7
Frequency Calibration
Equipment Required.
A Standard Frequency Source with accuracy ±0.05ppm or better at a frequency of 5MHz or
10MHz. Ideally use a 10MHz Rubidium source. The procedure below assumes 10MHz.
TF930 and TF960 Reference Oscillator Specification.
Temperature coefficient: ± 1 ppm over the full operating temperature range.
Ageing (first year): ± 1 ppm.
Initial accuracy: ± 0.2ppm (at 23ºC) when new.
Recommended recalibration accuracy: ±0.05ppm
Initial Set-up.
Place the unit in the controlled temperature calibration environment and switch on, running from
its batteries. The
Bat
annunciator should show. Allow at least 30 minutes to achieve temperature
equilibrium.
If
Lo Bat
is shown, connect the mains power unit and allow the unit to charge, which may take up
to four hours. Wait a further one hour after charging completes to allow any internal temperature
rise to dissipate.
Manual Reference Oscillator Frequency calibration.
Connect the standard frequency signal to the A input.
If the unit is on, press
Operate
to switch it off.
Hold down the
Input A/B
key and press
Operate
.
Release
Operate
when the display appears, then release
Input A/B;
the display should show
Cal ? Y n.
Press the left hand
Measurement Time
key to select Yes.
An annunciator
C
should be flashing in the display.
In calibrate mode an extra digit of resolution is shown (to speed the process), but note that the
last digit may be in error by more than one count.
Select 50
Ω
Impedance
(with the default AC
Coupling
) and centre the
Threshold
control at the
AC
marker. The display should show the frequency of the standard source, subject to any pre-
calibration inaccuracy.
If the reference signal is large it may be necessary to select the 5:1
attenuator.
The calibration setting is adjusted by pressing the
Frequency
and
Period
keys.
The
Frequency
key raises the oscillator frequency and makes the displayed value lower.
The
Period
key lowers the oscillator frequency and makes the displayed value higher.
The amount of adjustment for each press depends on the Measurement Time setting.
With 0.3s
Measurement Time
selected, note the difference between the reading and
10.00000MHz and press either the
Frequency
or
Period
key to get a closer result. Each step
moves the oscillator by about 0.2ppm (2Hz at 10MHz). Repeat as needed, aiming to be within
about 4Hz, then move to the next step.
Set the
Measurement Time
to 1s and repeat the process. The adjustment per step is now a
decade smaller and multiple presses may be needed. The measurement restarts after each set
of key presses. It will take a few seconds for the reading to stabilise because of the settling time
of a filter on the control voltage. Aim to be within about 0.5Hz and then move to the next step.
Set the
Measurement Time
to 10s and wait for the measurement to complete (
Measure
stops
flashing and an extra digit appears). Check that the reading is within ±0.05ppm (0.5Hz), making
further adjustment if needed (waiting for Measure to stop flashing after each adjustment). Each
key press moves the oscillator about 0.002ppm (2 counts in the last digit).
Although the calibration value can be set to this precision, this is more accurate than is
necessary, considering the medium term stability of the oscillator. The initial accuracy is specified
at ±0.2ppm on despatch from the factory, but after a unit has aged for a year or more, it is