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Original question:

Direct FM modulation at VCO with either LMX2531 or LMX2582 PLL lock at 2500MHz

LMX2694-EP: Direct FM modulation at VCO at Vtune of the LMX2694

Varaprasad Rayudu
Prodigy 150 points
Part Number: LMX2694-EP


Hello,

Could you please suggest suitable frequency modulation scheme to get the modulated output at the LMX2694 PLL chip output with below configuration.

  • PLL operating Mode: Fixed Frequency
  • Carrier Freq: 7.000 GHz
  • PFD Frequency: 20MHz
  • N Divider: 700
  • Loop BW: 50 kHz
  • Modulation Freq range: 30Hz to 1200Hz (Tunable and Fixed at given point of time)
  • Modulation Depth range: 30Hz -1500Hz (Tunable and Fixed at given point of time)

As shown in the above Fig, there are 3-ways (@ Refclk, @PLL and @VCO) to get the modulated output at the high frequency carrier signal and

With modulation input of 500Hz @ Refclk(10MHz) the modulation depth/deviation at carrier modulated output@7GHz multiplied by N divider value of 700.So to produce the modulation depth of 500Hz @7GHz, need to apply very low frequency modulation depth of around 0.7Hz(500Hz/700(N-div)) which is difficult to product such low frequencies.

Could you please suggest the suitable solution to resolve the issue with applying modulation signal either at the PLL side (using Digital Scheme) or at the VCO Vtune side(using Analog scheme using summing of the loop filter output with modulation input).

Regards,

RVP

  • 0
    TI__Guru** 110060 points

    Hi RVP,

    To modulate PLL, we have to continuously program PLL N and PLL NUM very fast. Whenever you program PLL N, the output frequency will change 10MHz. So modulate PLL does not work.

    To modulate VCO, given the Kvco at 14000MHz is approx. 200MHz/V. An 1mV modulation signal will change the VCO frequency by 200kHz. So modulate VCO does not work neither.

    In summary, there is no solution to meet your FM requirement.

  • 0 in reply to Noel Fung
    Prodigy 150 points

    Dear Noel,

    Thanks for your feedback and could you please explain little more on modulate PLL side what is meant by "Whenever you program PLL N, the output frequency will change 10MHz"

    Could you please suggest any other solution to modulate @Reference side to resolve the issue.

    Any test circuit or suggestion to feed low frequency modulation input at the reference side by dividing the modulating input by 700 to get the desired modulation at 7GHz.

    Regards,

    RVP

  • 0 in reply to Varaprasad Rayudu
    TI__Intellectual 1040 points

    Hi Noel,

    If we fix the N value at 1399 and used NUM as 1000000 and DEN as 1000000. The with every 1 increase in NUM value we can do 5HZ of step change in freq ?

    Do you see any issues in that ?

    Regards,

    Akshat

    FAE

  • 0 in reply to Akshat Garg
    TI__Guru** 110060 points

    Hi Akshat,

    NUM should always less than DEN.

    In addition, with 10MHz input and 7000MHz output, it is an integer channel, so NUM=0.

    We can increase NUM to increase the VCO frequency but we cannot reduce NUM to support -ve deviation, without changing the integer N.

    For example, N = 700, DEN = 1000 and NUM = 0. To reduce VCO frequency, we have to change N to 699 and then set NUM = 999.