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# Number of points, if bandwidth, sweep time ? - RF Cafe Forums

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 Post subject: Number of points, if bandwidth, sweep time ? Posted: Tue Jun 20, 2006 5:14 am
 Captain

Joined: Mon Jun 19, 2006 10:27 am
Posts: 13
Hello,

can anyone please explain to me, the relationship between
- number of points
- if bandwidth
- sweep time ?

For example, how do I know which bandwidth I need if the number of points and sweep time are given ?

 Post subject: Posted: Tue Jun 20, 2006 9:57 am

Joined: Sun Aug 03, 2003 2:02 pm
Posts: 476
Location: Erie, PA
Greetings mbergmann:

The relationship varies with each instrument. The usuer's guide usually provides a formula and/or table, such as Table 15-3 (page 149) in this Agilent manual for the ESA and PSA series spectrum analyzers. There are relational formulas throughout the programming examples.

http://cp.literature.agilent.com/litweb ... -90482.pdf

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 Post subject: Posted: Wed Jun 21, 2006 7:55 am
 Captain

Joined: Mon Jun 19, 2006 10:27 am
Posts: 13
Thanks Kirt for your answer, but does it really depend on the instrument ?

An explanation is given on

According to this, the number of points is determined by the maximum length of the impulse response and the bandwidth of measurement:
number of points = length of impulse response * bandwidth of measurement
First of all I don't understand this simple formula, secondly how do I know the impulse response of my DUT ? Generally, that's unknown I would say.

The necessary IF bandwidth is determined by the desired dynamic range (SNR).

Finally, the sweep rate is calculated by the bandwidth, number of points and group delay through the IF BW-filter (approximately the reciprocal value of the chosen IF BW). The sweep rate, together with the length of the impulse response results in the change in frequency during the impulse response, which must be lower than the IF bandwidth ("the sweep time is always okay if you are properly sampling the frequency domain!").

Posted  11/12/2012
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