[time-nuts] 8560E DC Coupling phase noise measurements

Loïc MOREAU Loic.MOREAU at eai.fr
Wed Oct 26 09:53:01 UTC 2011


Thanks, everything is clear now.

 I will have a try with a pair of  Schottky diodes as 10 db give 3.16 attenuation and so no enough for a +-15V rails , I want to be sure ;=) )
 
Regards
Loïc



-----Message d'origine-----
De : time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] De la part de John Miles
Envoyé : mardi 25 octobre 2011 22:55
À : 'Discussion of precise time and frequency measurement'
Objet : Re: [time-nuts] 8560E DC Coupling phase noise measurements


> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts- bounces at febo.com] 
> On Behalf Of Loïc MOREAU
> Sent: Tuesday, October 25, 2011 1:01 PM
> To: time-nuts at febo.com
> Subject: [time-nuts] 8560E DC Coupling phase noise measurements
> 
> Hi,
> In the process of measuring phase noise with a 'demphano' gear,  I am 
> a
bit
> reluctant to connect  my 8560E in DC mode to get 30 Hz low frequency
> sensitivity:  the SA input is very sensitive to DC voltage and may  be 
> destroyed by a voltage larger than 200mV.
> 
>  So with a LNA having no blocking DC output I would like to know the 
> best way to protect the SA.
> 
> I suppose that putting a 100µF capacitor between LNA and SA is not 
> especially a good idea as a charged capacitor to DC rail may have the 
> same effect that no caps at all.
> 
> I am looking for some advice in that domain to pursue phase noise 
> measurements,  for now I connect the SA with a caution after PLL lock 
> and measuring the SA input with a DC voltmeter before switching to DC 
> SA input but this mode of operation is a bit frightening.
> 
> Any idea ?

The 200 mV limit is only with 0 dB of RF attenuation.  With a quadrature PLL and LNA, you don't need to use 0 dB.  20 dB is probably OK, and if not, you can make up for it with more gain.

A pair of back-to-back Schottky diodes would be another alternative. 

-- john



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