achievable while driving a 3W amplifier into a dipole antenna.
Carrier Phase Noise
Better than -30dBc at 0.5Hz from carrier. Typically -50dBc. Some periodic variation due to PWM, but all within 0.5Hz of carrier, and all >50dB down. PWM and loop frequency (50Hz) components better than -50dBc. All noise components remain within 3 parts in 108 of the nominal carrier frequency.
Carrier Crystal Requirement
Fundamental mode high quality room-temperature crystal, 30pF parallel load, for exact output frequency required. HC6U case preferred. Oscillator is low power CMOS VCXO with a nominal frequency range of 1 part in 106. A commercial V-OCXO or V-TCXO can be used for better performance. Phase comparator version will operate from 1V RMS or TTL/CMOS input at 5MHz or 10MHz.
Reference Source
1V p-p CCIR composite video or composite sync into 75 Ohm, from a TV network using a Rubidium timing chain source. Received signal must be noise-free and stable, not retimed. Do not use sources without a Rubidium reference or better (in ZL use TV1 or TV2). Used without the sync separator, it can also be locked to any other local precision reference divided to say 100Hz. It will also lock reliably to GPS at 1pps or any integer fraction of 1kHz with no code changes.
10mW into 50 Ohm, semi-square wave isolated output (+13dBm). 5V p-p open circuit (10V p-p with ticks or 1kHz modulation on). AM modulation is 50% upward modulation, peak power 40mW. Carrier drop version is 40mW (+19dBm) when unkeyed or when keyed, with 6dB drop between keyed elements. Output is disabled when out of lock. Carrier drop version transmits CW dots at 1Hz when out of lock. With power amplifier, output is about 2.5W from 12V DC supply into 50 Ohm with 6 - 8dB carrier drop.

Tick sidebands and carrier phase noise are well suppressed
(Vertical axis dB, horizontal scale is Hz)
Spectral Purity
No measureable in-band components. Harmonics are not suppressed in the Exciter (use the suggested bandpass filter). With filter, or with suggested Power Amplifier, second harmonic is -50dBc, third is -40dBc. Second harmonic -20dBc, third harmonic -10dBc without filter. Tick modulation of carrier at 1Hz -30dBc. 1kHz tick modulation sidebands -12 dBc, third harmonic (3kHz) about -30dBc. -30dBc overall bandwidth less than 10kHz (1kHz ticks on), less than 0.5Hz (ticks off). Except during CWID, the "Clean" version is under 0.5Hz wide under all circumstances, and typically under 0.2Hz. During keying it has 4Hz sidebands.

Carrier (1000Hz) and 1 kHz modulation (2kHz) -12dBc
(Vertical axis dB, horizontal scale is Hz)

In the above picture the ~100Hz sidebands on the carrier at about -50dBc are PWM products.

Keying and ID
Carrier disabled when out of lock. Carrier optionally keyed with Morse ID every 10 minutes on zero minute, starting at second 1. Depending on option, seconds ticks may be disabled during keying. Carrier can be manually disabled or keying and lockout overridden. Optional 1000.000Hz seconds ticks square wave available separately or AM modulated. Tick duration 250ms (second 0), 5ms (seconds 55 - 58), 0ms (second 59), all others 50ms. 1000.000Hz modulation can also be on full-time, or disabled. Keying in "clean" version is -6dB between elements, with no break in carrier, no frequency or phase change. Keying is about 10WPM except QRSS version, which is 3 sec per dot element (QRSS3).

Minute tick (250ms) and second one tick (50ms)
Power Supply
External +12V DC nominal at 25mA, unregulated, any voltage from 7.2 to 15V suitable, negative earth. Internal backup battery supply 7.2 to 9V. External supply must be 1.2V higher than backup supply to take over load. Power Amplifier operates from +12 to +18V DC and can be at 50% AM high level (power supply) modulated when operating from 12V DC.
Time Setting
By PC application software, typically Windows Terminal, using a simple set of six instructions. UTC hours, minutes and seconds can be set. Final trimming of time to 10ms of UTC can take place while operating. The latest monitoring software also allows time setting.
Phase and Frequency Monitoring
Using software supplied, short-term phase, long term phase, variance and frequency can be monitored. Diurnal variations, thermal, long term ageing effects, and off-reference network operation can all be monitored. Software has phase resolution of 100ns (about 1 part in 107 at 10MHz), and frequency resolution exceeding 1 part in 1012. Frequency offset and variance are integrated over width of display, about 1 hour, 10 hours or more, depending on setting. Software also displays current time, phase control voltage (loop stress) and lock status.

Used with non-locking version, the software displays operating frequency and actual frequency offset to 1 part in 109, and can resolve to this accuracy in under 10 minutes.

Copyright Murray Greenman 1997-2005. All rights reserved. Contact the author before using any of this material.