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Precision clock generator has 16-digit resolution

Posted: 17 Jan 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Alex Mendelsohn? Stanford Research Systems? SRS? clock generator? CG635?

Test equipment maker Stanford Research Systems Inc. (SRS) has an extremely stable synthesized squarewave clock generator. The Model CG635 can be driven across an RS-232 serial line from a PC, or used in an automated suite using IEEE-488/GPIB (General Purpose Interface Bus) control. All functions can be controlled through either.

The need for clean clocking
Why is a clean clock source required? It's important if you're working with high-speed data converters, for example. In those systems, spurious clock modulation and jitter can create artifacts and noise in acquired signals and in reconstructed waveforms when using A/D (analog-to-digital) converters or DACs (digital-to-analog) converters.

Clean clocks are also important in communications systems and networks. Jitter, wander, or frequency offsets can lead to high BERs (bit error rates), or to loss of synchronization. The CG635's random jitter is less than 1ps.

Sub-audio to microwaves
With 16-digit resolution and fast settling time (less than 30ms), it delivers signals across a very wide range extending from sub-audio 15Hz into the microwave regime at 2.05GHz. Perhaps best of all, it's reasonably priced at about $2,500.

SRS's Model CG635 delivers low jitter signals with fast transition times of 80ps. It can be optionally outfitted with a variety of timebases, too (more on that below).

In use, the CG635 provides several clock outputs. Front-panel outputs provide complementary squarewaves for RS-485 levels as well as ECL and PECL logic, and (optionally) LVDS (low voltage differential signaling). Squarewave amplitude into 50-ohms can also be set from 200mV to 1V, with offsets between -2V and 5V. you can even drive devices such as RF mixers, using an output of 7dBm.

The timing of clock edges can also be modulated over a 15ns range. That's done using a rear-panel time-modulation input that exhibits a bandwidth extending from DC to over 10kHz. This input has a sensitivity of 1ns/V.

Letting an external analog signal control the phase of the instrument's clock output can be useful for characterizing a system's susceptibility to clock modulation and jitter.

A rear-panel RJ-45 is also provided to place differential squarewaves on twisted pairs for RS-485 use. It delivers signals up to 105-MHz, with LVDS levels up to 2.05GHz. This output also provides 5Vdc for optional line receivers. The clock outputs can drive shielded CAT-6 cables. The optional line receivers provide complementary logic outputs on SMA connectors.

Timebase options
As shipped, the CG635 includes a 10MHz crystal-controlled timebase with a stability of better than 5ppm (at ambient temperature). A 10MHz input also permits you to phase-lock to an external 10MHz reference of your choice. What's more, the 10MHz output can be used to lock two CG635s together.

You can also opt for other precision timebases. A $650 OCXO (oven-controlled crystal oscillator) option provides 100X better frequency stability than the standard crystal oscillator.

If you need even more stability, a $1,650 rubidium source can boost it to 10,000x better than what the standard oscillator can do. Either optional timebase reduces low-frequency phase noise of the instrument's synthesized output.

Phase and time modulation
The CG635's clock phase is also adjustable. Phase resolution is one degree for frequencies above 200MHz, and increases by a factor of ten for each decade below 200MHz (with a maximum resolution of one nano-degree). This precision lets clock edges be positioned with a resolution of better than 14ps at any frequency between 0.2Hz and 2.05GHz.

Other features include non-volatile memory. Ten sets of instrument configurations can be stored and recalled using this feature.

- Alex Mendelsohn

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