AD9914. Chirp. Logic of steps does not work

Hello,
We are working with AD9914 (DDS) Evaluation board to generate Frequency Chirp.
We encountered strange DDS behavior and request to explain it.

In short, Chirp duration does not linearly depend on GUI Sweep definitions.
The number of Chirp steps breaks simple steps' logic.

We prepared Chirp signal, with both "No-Dwell" checked options.
The Chirp period is prepared with 100 rising and 1 failing steps for ~100 us
duration. But we get the real Chirp period about 2 us, not 100!
(We used DROVER signal output for time measure.)

For reference purpose, we did the same Chirp with 10000 rising steps
and we get real 100 us duration.

So the simple steps' logic "works" for 10000 steps and "doesn't work" for
100 steps. More of that, we get incorrect results for other situations also
and do not include.

This is the strange behavior that we need to explain.

Details
We create 2 DDS setups (A and B) with these identical parameters.
Chirp frequency range: 100-200 MHz
Calculated Rising Chirp time: ~100 us
Calculated Failing Chirp time: ~10 ns (one step)
"No-Dwell" setting: Both are set
Internal PLL: 2500 MHz
External Clock: 125 MHz.

Different between setups:
A setup:
Number of rising steps 100
- 1 MHz Rising Step Size
- 1 us Rising Sweep Ramp Rate
B setup:
Number of rising steps 10000
- 0.01 MHz Rising Step Size
- 0.01 us Rising Sweep Ramp Rate

Result of Chirp period measured as time between 2 DROVER signals:
A setup: expected time 100 us, real time ~2 us !
B setup: expected time 100 us, real time ~100 us

The attached ZIP file includes 2 directories:
2500_100_200_100steps - for setup A
2500_100_200_10000steps - for setup B
Each directory includes
 GUI setup file
GUI images
Scopes images of the DROVER and Chirp signals and time measure
Also image of External Clock frequency measurement.

Note:
Also additional question appears, seeing on scopes images of the
Chirp signal. Its amplitude is not constant. Why?

ddsStrangeBehaviour.zip



Attached ZIP is moved to a better place
[edited by: Yakov_ at 2:51 PM (GMT 0) on 31 Jul 2019]