How does spur change after frequency divider?

[bageduke]

I have a question:

How does spur change after frequency divider? I know that offset frequency won't change, but how about spur power level? Also unchange?

Thanks,

 

[kspalla]

How does spur change after frequency divider? I know that offset frequency won't change, but how about spur power level? Also unchange?

How do you say that offset frequency is unchanged after frequency division?
let say I have frequency 1MHz and is divided by 100 then I have 10KHz.
Also let say the spur is at 900KHz then spur is located after division at what frequency?
it has few cases to understand in case of digital division.

 

[bageduke]

Yes, you are right. In the case you mentioned, the spur will fold back. When I talked about un-changed spur offset frequency, I meant that like multi-GHz clock divided down to multi-GHz clock and spurs are at MHz offset. Sorry, I didn make it clear.

Can you tell me, in this case, how does spur level change? Like phase noise, reduce by 6dB by every divide-by-2? Or stay un-changed?

Do you have any papers discuss this topic?

Thanks,

 

[biff44]

Well, imagine a carrier with some sort of small frequency modulation going on. If there is a single tone at frequency Fm modulating the carrier, the spectrum can be approximated by:

sideband power in dbc = 20 Log (Δf/(2* Fm))

where Δf is the frequency deviation, and Fm is the modulating rate.

If you were to pass this thru a divide by 2 circuit, the Δf frequency deviation would half, but the Fm would stay the same. So after the divide by 2, the sidebands would still be at the same +/- Fm distance from the carrier, but their level would be 6 dB lower.