# AD9361- Tx PLL High phase noise

Question asked by FreddyS on Jul 2, 2015
Latest reply on Jul 16, 2015 by tlili

Dear supporter

We are working with the AD9361 at 3.XGHz and recently we encountered a problem with Tx PLL phase noise.

When we transmit a frequency that has a fractional value of 0 we receive good SSB phase noise of about 5 - 6mRad (-46dBc measured at frequency offset of 1KHz-1MHz).

When we transmit a frequency that has a fractional value <> 0 we receive poor SSB phase noise of about 20mRad (-34dBc measured at frequency offset of 1KHz-1MHz).

Question: what can cause this poor SSB phase noise when the fractional value is <> 0 ?

Thanks

Freddy

We are using an external 40MHz clock connected to XTAN_N pin (Vp-p = 1.3V). (This is an OCXO 40MHz signal)

When we transmit a frequency that has a fractional value of 0 we receive good SSB phase noise of about 5 - 6mRad (-46dBc measured at frequency offset of 1KHz-1MHz).

When we transmit a frequency that has a fractional value <> 0 we receive poor SSB phase noise of about 20mRad (-34dBc measured at frequency offset of 1KHz-1MHz).

We did few test in order to investigate this problem: We transmit a CW signal using the internal BIST (Register 0x3F4 = 0xD3) at 3520MHz. We are using Analog devices Evaluation software ver 2.1.3 that control the AD9361BBCZ connected to our board.

1. In file 3520MHz_Integer = 88_Fractional = 0, you can see our CW. The noise level at frequency offset of 300KHz is -70.29dBc
2. In file 3520MHz_Integer = 88_Fractional = 1, you can see our CW. The noise level at frequency offset of 300KHz is -56.18dBc  (There is 14.1dB increase in noise level at frequency offset of 300KHz).
3. In file 3520MHz_Integer = 88_Fractional = 0_Phase_Noise, you can see that the integrated phase noise is 20*LOG(4.88mRad) = -46.23dBc
4. In file 3520MHz_Integer = 88_Fractional = 1_Phase_Noise, you can see that the integrated phase noise is 20*LOG(19.72mRad) = -34.1dBc  (There is 12.13dB increase in phase noise level).
5. In file 3520MHz_Integer = 88_Fractional = 0_vs_Fractional = 1, you can see that the blue graph is our CW tone when Fractional = 1. The yellow graph is our CW tone when Fractional = 0.
6. In file 3520MHz_Integer = 88_Fractional = 0_vs_Fractional = 1_Phase_Noise, you can see that the blue graph is our tone phase noise when Fractional = 1. The yellow graph is our tone phase noise when Fractional = 0.
7. In file 3520MHz_Integer = 88_Fractional = 0_vs_Fractional = 1_Zero_Span_Noise_300KHz, you can see a zero span image of our CW tone at frequency offset of 300KHz.
The blue graph is our CW tone when Fractional = 1. You can see two narrow pulses of 120uSec each. The distance between each pulse is 780uSec and the distance between each two pulses is 104.8mSec. (I don’t know what these pulses are)
The yellow graph is our CW tone when Fractional = 0,  You can see that the noise level is reduced.

1. Thanks.