LMX2595: LMX2595

Vikas Kumar40

Part Number: LMX2595
Other Parts Discussed in Thread: LMX2572

I want to use LMX2595 in the chirp mode with variable pulse width ie on time and off time for the chirp will change in system. As per the datasheet, when the chirp generation is over, it comes back to first frequency. Is there any method to switch off th frequency generation in this mode without switching off the PLL as settling time will come into picture. For the same pulse operation, if I start the chirp once again, how to make the phase coherency pulse to pulse.

over 7 years ago

0 Vikas Kumar40 over 7 years ago

Intellectual 670 points

As per the data sheet, only phase noise plots are attached . Is it possible to get spur plot around 16.5 GHz in integer mode.

0 Noel Fung over 7 years ago in reply to Vikas Kumar40

TI__Guru** 111060 points

Hi Vikas,

Could you draw your desired chirp waveform? This would help us answer your question more precisely.
In integer mode, there are reference spurs (due to OSCin) and fpd spurs (due to phase detector). These spurs are usually in the order of -80dBc.

0 Vikas Kumar40 over 7 years ago in reply to Noel Fung

Intellectual 670 points

Noel

I have attached the snapshot for the requirement

0 Vikas Kumar40 over 7 years ago in reply to Vikas Kumar40

Intellectual 670 points

Noel
I have attached the plot. You can see that, I have made two comments, one for the phase during ramp up start and other one, no output from PLL. As the overall band is 1 GHz, so do you mean to say, other than harmonics, I will be getting spurs 80 dBc apart from the power supply. Or any other spur will come into picture for wideband.

0 Vikas Kumar40 over 7 years ago in reply to Noel Fung

Intellectual 670 points

Noel

What about the fractional mode. As the doubler is enabled after 15 GHz, the half of the required frequency will be coming out ?

0 Vikas Kumar40 over 7 years ago in reply to Noel Fung

Intellectual 670 points

Noel,
Any feedback on the pulse to pulse phase ?

0 Noel Fung over 7 years ago in reply to Vikas Kumar40

TI__Guru** 111060 points

Hi Vikas,

assume you use 100MHz OSCin and fpd, then at steady state (16GHz), you will have spurs at multiples of 100MHz offsets (due to OSCin and fpd) as well as spurs at 8GHz (due to VCO). only the 100MHz spurs will fall into the 1GHz band. During ramp, the PLL will be in fractional mode, there will be fractional spurs as well.
As for the ramp up, you need +/-62.5MHz variation from the VCO, multiple VCO calibrations are necessary during the ramp. So the ramp up waveform is not a linear ramp, it would be a staircase ramp. LMX2572 datasheet section 8.1.7.2 has more details on this. The only way to have no output from the device is to powerdown the output buffer, this has to be done manually.

0 Vikas Kumar40 over 7 years ago in reply to Noel Fung

Intellectual 670 points

Noel

Anyhow , the subharmonic will be out of band, so its not much of the trouble. Regarding the staircase ramp, In the LMX2595 data sheet, where the plot is for 120 MHz band, its showing linear. Where can I simulate the situation, depending on the pulse chirp bandwidth as well as the pulse width, I can get the spec for the time for which staircase will be there.

0 Noel Fung over 7 years ago in reply to Vikas Kumar40

TI__Guru** 111060 points

Hi Vikas,

As datasheet mentioned, the linear ramp range is not a constant, it varies with temperature as well as VCO core. At lower VCO core, the range will also be smaller. In your application, you will need to operate it at VCO1. I just checked with an EVM, the linear range is approx. 90MHz. This is sufficient in your application. However, you have to caution about the variation from part to part and temperature.

0 Vikas Kumar40 over 7 years ago in reply to Noel Fung

Intellectual 670 points

Hi Noel, Till 90 MHz, its ok I feel. But, when I want to use 150 MHz chirp Bandwidth, I feel I should go with one up-conversion rather than directly chirp .

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LMX2595: LMX2595