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AD9854 difference between RFSK and CHIRP

Question asked by RobS on Jan 20, 2012
Latest reply on Oct 16, 2012 by Sudhanshu

Hello,

 

 

We at our company are using the AD9854 and have encountered some strange behavior between the CHIRP mode and the RFSK mode.

In the RFSK mode the delta frequency can be clearly seen when the signal is analyzed on the SA. However when changing the mode from RFSK to chirp we get a completely different response.

 

Are these measurements the expected behavior?

Are there some special things we need to take into account when using the CHIRP mode or in general on working with DDS?

Are we preforming the measurement correctly?

 

Kind Regards

Rob

 

100krfsk100nsPT.jpg

Mode: RFSK

Ref clock: 30MHz

Ref. multiplier:10

Delte frequency: 100 kHz

Start frequency : 10MHz

Stop frequency: 35MHz

Rampe rate (pulse time): 100ns

 

With these settings we can clearly distinguish the delta frequency

 

100kchirp100nsPT.jpg

Mode: Chirp

Ref clock: 30MHz

Ref. multiplier:10

Delte frequency: 100 kHz

Start frequency : 10MHz

Updateclock: 34µs <=> Stop frequency: 35MHz

Rampe rate (pulse time): 100ns

 

The measured deltafrequency  is now only 30 KHz while the only thing changed is the mode.

 

100kchirp30nsPT.jpg

Mode: Chirp

Ref clock: 30MHz

Ref. multiplier:10

Delte frequency: 100 kHz

Start frequency : 10MHz

Updateclock: 10.2µs <=> Stop frequency: 35MHz

Rampe rate (pulse time): 30ns

 

When changing the ramp rate to 30ns we get to see the deltafrequency we expected.

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