[time-nuts] crystal notch filter // numbers
adrian.rus at broadhurst.ro
Mon Oct 3 13:29:16 EDT 2016
The setup is the following:
make name type level/attenuation
Tesla [?] T284 SWR bridge
Rohde & Schwarz SMIQ 03 Generator +10dBm
Rohde & Schwarz FSP 13 Spectrum Analyzer
Agilent N9355B Power limiter -0.98dBm at 10MHz
The notch filter attenuation @10MHz and @10dBm input is the following:
Att [dB] delta f [Hz]
12.22 904.1 [this is the far out attenuation of the setup]
Disclaimer: I am not an advocate of this [notch filter] PN or PN+AN measurement method. I just have the idea of compensating a SWR bridge having on its DUT port an XC by a resistor at its reference port and I was amazed by its simplicity [compared with other notch filter setups]. For the particular XC I have used, the VSWR is not top notch as the XC proved to have some 15ohm series resonance. So, losses given impedance mismatches shall be considered [@ the 12dB far out attenuation of the setup].
I have no deep knowledge about XCs at all and very little about their close in noise [I can extend why, in private emails]. If this setup is useful for somebody, it is fine. If not, it is also, fine. Do not shoot [on] me for posting these findings.
From: time-nuts [time-nuts-bounces at febo.com] on behalf of Bob Camp [kb8tq at n1k.org]
Sent: Monday, October 03, 2016 12:17 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] notch filter for close in phase noise measurement
If you do a power sweep on a crystal resonance, it’s a very predictable sort of thing.
Essentially you “chase” the resonance up (or down) in frequency with the sweep. At some
point, it “snaps” and drops back to the low power line. I suspect that what is being observed
is the “snap” as the crystal stops accepting power.
In some crystals, you can get a very observable effect at a few hundred microwatts. With
other designs it’s 10’s of microwaves or maybe even up around a milliwatt.
Liquid nitrogen cooled crystals anyone? :)
> On Oct 2, 2016, at 4:49 PM, Chuck Harris <cfharris at erols.com> wrote:
> That is a most interesting suggestion.
> Suppose the filter crystal was pulled to the DUT frequency, and due
> to the inertia of its very high Q, was able to show you the phase noise
> variations of the DUT better than one might expect?
> -Chuck Harris
> Bob Camp wrote:
>> .....One thing you may be seeing is the crystal shift frequency as it is tuned to “accept” power from the source.
>> With milliwatts of power flying around, that would not be unusual.
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