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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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Wolfcat
Post subject: Bandwidth Measurements
Unread postPosted: Wed Jun 08, 2005 3:26 pm
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Lieutenant

Joined: Wed Jun 08, 2005 3:17 pm
Posts: 2
Location: Chesapeake, VA
I would like to know the following if anyone can help:

1. Why are 160 MHZ IF's measured at the 3dB point and 1 Ghz IF's measured at the 6dB points?

2. Is there such a thing as Effective Bandwidth? If so, how is it actually measured?

3. Using an HP83640 and an HP8757 what is the best /preferred way to measure Bandwidth? I.E., Search Bandwidth, Search Left, Search Right, or Delta it and do it by eyeball.

4. Are there any good books out there on performing RF, IF test measurements?

Any help would be greatly appreciated.


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Wolfcat
Post subject:
Unread postPosted: Thu Jun 16, 2005 9:16 am
Offline
Lieutenant

Joined: Wed Jun 08, 2005 3:17 pm
Posts: 2
Location: Chesapeake, VA
Anybody? Anybody? Bueller, Bueller?


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Guest
Post subject: Bandwidth Measurement
Unread postPosted: Thu Jun 16, 2005 1:08 pm

Unfortunately, the definition of bandwidth depends on its use.

For filters, you use the Equivalent Noise Power Bandwidth: The bandwidth of the perfect "brick-wall" filter which passes the same noise power from an Additive White Gaussian Noise source.

For communications systems planners, the amount of acceptable interference determines the bandwidth. The 6 dB, 20 dB, 40 dB, and 60 dB points have all been used historically.

For digital transmitter designs, you have several choices:
1. The minimum-possible bandwidth, below which errors cannot be reduced arbitrarily close to zero by controlling Eb/No and group delay
2. The first-null to first-null bandwidth (for ASK and PSK)
3. The bandwidth containing x% of the total transmitted energy

For analog FM, the Carson's Rule Bandwidth contains 98% of the transmitted energy.

For analog AM, twice the highest frequency present in the baseband.

You need to make a bandwidth measurement - I presume that's because it matters to someone. Unless that someone is a marketing droid with no technical knowledge looking for brochure fodder, that person would be appropriate to ask.

Good Luck!



Posted  11/12/2012
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