AFE7071: output of AFE7071 has unexpected spurious at 0.5LO and 1.5LO frequency

user6150974

Part Number: AFE7071
Other Parts Discussed in Thread: LMX2581, AFE7070

hi，i have some problem about AFE7071, i find the output of AFE7071 have some unexpected spurious at the 0.5*LO and 1.5*LO frequency.

when i change the LO frequency, the frequency of spurious will change also. i use LMX2581 as the LO source, and check the output power

of LMX2581 between -5dbm to +5dbm. and different LO power didn't affect the spurious much.

attached is the test photo of spectrum analyzer. the output frequency is 400MHz and 600MHz.

anyone can help me to resolve this problem?

over 5 years ago

0 Yusuf%20Agoro over 5 years ago

TI__Expert 4880 points

User,

Are you sending a pattern to the AFE7071? If so please specify the frequency & the number of tones.

Thanks

Yusuf

0 user6150974 over 5 years ago in reply to Yusuf%20Agoro

Prodigy 110 points

i send a QAM modulation baseband signal to AFE7071, sample rate is 64Mhz, baseband signal bandwidth is 8Mhz. DVB-C standard.

i make sure the baseband signal is no problem , it's a clear spectrum.

the photo is the RF output spectrum. i want to know why i find unwanted spectrum at 0.5x and 1.5x of the LO/RF frequency?

how to avoid this ?

0 Yusuf%20Agoro over 5 years ago in reply to user6150974

TI__Expert 4880 points

User,

can you send me the pattern file you are using so i am try and replicate results on my end?

Yusuf

0 user6150974 over 5 years ago in reply to Yusuf%20Agoro

Prodigy 110 points

dac_in_data.rar

attached is the pattern file. it 's txt file with 2 columns, the first is 14bits signed I data, the second column is Q data.

0 user6150974 over 5 years ago in reply to Yusuf%20Agoro

Prodigy 110 points

hi Yusuf , when can i get your help? i want to resolve this problem in urgent.

0 user6150974 over 5 years ago in reply to Yusuf%20Agoro

Prodigy 110 points

hi Yusuf , when can i get your help? today is Sep 24 now, and i want to resolve this problem in urgent.

0 Yusuf%20Agoro over 5 years ago in reply to user6150974

TI__Expert 4880 points

User,

Can you send me a shematic of your board? I am not able to see this issue on my setup. I suspect that there is noise coupling onto the LO sourced by the LMX. To test this, disconnect the LMX and use another signal source for LO on the AFE. Is this possible to do?

Yusuf

0 user6150974 over 5 years ago in reply to Yusuf%20Agoro

Prodigy 110 points

i don't think the hardware design has some problem. yesterday the local TI FAE bring the AFE707XEVM Evaluation Module to my office, and

we did some test together. the result is same.

the LO frequency is 400Mhz, we use a signal generator as LO source, the LO is clear sine wave. when we output to the AFE7070 chip a fixed

DC signal. the wanted signal is 400Mhz single tone, and the unwanted 200Mhz signal (0.5xLO frequency) is -57dbc.

but when we changed the data source to AFE7070 to a QAM(8Mhz bandwidth) signal(like the data file i provided last time), the signal at 200Mhz

is about -38dbc compare to the main output signal at 400Mhz, like the photo i attached before. the same result as my own board.

so can you test use my data file as source data to DAC? and it's the design or performance defect of this chip? or some special/reserved register setting will

help to resolve this ? can you show me your test photo?

0 Yusuf%20Agoro over 5 years ago in reply to user6150974

TI__Expert 4880 points

User,

I was able to replicate the issue and find the solution. Please power down the LVDS output buffer by asserting bit 7 in register address 0x06.

Reference the following picture.

Thanks

Yusuf

0 user6150974 over 5 years ago in reply to Yusuf%20Agoro

Prodigy 110 points

thanks a lot. i have already resolve this problem after set pd_lvds to 1, the spectrum at 0.5*LO and 1.5*LO is clear now.

RF & microwave

RF & microwave forum

AFE7071: output of AFE7071 has unexpected spurious at 0.5LO and 1.5LO frequency

RF & microwave

RF & microwave forum

AFE7071: output of AFE7071 has unexpected spurious at 0.5*LO and 1.5*LO frequency

AFE7071: output of AFE7071 has unexpected spurious at 0.5LO and 1.5LO frequency