Philips Pm 5326 Bedienungsanleitung Seite 29

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range can be adjusted by means of control RF CENTER with reference to the frequency of the RF
generator. An example is given in fig. 4, at the top.
Fig. 3.3-13 Wobbulator setup
With actuated button SWEEP OFFION, the RF sweep generator is wobbled about the centre frequency
at a frequency deviation adjustable by means of control WIDTH. The display width i s set by using
control LF AMPLITUDE. For adjusting the wobbulating frequency with
A-
mode, use control LF
FREQUENCY. The same control (PHASE) i s used for phase shifting with N-mode, when button
Aim
is actuated.
3.3.6.2.
Wobbled signals
The wobbled frequency-modulated HF signal with large amplitude (e. g. fig. 4, at the bottom) 's supplied
to the low-ohmic input of the test object, ideally without reaction. Typical test objects are:
AM,
FM and TV-IF amplifiers and VHF channel selectors matching t o the frequency ranges and the mean
frequency deviation of the RF sweep generator. The signals are supplied, for instance, to the low-ohmic
base connections of the IF stage. So-called capacitive test probes have been proved good for valved
mixer and IF stages, which transfer the wobbled signals t o the anodes of the valves without reaction.
The signal is semi-rectangular due to the decoupling at the sweep oscillator; this does not mean any dis-
advantage, as the important information of an FM signal i s inherent in the zero-crossings of the signal.
The wave form i s not so important, as the amplitude i s mostly limited by the IF amplifier.
The portion of the harmonic waves is considerable, especially of the uneven harmonics. This is not incon-
venient, as the selective measuring or wobbled object itself extensively suppresses the harmonics.
Instead of the fundamental wave i t is possible to operate with the harmonics; but it must be taken into
account, that the frequency width increases with the ordinal number of the harmonic. In special cases
the harmonics can considerably be reduced by inserting a low-pass filter between output socket and mea-
suring cable, especially valuable for applications according to chapters 3.3.6.7 and 3.3.6.8.