peculiar vco phase noise

[analog_chip]

Hi,
The phase noise of my vco looks weird with a peaking near the center.
The vco is followed by a Power amplifier followed by offchip matching network. I have control on the matching network components.

Is it parasitic oscillation? How can I fix this? Any materials that discuss solving such issue?

 

[RCinFLA]

That look like a low frequency spurious signal. There may be some low frequency bias oscillation within the oscillator, but first I would make sure it is not from the power supply, like an unstable regulator.

It is common problem for RF PA's to have low frequency oscillation in their biasing circuitry.

If it is within a synthesizer what are reference frequencies and phase detector frequency.


Look for anything around 3.7 kHz.

 

[jiripolivka]

Check your DC power supply. If you can, never use a switched AC-DC converter, only a linear power supply. Switchers usually generate such peculiar noises.

 

[analog_chip]

Hi,
Thanks everyone for the reply.
1. I forgot to mention that this an injection locked oscillator locked to the the third harmonic of 800 MHz i.e. 2.4 GHz

2. The PCB has not been designed very well since it was my first RF PCB. The decaps (5pF, 1nH, 10nH) are far away IC pins. I shall design a new one though.

RCinFLA: How would low frequency supply noise (3.7KHz) appear near 2.4 GHz. Does it get modulated & upconverted?
jiripolivka: Do you mean the supply may have noise at 2.4GHz + 3.7KHz?

 

[jiripolivka]

Hi,
Thanks everyone for the reply.
1. I forgot to mention that this an injection locked oscillator locked to the the third harmonic of 800 MHz i.e. 2.4 GHz

2. The PCB has not been designed very well since it was my first RF PCB. The decaps (5pF, 1nH, 10nH) are far away IC pins. I shall design a new one though.

RCinFLA: How would low frequency supply noise (3.7KHz) appear near 2.4 GHz. Does it get modulated & upconverted?
jiripolivka: Do you mean the supply may have noise at 2.4GHz + 3.7KHz?

Yes, I have seen several similar problems due to power supply interference hat degraded PN at the switching frequency. Poor DC filtering also brings 50 or 60 Hz and harmonics to PN spectrum.

As you mention your VCO is injection locked, check the injection level and signal quality. PN always depends on reference PN. Also parasitic oscillation is possible, often above the RF output frequency.