Crest Factor for FM signal???

[nazanin.gharib62]

Crest Factor for FM signal???

Hi All,

Crest factor for a sin wave is 3.01 dB.
Why Crest factor for a FM modulated signal is 0dB.
the FM signal basically is a sinusoidal carrier.


Is it right to say that the constant envelope modulations such as FSK or GMSK PAR value is equal to 0dB?

Could you please clarify the difference in PAR values for sine wave and FM signal??

Thanks in advance...!!!!

 

[FvM]

I can only guess that different definitions of crest factor are in use. Where do you see the 0 dB specification?

 

[Dan Mills]

In a modulated signal the crest factor is taken to be the db ratio of the rf power under peak modulation to the rf lower with the modulation removed.

As these two power levels are the same in FM, the crest factor is zero.
The same consideration for say AM will give 6dB, and for SSB the factor is theoretically infinite.

73 Dan.

 

[vfone]

If crest factor depends by modulation (peak and removed modulation) how can people spent time defining crest factor for un-modulated signals as: sine, square, triangle?

https://en.wikipedia.org/wiki/Crest_factor

Above is mentioned that: Crest factor is the peak amplitude of the waveform divided by the RMS value of the waveform.

You mentioned 6dB crest factor for AM. This is for what modulation index: 25%, 50%, 75%?
SSB crest factor is infinite !?

 

[Dan Mills]

Better in modulated systems to speak of PBR really (Power backoff ratio) which is the ratio of the peak envelope power to the average power, it is pretty much the same idea and you see the term crest factor being used occasionally as a near equivalent.

100% mod AM, the rf voltage peaks at twice the unmodulated level, hence 6dB, but interestingly the average power for an AM transmitter rises by 3dB under sinewave modulation, as the envelope swings from 0 to twice the voltage of the unmodulated carrier, this makes the crest factor strongly dependent upon the exact wording of the definition you are using (PEP to Average power under modulation gives 3dB, PEP under modulation vs unmodulated carrier gives 6dB).....

When discussing modulated RF you have to be very careful as PEP, Peak and average powers (Not to say amplifier efficiency) all behave differently under modulation (And I will admit to being somewhat unhelpfully loose with the terminology upthread).

Note that 'RMS Power' is a nonsense term, you can have RMS Voltage or RMS Current, but the term that should be used is 'Average Power'.

Regards, Dan.

 

[vfone]

I am totally lost. I am afraid there is a lot of misunderstanding of the terminology, mixing "Power backoff ratio" (which I never heard until now), with 100% AM (which actually is never used in real life because is close to over-modulation), and with AM average power with and without modulation.

'RMS Power' is a nonsense term !? Tell this to Analog Devices, one of the biggest semiconductor companies in the world, which for more than 25 years they have a chain of chipsets: RMS Power Detectors.

 

[nazanin.gharib62]

I can only guess that different definitions of crest factor are in use. Where do you see the 0 dB specification?

In the below Link, the PAR is 0dB for FM modulated signal.
please see page 12 (part 1.3.2 Phase and Frequency Modulation, PM and FM).

**broken link removed**

Is it right to say that the constant envelope modulations such as FSK or GMSK PAR value is equal to 0dB?

 

[FvM]

I have no doubts that the definition of crest factor given by Dan Mills is used in the paper (unfortunately no valid link).

In so far your questions have been already answered.

 

[nazanin.gharib62]

In the below attachment, the PAR is 0dB for FM modulated signal.
please see page 12 (part 1.3.2 Phase and Frequency Modulation, PM and FM) of steer_rf_chapter1.pdf.

and please see the second attachment too (7TS02_2E.pdf).

Thanks in advance...!!!!

 

[FvM]

I wonder what you're presently asking beyond the qustions that have been already answered?

By the way, there's a definition of crest factor in the R&S paper (Ppep/Pavg) which is obviously different from the said Vpeak/Vrms definition as e.g. derived in Wikipedia. So everything seems to be clear, there's probably a problem of reading.