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Thread: RF ripple and flatness. Why and how?


Permlink Replies: 1 - Pages: 1 - Last Post: Feb 21, 2008 11:27 PM Last Post By: Dr_joel
ahgan82

Posts: 6
Registered: 02/18/08
RF ripple and flatness. Why and how?
Posted: Feb 18, 2008 2:46 AM
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Hi Guys,

Appreciate if you guys manage to clear my doubts. Anyway, just for your guys information, i am doing power sensor products. Below is my doubts.


1. What is RF ripple?
2. How this RF ripple will affect the measurement?
3. What cause the RF ripple occur?
4. How to eliminate RF ripple?

Below is my understanding of flatness, please correct me if i am wrong. Thank you.

1. What is Flatness?
IF amplitude flatness would act as a frequency discriminator and convert frequency changes into amplitude errors.

2. How this flatness will affect the measurement?
convert frequency changes into amplitude errors.

3. What cause the flatness happened?
due to SWR across any 10 MHz span, the contribution due to the AC coupling capacitor and the effect of the YTF (YIG tuned filter). and linearity???
Dr_joel


Posts: 2,688
Registered: 12/01/05
RF ripple and flatness. Why and how?
Posted: Feb 21, 2008 11:27 PM   in response to: ahgan82 in response to: ahgan82
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Mostly when we say RF ripple, we mean an unflatness of response vs. frequency.

This is very often caused by mismatch between measurement equipment and the DUT, with the ripple frequency inversely propotional to the cable length between instrument and DUT.

The effect on the measurement is to make the DUT appear to have more freuquency response than it does.

In most cases, it can be removed with post correction, if the source, DUT S11 and S22 and load match are known. Alternatively, using high quality attenuators can reduce the mismatch appear from the source or the load. It is best to add the attenuators onto the ends of the cables, as cables are a source of mismtach as well.

Ripple can also be caused by bad RF cables, bad flatness response in the source, bad flatness response in the receiver.

If you are using an SA to measure amplitude, the YTF can be non-repeatable, and can be a source of mismatch error.

For the most part, any measurments done on a DUT to measure gain or flatness would be better done with a network analyzer, provided the stimulus and response are appropriate (eg, won't work too well on an optical to RF transducer).

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