Operating instructions

OPERATING MANUAL 2945A
46882-311D 3-31
External attenuators
Compensation for an external attenuator or test fixture included in the RF input path of the
monitor can be programmed. The value, in dBs, of the attenuator should be entered in the
RF Power Ext Atten: field of setup page 1. The value of the attenuator will then be included in
power calculations and the power indicated by the power meter will be that at the input to the
external attenuator.
This feature makes use of the extended ranges of the power meter.
The value of the attenuator will also be included in RF level calculations and readouts when in
the Rx TEST mode and in Dx TEST mode.
‘
’ is shown against any signal level which has been calculated to include the external attenuator.
*OSCILLOSCOPE*
With the oscilloscope facility selected, the POWER bar chart is removed from the display but the
digital read-out of the power level is retained.
*DUPLEX*
With the monitor in the Dx TEST mode the POWER bar chart and digital display remain in the
same location and the indications are identical to those given in the Tx TEST mode.
Transmitter frequency
The digits adjacent to the Tx Freq legend on the Tx TEST display refer to the frequency to which
the Service Monitor receiver circuits are tuned.
If the frequency of the transmitter is known, this figure can be entered manually. This is done by
firstly pressing the [Tx Freq] key in the Tx TEST mode menu and then entering the figure using
the data input keys. Complete the entry by use of the appropriate terminator key.
The receiver circuits of the monitor will then be tuned to that frequency and any signal within the
IF pass band will be examined. The frequency difference between the set frequency and the true
frequency will be displayed as ‘offset’.
Hint
When testing a transmitter using sequential tones, it may be necessary to pre-tune the Service
Monitor receiver circuits in this way in order to successfully decode the initial tones.
If the transmitter frequency is unknown, or it is to be measured, key up the transmitter and use
the key sequence [Tx Freq], [Autotune].
The receiver circuits of the monitor will then carry out a sweep to locate the signal and tune to it.
The frequency of the signal is displayed against the
Tx FREQ legend.
*DUPLEX*
The same frequency measurement procedures are applicable when in the Dx TEST mode.
RF distortion, and
Spurious outputs
Distortion of the transmission RF output may be deliberate (modulation is a form of deliberate
distortion), it may be due to a fault or because of design limitations. Whatever the reason the
distortion produces harmonics or other spurious outputs.
These can be located using the SPEC ANA mode of operation.
By viewing the output signals from the transmitter on a voltage vs frequency plot, any
transmissions outside the transmitter's specified bandwidth can be identified.
The use of the SPEC ANA mode is explained fully under that heading.