LMX2820: Error Signal Output

Euan Tough

Part Number: LMX2820
Other Parts Discussed in Thread: LMX2595EVM, LMX2595

Tool/software:

Hi, we are investigating using the LMX2820 chip to replace a discrete component PLL circuit in our system. We wish to mix the RF signal response from our device at RF_in with the VCO on chip to recover of the actual error signal response which will then be used to drive a component on our device. I think if I have understood the behavior of the component correctly, this can be achieved by reading the output of the Charge Pump (CPOUT) pin.

We were wanting to test this on an evaluation board we had in the lab (LMX2595EVM), but it appears the CPOUT pin is not connected/accessible? Is there any way to access this output?

Many thanks.

over 2 years ago

0 vicente flores prado over 2 years ago

TI__Mastermind 32152 points

Hi Euan,
LMX2595EVM does not have a direct access to the CPOUT pin per say -

There's a test point for VTUNE, you can also probe the CPout pin by probing the top pad of C1_LP but no direct access to the pin.

Regards,

Vicente

0 Euan Tough over 2 years ago in reply to vicente flores prado

Prodigy 10 points

Thanks for the response. I had considered probing with a pin probe.

So is the CPout the correct output to recover the error signal?

0 vicente flores prado over 2 years ago in reply to Euan Tough

TI__Mastermind 32152 points

Hi Euan,
Can you please elaborate on signal error? I want to ensure I understand what you're referring to and what your application is.
Are you trying to figure out how the CPout current becomes a voltage after the loop filter which is seen at the VTUNE pin which will correspond to a certain VCO frequency?

Regards,

Vicente

0 Euan Tough over 2 years ago in reply to vicente flores prado

Prodigy 10 points

By signal error, I mean the output of the loop filter, which we will use to drive a component in our system. This would be a current ideally.

0 vicente flores prado over 2 years ago in reply to Euan Tough

TI__Mastermind 32152 points

Hi Euan,
I see what you mean, you mean the phase error between the R & N divider which gets compared by the PLL and produces a correction current based on the phase error between the dividers - got it.
What exactly do you want to drive here or accomplish? typically the CPout current becomes a voltage after going through the loop filter which becomes your VCO frequency.
From what I understand, you don't wish to use the internal VCO of LMX2595? Could you provide a block diagram or more information on your system so I can understand your requirements and better assist you?

Regards,

Vicente

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LMX2820: Error Signal Output