Continue to Site

# nV/Hz to dBc/Hz conversion for phase noise

Status
Not open for further replies.

#### deepsetan

Hi guys,

I've problem in calculating the phase noise of my 2.4-GHz local oscillator for my receiver. In LTSPICE simulation, the phase noise generated is in V/(Hz)^1/2. How to convert V/Hz to dBc/(Hz)^1/2. I don't found any enough information for this conversion. I try to use 20*log (Vn/Vc) where Vn and Vc is voltage at offset frequency and carrier frequency but I think there is something wrong. This is my phase noise simulation.

Vc = 85.8408pV/(Hz)^1/2 at 2.4-GHz
Vn = 85.8409pV/(Hz)^1/2 at 2.401-GHz for 1-MHz offset frequency

Looks like the noise number is 0 dBc at 1 MHz. Possible in some kind of architectures for VCO.

BTW what kind of oscillator are you using?

Looks like the noise number is 0 dBc at 1 MHz. Possible in some kind of architectures for VCO.

BTW what kind of oscillator are you using?

Hi,

Actually the peak spectral voltage Vc = 160mV/Hz at 2.4-GHz. So the phase noise is around 180 dBc/Hz @ 1-MHz. I'm using pierce oscillator circuit topology with CMOS MEMS SAW resonator.

In LTSPICE simulation, the phase noise generated is in V/(Hz)^1/2. How to convert V/Hz to dBc/(Hz)^1/2.

You have specified the noise level, but not the carrier level. To calculate the phase noise in dBc (where c stand for carrier), you need the noise level and the carrier level.

Vc = 85.8408pV/(Hz)^1/2 at 2.4-GHz

Makes no sense. The carrier level is not specified in V/Hz^1/2 because ideally, it is a single tone. The carrier level is an absolute level, so it should be something like Vc=xxx mV.

You have specified the noise level, but not the carrier level. To calculate the phase noise in dBc (where c stand for carrier), you need the noise level and the carrier level.

Makes no sense. The carrier level is not specified in V/Hz^1/2 because ideally, it is a single tone. The carrier level is an absolute level, so it should be something like Vc=xxx mV.

Hi,

Actually my problem already solved and I got the value of Vc which is 160mV
and I managed to convert V/Hz into dBc/Hz. After did my simulation and some calculations, the phase noise is around 180dBc/Hz @ 1-MHz.

After did my simulation and some calculations, the phase noise is around 180dBc/Hz @ 1-MHz.

Congratulation!

You now have a world record oscillator - or a mistake in your calculations.

- - - Updated - - -

Vc which is 160mV

If your carrier is 160mV into 50 Ohm, that is -6dBm. Thermal noise at room temperature is -174dBm/Hz, so oscillator noise floor can't be less than -174dBm/Hz (absolute level) or -168dBc/Hz (noise level relative to -6dBm carrier).

Congratulation!

You now have a world record oscillator - or a mistake in your calculations.

- - - Updated - - -

If your carrier is 160mV into 50 Ohm, that is -6dBm. Thermal noise at room temperature is -174dBm/Hz, so oscillator noise floor can't be less than -174dBm/Hz (absolute level) or -168dBc/Hz (noise level relative to -6dBm carrier).

Hi,

1.Today I changed some of the values and I found that my peak spectral voltage of Vc at 2.4-GHz is actually 115.02mV. This make the phase noise of the oscillator becomes around 182.48dBc/Hz @ 1-MHz. If you don't mind, can you explain to me how you relate the phase noise with the -6dBm value.

2. The phase noise of Bluetooth is -110dBc/Hz @ 500-KHz offset frequency. GSM850 and GSM require -118dBc/Hz @ 600-KHz offset frequency. If I want to make the oscillator operate in bluetooth and gsm, does it mean that my local oscillator does not meet the requirement?

Last edited:

Status
Not open for further replies.