LMH2120 Questions on the output voltage

Richard Elmquist

Other Parts Discussed in Thread: LMH2120

I have a customer who is seeing the following when testing the LMH2120:

I am using an AN-2064 eval board with a 2.5 Gb/s PRBS input signal as the RF source. I see a strong 6.25 MHz tone on a DC pedestal. The tone amplitude is about 33% of the DC level. Any idea why I might be seeing this tone? I don’t think it’s related to the spectral content of the PRBS signal? I tried lower data rates and get the same tone, so I think the problem is in something I’m doing wrong. I am just sending the output into an o-scope. That part is pretty simple.

I have looked at it with a DVM and that reading is fine. I will however be reading this signal with an ADC and the sampling could become an issue. I have made an LP filter to eliminate the 6.25 MHz tone. It’s still surprising to me that this would show up. I’m not sure if you’re planning to look further into this, but I’m sure than anyone doing high-speed sampling (~ 1MHz) would benefit from its resolution. I did run the fft analysis on the scope and it detected a very pure 6.25 MHz tone and this tone rode on a DC level. The tone did not change amplitude, the the DC level changed as one would expect with a changing RF power level.

Is this something the customer should be seeing? Is this signal coming from the part ar is there something else happening?

Thanks for your help with this!

Richard Elmquist

over 7 years ago

0 Tomi Koskela over 7 years ago

TI__Genius 11020 points

Hello,

Is this with one unit or have multiple devices been tested? There can be some residual ripple at the output, but it should be low enough that usually low pass filtering is not needed. In datasheet page 21 is described the low pass filtering which can be used for certain type of input signals.

The oscilloscope can load the output which can have negative side effects. Active probe could be tried does it effect the behavior.

Thanks,

Best regards,

Tomi Koskela

0 Richard Elmquist over 7 years ago in reply to Tomi Koskela

TI__Mastermind 45250 points

Tomi,

So the signal he is seeing is normal and is always present? He can filter it and has done so, but I wanted to make sure that this was not something else that was happening with the circuit. So the 6.25 MHz signal is normal and is always present?

Thanks for your help with this!

Richard Elmquist

0 Richard Elmquist over 7 years ago in reply to Richard Elmquist

TI__Mastermind 45250 points

Tomi,

Can you please look at the questions I posed above?

I just wanted to make sure that the 6.25 MHz signal is something that is normal. I told the customer that I would verify if this signal was always present. If not, can you explain where the signal might be coming from?

Thanks for your help with this issue!

Richard Elmquist

0 Tomi Koskela over 7 years ago in reply to Richard Elmquist

TI__Genius 11020 points

Hello,

I just checked on EVM and I don't see 6.25MHz signal coming from the output. So it must be coming from somewhere else.

Thanks.

Best regards,

Tomi Koskela

0 Richard Elmquist over 7 years ago in reply to Tomi Koskela

TI__Mastermind 45250 points

Tomi,

Thanks! That is what I needed to know.

I will let you know if the customer has any further questions.

Thanks again!

Richard Elmquist

RF & microwave

RF & microwave forum

LMH2120 Questions on the output voltage