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Clock Bias

Other Parts Discussed in Thread: LMH6702, TINA-TI, VCA821

I am looking for a solution to the following:

An LVDS pulse is provided by an FPGA that is 200MHz (5 ns).  A DAC is providing two inputs that are a DC bias 0-5V and an amplitude adjustment also 0-5 V.  The circuit would allow amplitude adjustment of the FPGA clock and a DC  bias adjustment of the FPGA clock without changing the pulse width (5 ns).  The output of this circuit would be 0-1V.  As the DAC is changed the output pulse would reflect the adjustment.

Thanks,

Earl

  • Hello Earl,

    Is it possible to reduce the amplitude of the inputs coming out of the DAC to more conform to the control voltage signals of the amplifiers? I have attached a TINA-TI schematic of what I think you are looking for using the LMH6702 and VCA821. The solution below if for dual supplies (+/-5V) but I think it should work for single supplies as well.

    There are two knobs which you can use for clock DC bias control and clock amplitude control as noted in the below diagram. The clock DC bias control is fairly limited to +/-1V or 0 to 2V common mode depending upon the gain set for VCA821, so as to avoid clipping at the VCA821 outputs. As a result, the DAC bias ctrl output would need to be limited to within this range for DC bias control. The maximum gain of VCA821 is configurable from 6dB to 39dB with >40dB dynamic range adjustable through the VG pin, which will be the second DAC input for amplitude control.

    The large-signal bandwidth of the VCA821 is close to 320MHz for 4Vpp output, so you are seeing a sine-wave at the output. However, the frequency has not been modulated. If the output amplitude of the VCA821 seems low, you could increase the gain of VCA821 to achieve the right signal swing as the VCA821 is not slew-limited.

    Best Regards,

    Rohit

    Clock_Bias_Ctrl.TSC

  • Thanks, Rohit, I'll get a chance to breadboard something up next week.

  • Hello Earl,

    I would definitely recommend ordering evaluation boards for the individual parts and trying out the circuit, since you could run into oscillation issues with breadboard due to parasitic.

    Best Regards,

    Rohit