User Manual

13
This setting should count most signals, but on very small signals some slight adjustment may be
needed for maximum sensitivity. The usable adjustment range from this position is approximately
±50mV (1:1 attenuation) or ±200mV (5:1 attenuation).
If DC coupling is in use then the feedback mechanism is disconnected and the threshold is
directly adjusted by the control over the range of nominally 0 to 2V (1:1 attenuation) or 0 to10V
(5:1 attenuation).
There is some over-range at each end of the control. The Threshold control should be adjusted in
the direction which brings both yellow LED lamps on and then finely adjusted to get the most
stable measurement.
For waveforms with slow edges adjusting the threshold will, of course, affect the Width and
associated Ratio and Duty cycle measurements but not Frequency, Period and Count.
The Threshold control should always be adjusted slowly, as there is a noise rejection filter with a
long time constant in the circuit.
Input B
Input B is used for frequency measurements in the range 80MHz to >3GHz. The input
impedance is nominally 50Ω. The maximum input voltage from 20MHz to 3GHz is 1Vrms and the
input is diode clipped with inputs over 250mVrms.
The input is protected against temporary accidental connection of mains voltages up to 250Vrms
at 50/60Hz.
The signal being measured should have a 50Ω source impedance to avoid standing waves which
could give spurious results. The input cable should be kept as short as possible and 50Ω coaxial
cable should be used.
Note that, because of the wide bandwidth of this input, signals mixed with other components
which fall within the frequency and sensitivity range of the input can cause incorrect counting;
externally attenuating or filtering the signal before presenting it to the counter may help to obtain
a correct reading. In particular, when attempting to count the highest frequency component of a
signal with broadband noise or other interference, an external high pass filter may be needed,
especially with small signals above 2GHz. The C input provides better performance above this
frequency.
Input C
Input C is used for frequency measurements in the range 2GHz to >6GHz. Although the
sensitivity is not specified outside this range, it will typically count frequencies from 1.8GHz up to
7.5GHz. The input impedance is nominally 50Ω. An input coupling capacitor is followed by a
resistive attenuator and a PIN diode limiter. The maximum input voltage for correct counting is
1.5Vrms (+16dBm) and the maximum input without damage is 4Vrms (+25dBm). There is a
250kΩ DC coupled bleed resistor to minimise the risk of static build up destroying the input
coupling capacitor. This input is also protected against temporary accidental connection of mains
voltages up to 250Vrms at 50/60Hz.
The signal being measured should have a 50Ω source impedance to avoid standing waves which
could give spurious results. The input cable should be kept as short as possible and 50Ω coaxial
cable should be used.
This input has a sharp low frequency cut off below 1.5GHz and has much better noise immunity
than the B input. It also has much better large signal handling capabilities. Unless the Ratio B:A
capability is needed the C input should be preferred for all signals above 2GHz.
Function and Measurement Time Selection
Function and measurement time are selected using the keys immediately below the display. The
annunciators in the display show the current settings.
Function Selection – A input
Pressing the FREQUENCY, PERIOD or WIDTH key will immediately set the instrument to that
function; pressing and holding the key down for more than 1 second will change the function to