(LT6990)Frequency Modulated Operation (Voltage-Controlled Oscillator)

Hi !

I have some questions about LTC6990.

Q1)

One of our customer want to use LTC6990 as Voltage-Controlled Oscillator.

And I am trying to calculate the error.

The error factors that can be read from this figure and the equation are the dispersion of the resistance and the variation of the voltage of VCTRL.

Other than the above error, it is recognition that it becomes only the voltage error of VSET.

Is my understanding correct?

Q2)

Considering the temperature change, I think that it is necessary to add the following error to the above error, is that enough?

Q3)

calculated the error but it look the error is big.

But the datasheet said this.

Does this means this error include VSET voltage and drift error?

I mean the error of the external resistor is not calculated ,correct?


Best regards

Kawa

  • +1
    •  Analog Employees 
    on Nov 14, 2019 1:36 AM

    The error on Vset does not apply if you use only a single resistor to ground. Vset changes are self compensating in this case. From the Operation section of the data sheet on page 10 "From this equation it is clear that VSET drift will not affect the output frequency when using a single program resistor (RSET). Error sources are limited to RSET tolerance and the inherent frequency accuracy ΔfOUT of the LTC6990". The inherent accuracy is the 2.2% spec that you note above.

    However, in a two resistor VCO type of application the Vset error does matter as described in the "Frequency Error in VCO applications Due to Vset Error" section of the datasheet on page 17. As you note, the errors can be high in some cases especially when the voltage range is so extreme. The error due to Vset variation goes to zero when the VCO voltage is at 0V as you essentially have two resistors in parallel. I am not sure if the calculations shown are correct as I don't know what all of the resistors are doing. For more accuracy you can use the circuit shown in Figure 13 of the data sheet.