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Part Number: LMX2571
On Evaluation Module "LMX2571EVM" (www.ti.com/.../lmx2571evm) I see spur with frequency of Fout/2 (subharmonic) and level - 52dBc.Also I see harmonics of frequency of operation of the phase detector with level - 75dBc.From where do they undertake?
The output spectrum at generation frequency 1008 MHz (OSCin=20MHz, Fpd=120MHz, N=21.0, CDIV=5):
TICS_Rg 2018-11-07 1008 120pd 4sd 2del 325uA 800 - F12.rar
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In reply to Noel Fung:
It doesn't help:- Disable Dithering- Set MASH order to Integer- use smaller charge pump current- use bigger charge pump current- MULT_F1=3 (was 6) and N=42 (was 21)
I dissabled OSC (I connected Y1:1 to GND) - remained only the output frequency, which became a little less, and her subharmonica 1/2:
I see a subharmonica 1/2 as well on my PCB
In reply to Tolchinsky Alexander:
In reply to Dean Banerjee:
I connected the R&S SMB100 generator to input of spectrum analyzer: the subharmonica 1/2*Fout – is absent.I set OUTBUF_RX_EN_F1 = to dissable - out Rx is clear.
With my settings I see a subharmonica 1/2*Fout only for CDIV1*CHDIV2 = 5*1 (-52dBc) and 7*1 (-58dBc). Change of MULT_F1=3 (Fpd=60VHz), N=21*2=42=0x2A doesn't change a situation.
I set PLL_N_PRE_F1=4 and N=21=0x15 – the situation has changed. CDIV1*CHDIV2:5*1 -> Fout*3/4@-60dBc6*1 -> Fout/2@-66.5dBc7*1 -> Fout/4@-67dBc
Subharmonicas go from LMX2571. What to do?
For me is more suitable LMX2572LP (e2e.ti.com/.../713608 and I will try this chip before the following redesign of my project. At this chip CHDIV changed - now it is only 2^n. It is possible to hope that LMX2572LP does not have this problem of subharmonics?
Can you give settings for LMX2571 for the frequency of 1008 MHz with the reduced level of subharmonic 1/2?
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