Hi Martyn,

For such measurement you may shield your DUT and instruments. Ever using
a shielded room which might not be easy for everyone or using shielded
tents. You can find them made custom for 600-1000€ depending on the
size. I've just ordered two of them from a company in netherlands

For main line frequencies, you may differentiate what is conducted from
what is radiated. For EMI, I'm using now active low pass filter at very
low frequency to follow my low noise regulators and I get very good
result, but honestly not tested as low as -130dBc... But on low noise
PLL or oscillators I can get rid of any spurious from PSU but at 50Hz,
my noise floor has never reached -130dBc...  I'm working on a very low
noise generator (20fs jitter 10Hz-1Mhz) at the moment and at 50 Hz, the
phase noise is  -80dBc/Hz which is already not bat at 6 GHz

I've measured last week a Wentzel VCXO giving -150dBc/Hz @ 50 Hz and I
could measure the 50 Hz at -130dBc/Hz but this had not the active filter
on it. I test it again using the shielded tent exactly to know what is
radiated from conducted. If still there under the tent I will try using
my active filter to see if this makes a difference.

Cheers
Stephane

------ Message d'origine ------
De : "Martyn Smith" martyn@ptsyst.com
À : time-nuts@febo.com
Envoyé 12/07/2016 12:44:31
Objet : [time-nuts] OXCO Spurious Output at Line Frequencies

Unless your DUT is in a mu-metal enclosure, I wouldn't say observing the
same results while battery powered provides any conclusive statement as to
whether or not its the DUT. The terminals around a crystal are pretty high
impedance at line frequencies.

On Tue, Jul 12, 2016 at 6:44 AM, Martyn Smith martyn@ptsyst.com wrote:

I retired in 1999 so it's been a long time since I used one, but I don't recall
ever seeing a device measured using an HP 3048 system that didn't exhibit line
frequency (and harmonic) spurs.

See the "live" plots at the end of this page:
http://hpmemoryproject.org/news/3048/hp3048_01.htm

Wes

On 7/12/2016 3:44 AM, Martyn Smith wrote:

Hi

The “practical” solution often is to ship it to an office of the same company that happens
to have 60 Hz power. The spur moves to 60 / 120 / 180 and they move on with their
evaluation.

I have also played the “let’s do it with batteries" approach. Screen rooms were of little benefit.
Simply running test gear on batteries did not do the job. Ultimately we wound up in the
middle of an Illinois corn field with a bunch of gear modified to run purely on batteries. The
spur did go down, but it never fully went away.  It’s been 40 years since that adventure so I
really could not say just how far down we got it ….

Anything past -120 db at line frequency is in the “don’t bother” category.

Bob

Bob the VLF guys in Europe can receive the 60Hz signal over here! so you
probably needed a test site on the far side of the Moon :-))
Alan
G3NYK

----- Original Message -----
From: "Bob Camp" kb8tq@n1k.org
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Tuesday, July 12, 2016 10:53 PM
Subject: Re: [time-nuts] OXCO Spurious Output at Line Frequencies

On 7/12/16 3:44 AM, Martyn Smith wrote:

It's impossible to get rid of that.  You'd need a magnetically shielded
enclosure, because most of the line frequency is in the form of the
magnetic fields from the building wiring, transformers, etc..  It's
almost impossible to build your oscillator and measuring system without
any loop area, into which those magnetic fields couple.

Let's assume you're at 0dBm which is 0.223 Vrms (It's easier for me to
work in voltage, although for magnetic field, you should be working in
current).

So, 130 dB down is 3E-7 times less, or about 74 nV.  It doesn't take a
very big field to induce a few nV  in a circuit, even if it's all low
impedance stuff.

Hi

Bottom line:

This is why you often see spur specs stated with a lower frequency limit (like 250Hz) or
an exclusion for line related spurs.

One really fun one not (yet) mentioned:

All of those motors in the building are rotating at line frequency. You don’t just have an
electromagnetic field. You can have an acoustic / vibration field as well. Probably not a big
deal, but it’s there ….

Bob

I can't seem to get the line related spurs much below -170dBc/Hz for the fundamental and -180dBc/Hz for the line frequency related harmonics. Everything was bolted down to a 12mm thick aluminium slab. Avoiding an Earth loop by isolating the Timepod Ch0 and CH2 inputs from the aluminium plate seemed to help.
Unfortunately I don't have a cornfield or equivalent readily available, although there is a carpark and paddock at the local zoo to which I have access after hours should I want.
Bruce

  From: Bob Camp <kb8tq@n1k.org>

To: Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Wednesday, 13 July 2016 9:53 AM
Subject: Re: [time-nuts] OXCO Spurious Output at Line Frequencies

Hi

The “practical” solution often is to ship it to an office of the same company that happens
to have 60 Hz power. The spur moves to 60 / 120 / 180 and they move on with their
evaluation.

I have also played the “let’s do it with batteries" approach. Screen rooms were of little benefit.
Simply running test gear on batteries did not do the job. Ultimately we wound up in the
middle of an Illinois corn field with a bunch of gear modified to run purely on batteries. The
spur did go down, but it never fully went away.  It’s been 40 years since that adventure so I
really could not say just how far down we got it ….

Anything past -120 db at line frequency is in the “don’t bother” category.

Bob

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