Johnny, The issue is not trivial, as it depends on the DUT and the VNA architecture. There are two aspects, the number of points and the sweep time. If the DUT has a long delay, e.g., a very long cable, or a fiber optic you need to be aware that some VNAs may show wrong values (auto sweep time may not then), the reason is that the VNA sweeps by a frequency fast (so large span, fast sweep time makes the problem worse), then the delay of the DUT can cause that the VNA is receiving already on a different frequency point then the frequency that is present at the output of the DUT, as the frequency of the output of the DUT. This happens if you have a large span (like 1-10GHz) and a fast sweep time and a long delay DUT. You need to check if changing the sweep time changes the S21 waveform. There are many methods to avoid this, like stepping through frequencies, compensating the delay of the DUT in the reference channel, slow sweep times. Not every VNA has this property, but we aware. The other is that the VNA usually has no "binning" or "detector" process, spectrum analyzers have a binning process. In the following I will contrast how a spectrum analyzer vs. a VNA react to a certain set of settings: If you set a spectrum analyser as follows: Span 100 MHz RBW 10 kHz NumPoints 101 Then this looks as if you only measure one point every 1 MHz and this point is only 10kHz wide, so you may miss a signal in between points. In reality, the spectrum analyzer will measure many more points, so it cannot miss a signal between points. After measuring like 30000 points it has to condense these 30000 to 101 points, the points it will report to the user. This is done by a process called "detector", or "binning". Check http://www.books.rohde-schwarz.com/go/rohdeschwarz/_ws/resource/_ts_1327929157000/rO0ABXQAGnN0YXRfcHJpdjpwcm9kdWN0czpjcl9mb3Nh/data_info/spa_e_lp.pdf (from the very good by Rhode & Schwarz on spectrum analyzers) Or http://cp.literature.agilent.com/litweb/pdf/5952-0292.pdf (older HP reference) The user can select how this process is performed. If you have to condense 300 measurement points to 1 reported point then some methods are: Max(300) is reported called "maximum detector" Min(300) is reported, called "minimum detector" (rarely used) Other detectors are sample, auto etc. As far as I know, in a network analyzer the process often different If you set a VNA Span 100MHz RBW 10kHz NumPoints 101 And your DUT has some interesting feature BETWEEN measurement points, so like a narrow resonance, you may miss it. As the VNA will not measure between measurement points and then perform a detector process. I am sure there are people in the SI-LIST who know this issue better, I would be happy to learn more about it. David Pommerenke ****************************************************************************** Dr. David Pommerenke, Professor, EMC laboratory, 4000 Enterprise Dr. Missouri University of Science and Technology, Rolla, MO, USA , 65401 Cell: 573-308-2019 Tel: 573-341-4531 davidjp@xxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Yejun Fu Sent: Monday, January 06, 2014 9:17 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Number of points for your VNA Hi all: I want to know how to define your VNA's # of points, some person told me it is depends on the your DUT's length, some times if you have fibers in your DUT you have to change your points value. Could any one help to clear this question? Thanks Johnny ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu