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How to make the phase noise monolithic?

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Advanced Member level 5
Jul 28, 2010
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I designed a freq source, and the phase noise is as attached.

But the problem is the phase noise is not monolithic, I don't know what is the cause? The 1899M signal is coming from X3 of 633MHz signal, and the 633MHz signal phase noise is monolithic.


  • 1899-001.JPG
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I think the word you are thinking of is "monotonic".

It is very common to see bumps/spikes in a phase noise measurement. That bump at 3 Khz, is likely due to a pll control loop closing out at an open loop frequency of around 3 KHz. When a control loop loses its open loop gain, there is often closed loop peaking of noise at that point due to insufficient phase margin.

The transfer function of a simple closed loop system is G(s) = G(s)/(1 + GH(S))

when |GH| = around 1, and angle(GH) is around 180 degrees, you can see that G(s) blows up.

If you can not see the bump at 633 mhz, but it is there at 3x the frequency, remember that that bump will be 10 dB lower at the input frequency, so your measurment system may hit a noise floor.

It is also not that uncommon for a X3 multiplier to be have some inherent instabilities too. if it is an active multiplier, there could be a noise bump in the active device. If it is a passive diode multiplier, there could be parametric oscillations.
Hi, biff44,
thank you very much.
I used SRD as multiplier, two stages.
If it is because of parametric osc, how to cancel it?

You would have to explain the srd circuit in detail. If you are driving the srd with high power I would first look at the driver amplifier and make sure it is really stable. Start with adding all sorts of bypass caps on the base/collector bias points (like 47 uF tantalums) and see what happens. I would try some loading impedances on input and output of that driving amp to see if that snubs it. A good first step is to grab a high impedance oscilloscope probe and probe everywhere to look for a 3 KHz sine wave oscillation (even a few millivolts would be important to find).

Really check out the measurement system and make sure none of the rf components you are using have a 3 khz sideband.

You can try replacing the srd with a schottky diode tripler and see if the problem goes away. If so, that tells you for sure it is in the X3 or in the 633 MHz source.

Measure something really clean at around 600 mhz, and verify that your measurement system noise floor is at least 20 dB better than the phase noise you are measuring on the 633 mhz source.

Maybe you take two identical 633 mhz sources and beat them together in a mixer, and look at the baseband output to see if the 3 khz bump is there or not.

An SRD IS susceptable to parametric oscillations. They are just varying reactances, and if you drive them right, the diode can exhibit a negative resistance at some unexpected frequency. Look carefully at the x3 output over a broadband. Are there ANY frequencies present other than 633 or nx633. For instance, if there is a spike at 57 Mhz, you should investigate it! Sometimes varying the resistance to ground in the bias circuit can make the instability come and go.

You might try an inside/outside DC block on the measurement system conneciton, just in case it is somehow leaking a 3 khz signal back to your X3 circuit.

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I guess the PLL filter has larger bandwidth than wanted..
LO Phase Noise is decreasing with 1/f slope but because of bandwidth of PLL Loop Filter the other noise sources appear in-band
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