Stanford Research Systems SR620, Руководство пользователя

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Specification Guide 25
SR620 Universal Time Interval Counter
accuracy of the SR620 and determines the calibration interval necessary to maintain a desired error limit.
The long-term stability consists of two components: oscillator aging and oscillator temperature response. The
aging of an oscillator is the change in frequency over time due to physical changes in the components
(usually the crystal) and is usually specified as a fractional frequency change over some measurement period.
Temperature response is due to changes in the oscillator characteristics as a function of ambient temperature
and is specified as a fractional frequency change over some temperature range. The timebase for the SR620
is specified as:
standard oscillator oven oscillator
aging 1x10
-6
/yr 5x10
-10
/day
temperature response 1x10
-6
0 to 50°C 5x10
-9
0 to 50°C
So, for example, with the oven oscillator 30 days after calibration the oscillator may have drifted at most 30 x
5x10
-10
x 10MHz = 0.15Hz. Also, a worst case temperature variation must be assumed when evaluating the
worst case error. That is, for example, the optional oscillator must be assumed to be at worst 5ppb in error
because the conditions when the SR620 was calibrated are unknown.
EXTERNAL TIMEBASES
The SR620 has a rear panel input that will accept either a 5 or 10Mhz external timebase. The SR620 phase-
locks its internal timebase to this reference. The phase-locked loop has a bandwidth of about 20Hz and thus
the characteristics the the SR620's clock, for measurement times longer than 50ms, become that of the
external source. For shorter measurement times the clock characteristics are unimportant compared to the
internal jitter (25ps rms) of the SR620. Thus, if the signal from a Cesium clock is input into a SR620 with a
standard TCXO oscillator the short-term and long-term stability of the SR620 will become that of the Cesium
clock.
TRIGGER INPUT SPECIFICATIONS
There are two ways that the inputs can affect the resolution and accuracy of a measurement. The first is
called trigger jitter and is due to random noise on the A and B input signals and the trigger input buffers. This
random noise causes the input to trigger at a time different than it otherwise would in the absence of noise.
Because this is a random process this affects the resolution just as the other random noise sources do.
Trigger timing jitter can be minimized by careful grounding and shielding of the input and by maximizing the
input slew rate. Note, however, that the slew rate is limited by the SR620's 1ns input rise time. The trigger
timing jitter can be described by the equation:
Trigger Timing Jitter
=
E
internal
( )
2
+
E
signal
( )
2
Input Slew Rate
where
E
internal
=
internal input noise 350
µ
V rms typical
( )
E
input
=
input signal noise
If the trigger level is set to a value other than the intended value the time interval measured will be in error.
This error, trigger level timing error, is a systematic error that affects only the error of the measurement and
not its resolution. The SR620's trigger thresholds are set to an accuracy of 15mV + 0.5% of value. The effect
this has on the measurement is given by:
Trigger Level Timing Error =
15 mV + 0.5% of setting
Input Slew Rate