7
Socket B is used for frequency measurements in the range of 80MHz to 3000MHz. The input
impedance is nominally 50
Ω
. The signal is diode clipped with inputs over 350mV rms. Ideally, the
signal being measured should have a 50
Ω
source impedance to avoid standing waves which could
give spurious results. The input cable should be kept as short as possible and 50
Ω
coaxial cable
should be used.
Both inputs are protected against accidental connection of mains voltage up to 250V rms at
50/60Hz. Note that the instrument is not intended for the continuous measurement of the mains
supply because the occasional high voltage transients which occur on mains supplies could
damage the input. The maximum input voltage at frequencies above 1MHz is 1V rms.
Note that because of its high sensitivity the instrument may give a spurious reading when nothing is
connected to its input. This does not affect normal operation and should be ignored.
Note also that the signals mixed with other components which fall within the frequency and
sensitivity range of the input being used can cause incorrect counting; externally attenuating the
signal before presenting it to the counter may help to obtain a correct reading. Signals with slow
edge speeds (such as low frequency sinewaves) will show some jitter on the reading because of
random noise.
TIMEBASE & OTHER ACCURACY CONSIDERATIONS
The instrument has an internal oscillator of good stability but nevertheless the operating frequency
is affected by temperature and time (ageing). The oscillator has been factory set such that it is
within ±2ppm (parts per million) after warm-up in an ambient of 23°C. At ambient temperatures
other than 23°C the additional error should be less than ±0.3ppm/°C for the operating range 18°C
to 28°C. The crystals are pre-aged before assembly but a further ageing of up to ±5ppm can occur
in the first year. Since the ageing rate decreases exponentially with time it is an advantage to
recalibrate the instrument after the first 6 months use, see Calibration and Maintenance section.
Calibration and Maintenance
Recalibration may be carried out without dismantling the instrument by accessing the trimmer
through the hole marked CAL on the back of the case. Use a frequency standard or standard
frequency receiver to provide a high accuracy signal at the input and adjust the trimmer slowly for
the correct reading.
Cleaning
If the instruments requires cleaning use a cloth that is only lightly dampened with water or a mild
detergent.
WARNING! TO AVOID DAMAGE TO THE CASE OR DISPLAY WINDOW NEVER CLEAN WITH
SOLVENTS.
