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regulator oscillating problem - RF Cafe Forums

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pigger

Post subject: regulator oscillating problem

Unread postPosted: Sat Jun 19, 2004 1:32 pm

our PLL (2GHz) is supplied by a voltage output form a regulator, but the phase noise of this PLL is huge when it is tied to the regulator. It is working very well when there is no regulator. So we can guess it is the regulator oscillating that makes the PLL working inproperly. We followed the regulator data sheet for its peripheral circuit, but it did not work out.

can anybody give me some idea on how to solve this prblem?

Top

Guest

Post subject: Regulator output

Unread postPosted: Sun Jun 20, 2004 9:56 am

Did you check the regulator output on oscilloscope? We have a couple of top notch PLL designers here, I will see if they can recommend a regulator or remedy. Also, do you know what the pulling or puhing spec to the VCO is? ( I forget which it is pulling or pushing for Vcc sensitivity).

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Itay

Post subject:

Unread postPosted: Mon Jun 21, 2004 11:01 am

Have you put enough decoupling capacitors on the regulator? Usually you will need to put a super filter circuit on the supply of the VCO - this is extremely important. Super filter circuit is based on an NPN transistor, which is bypassed with decoupling capacitors at the collector and emitter and its output at the emitter is connected to the Vcc pin of the VCO. It drops the supply voltage from the regulator (Which needs to provide a highr voltage than the supply voltage of the VCO) to the voltage level required by the VCO.

In addition you will need at least one tantalum capacitor with low ESR as part of your decoupling capacitors battery. A value of 10uF is adequate for this purpose. You have to choose a regulator with low output noise, for example: MICREL's MIC5205 is a good choice, this device has a low output noise and by connecting an external capacitor you can reduce the output noise even lower, this is excellent for low current applications as VCO's/PLL's.

http://www.micrel.com/_PDF/mic5205.pdf

Good luck,

Itay

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Guest

Post subject: Re: Regulator output

Unread postPosted: Mon Jun 21, 2004 4:12 pm

Thanks for replying. I checked the VCO spec, pushing figure is 1MHz/V

Anonymous wrote:

Did you check the regulator output on oscilloscope? We have a couple of top notch PLL designers here, I will see if they can recommend a regulator or remedy. Also, do you know what the pulling or puhing spec to the VCO is? ( I forget which it is pulling or pushing for Vcc sensitivity).

Top

pigger

Post subject:

Unread postPosted: Mon Jun 21, 2004 4:15 pm

we put 2 tantalum caps, one is 10uF, the other is 0.1uF.

and there are 3 decouping caps for the VCO vcc pin, all are tantalum, (1u, 0.01u and 10pF). But nothing helps

Itay wrote:

Have you put enough decoupling capacitors on the regulator? Usually you will need to put a super filter circuit on the supply of the VCO - this is extremely important. Super filter circuit is based on an NPN transistor, which is bypassed with decoupling capacitors at the collector and emitter and its output at the emitter is connected to the Vcc pin of the VCO. It drops the supply voltage from the regulator (Which needs to provide a highr voltage than the supply voltage of the VCO) to the voltage level required by the VCO.

In addition you will need at least one tantalum capacitor with low ESR as part of your decoupling capacitors battery. A value of 10uF is adequate for this purpose. You have to choose a regulator with low output noise, for example: MICREL's MIC5205 is a good choice, this device has a low output noise and by connecting an external capacitor you can reduce the output noise even lower, this is excellent for low current applications as VCO's/PLL's.

http://www.micrel.com/_PDF/mic5205.pdf

Good luck,

Itay

Top

Alberto

Post subject:

Unread postPosted: Tue Jun 22, 2004 8:23 am

Are the regulator and the decoupling capacitors close enought to the VCO ? How did you check the synthesizer without the regulator ? (I mean if did you use the same PCB track to fed the VCO with or without LDO).

I think you also have to check if your GND is good and well connected between regulator and VCO (f.i. using a filled area with many vias).

Just another note: some LDO requires not too low ESR output capacitors, otherwise they can become unstable: check the data-sheet of your regulator.

Top

pigger

Post subject:

Unread postPosted: Tue Jun 22, 2004 11:48 am

we have decouping caps close to VCO and regulator, but they are not quite close.

we have a test point next to the regulator, so we just lifted the output pin of the regulator and hooked up the test point to a power supply.

I checked the datasheet, they ask for low ESR caps.

Alberto wrote:

Are the regulator and the decoupling capacitors close enought to the VCO ? How did you check the synthesizer without the regulator ? (I mean if did you use the same PCB track to fed the VCO with or without LDO).

I think you also have to check if your GND is good and well connected between regulator and VCO (f.i. using a filled area with many vias).

Just another note: some LDO requires not too low ESR output capacitors, otherwise they can become unstable: check the data-sheet of your regulator.

Top

Guest

Post subject:

Unread postPosted: Tue Jun 22, 2004 11:38 pm

Hi,

Is your circuit already exists on a final board layout, or is it still a prototype? If it still can be modified, I suggest you will add a super filter to the VCO supply, as I suggested. You can also put the VCO and PLL into an enclosure of shielded metal can, which will be soldered to the GND plane of your board. You will surround the VCO, PLL and the loop filter with a stripe of GND stitched with via holes and to this stripe you will solder the shiedling can. This will provide a high isolation to the sensitive circuitry from the external noisy world. This is a good way to protect a sensitive circuit.

Good luck,

Itay

Top

guest

Post subject:

Unread postPosted: Mon Jul 05, 2004 4:16 pm

hope you dont use LM317? Measurement-=> knowledge. If everything is perfect, your system may oscillate.

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pigger

Post subject:

Unread postPosted: Sat Jul 10, 2004 2:00 am

no, we don't use national semi's stuff

guest wrote:

hope you dont use LM317? Measurement-=> knowledge. If everything is perfect, your system may oscillate.

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