[time-nuts] 5120A and PM signal

Grant Hodgson grant at ghengineering.co.uk
Mon Jan 14 20:00:34 UTC 2013


Filip

What exactly are you trying to measure?

A phase-modulated carrier will have an infinite number of sidebands 
spaced at the modulating frequency, the amplitude of the sidebands 
gradually reducing away from the carrier.   The amplitude of the 1st 
pair of sidebands (closest to the carrier) is a function of the 
modulation index.  In your case the 10MHz carrier signal has it's phase 
modulated by +/- 1 rad, at a rate of 10kHz. Note that the level of the 
carrier is itself a function of the modulation - i.e. as you change the 
level of the peak deviation, the level of the carrier will change.  With 
1 rad deviation, the carrier will be approx. 2.3dB down compared to with 
no modulation.

So, if you want to measure the phase noise of the 2024, then you should 
turn off the modulation.

Are you getting confused about the dBc/Hz measurement?  'dBc' means 'dB 
relative to the carrier' - that's why it's called dBc.  dBc/Hz is 
usually used for noise measurements only, and means the level of noise, 
relative to the un-modulated carrier (in dB), if measured in a 1Hz 
bandwidth, at a given offset from the carrier.

The level of modulation sidebands is independent of measurement bandwidth.

I may have missed something here - please could you give more details of 
what you are trying to do?

regards
Grant


> Date: Mon, 14 Jan 2013 11:35:02 +0100
> From: Filip Amator <filip.amator at gmail.com>
> To: time-nuts at febo.com
> Subject: [time-nuts] 5120A and PM singal
> Message-ID:
> 	<CABZTLJCZ+vhT+EhiJZgm0Uky_Lio9yzPhOgxNgmg7YaP4RTn6A at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Hello,
>
> I made a simple measurement using Symmetricon 5120A phase noise
> measurement set and a Marconi 2024 signal generator and I don't
> understand the results. I measured the phase noise of 10 MHz signal
> with 1 Rad phase modulation at 10 kHz, and I got from mesurement peak
> at 10 kHz with level about -26dBc/Hz. According to the current
> definition of dBc/Hz, the value of -26dBc/Hz should be considered as a
> -26dB of modulation relative to 1 Rad. But I would expect that the
> peak level will be at 0 dBc at 10 kHz. Does anyone knows how to
> explain this difference?
>
>
> Filip Ozimek
>
>
>
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