65 Aleksandar Stamboliyski Str., 8000 Bourgas, Bulgaria
Tel: +359 56 820027, Fax: +359 56 836700
E-mail: [email protected] ,Web: www.devabroadcast.com
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Operation
A WORD ABOUT MEASUREMENT LIMITATIONS
Signal Strength
Signal strength is a major factor when a signal is measured and evaluated. FM as compared to
the AM is a much less susceptible to the inherent noise in radio reception.
On the other side speaking about FM broadcasting, it is the monaural reception which can
tolerate and profit from a much narrower IF bandwidth. Stereo reception adds all the noise present
in the 23 – 53 kHz sub-band. Moreover it is the amplitude component (AM) of this sub band
that is converted down to audible noise to be added to the program signal. That is why the noise
performance of FM-stereo can be as much as 20 dB worse than that of monaural broadcast.
As a general rule better reception and measurement can be obtained by using highly directional
outdoors antenna. Such antenna besides improving the signal strength, will contribute in eliminating
the multipath distortion.
A bargraph indicator used in DB4004 allows measuring the strength of the signal being received,
where the rule “the more the better” is in force. To reach this indicator, activate
LEVELS
menu.
Multipath Distortion
In FM reception, regardless of overall signal strength, a very important consideration is
that obstacles, (which an FM signal may encounter in the line-of-sight path) tend to reflect and
disperse the signal in many directions. In many locations, especially in urban areas where many
tall buildings interfere with the direct transmitted signal or in suburban areas surrounded by hills
or mountains, an FM receiver may pick up a station’s primary signal but, also, several secondary
reflections coming from various directions. These reflections arrive at the receiver out-of-phase,
slightly delayed in time with the primary signal and tend to blur or distort the principal signal. The
degree of distortion depends on the number and relative strength of the reflections.
The result, known as multipath distortion in FM reception can range from a low-level fuzziness
to a severely distorted sound quality, particularly at the high frequencies or treble. Multipath
distortion is especially troublesome in FM stereo reception.
DB4004 incorporates a multipath distortion detector and multipath bargraph indicator to show
the multipath level of received signal. The so mentioned multipath bargraph can be reached under
the
LEVELS
menu.
