User Manual

Timebase and Other Accuracy Considerations

The following is intended as a guide to determine the limits of measurement error.

Internal Oscillator

The instrument has an internal temperature compensated crystal oscillator (TCXO) which has

been factory set from a Rubidium reference standard such that it is within ± 0.2ppm (parts per

million) after warm-up in an ambient of 21ºC. At ambient temperatures other than 21ºC the

additional error is less than ± 1ppm over the whole operating range 5ºC to 40ºC.

The ageing rate is less than ± 1ppm in the first year and decreases exponentially with time. The

recommended calibration period is 1 year, see Maintenance section.

External Reference

If measurements are to be made which require still greater accuracy than can be obtained using

the TCXO, an external 10MHz frequency standard may be applied to the External Reference

input. The signal should be TTL, 3Vpp to 5Vpp CMOS or 1 to 2Vrms sinewave. The external

reference is used to phase lock the internal oscillator and must only be a high accuracy 10MHz

signal. It is not possible to make ratiometric measurements by applying a non-standard signal.

The presence of an external reference signal of adequate amplitude is automatically detected

and phase lock is attempted; the Ext Ref display annunciator is shown when the external

reference is detected. Note that if an improper signal is applied then the internal oscillator will be

pulled off frequency and measurement accuracy significantly impaired.

Noise

When measuring low amplitude, low frequency sinewaves noise will cause variations in the

displayed result at each display update. Users should make every effort to maximise the

amplitude of the signal presented to the input. The internal noise of the instrument is random,

with a significant low frequency (1/f) element. Selecting a longer gate time will reduce the effect

of this noise, and allow the user to see the extremes of the variation and establish an

approximate average. This method may be less effective on signals with externally induced

intermittent or non-random noise (such as supply frequency interference).

Signal level

In general it is obvious from the variations of the display value that a signal is too small for

reliable counting, but on the B input at frequencies over about 2GHz and on the C input at

frequencies over about 5GHz the effect of insufficient signal can be very subtle. A signal 2 or 3dB

below the true threshold might only show an error in the eighth digit in a consistent way that is

not obviously detectable; for true accuracy users are recommended to ensure that the signal

level meets the published specification even though the instrument is typically notably more

sensitive.