# [time-nuts] Question for any HP105 owners

Tom Van Baak tvb at LeapSecond.com
Tue Aug 30 14:25:29 EDT 2011

```Mark,

Let me explain the sqrt(2) thing.

Since you have a number of oscillators you have probably already
compared them to each other to see which one is best, or which
pair is best.

When you compare the FTS1050 against the HP105 remember
that the ADEV numbers you obtain are always the rms sum of
the instabilities of the DUT, the REF, and the 5370B. At first you
can't quite know how much of the measured instability at each
tau is due to each of the three parts.

So the trick -- if you are comparing two of your best oscillators
at tau 100 s and get something like 1e-12 you can assume that
worst-case they are sqrt(2)*1e-12, or 7e-13 each.

Why? Because your 1e-12 measurement is the rms sum of DUT
and REF and comparator noise. At 100 s the 5370B noise is down
to, say, 2e-13. The rms of 2e-13 plus 7e-13 plus 7e-13 = 10e-13,
or 1e-12.

In other words, when you get a DUT that approaches to how good
good the DUT really is.

Once you've played with this you can try the next trick which is the
so-called 3-cornered hat. The goal in the end is to better determine

/tvb

----- Original Message -----
From: "Mark Spencer" <mspencer12345 at yahoo.ca>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>; "Tom Van Baak" <tvb at leapsecond.com>
Sent: Tuesday, August 30, 2011 8:00 AM
Subject: Re: [time-nuts] Question for any HP105 owners

Thanks Tom, the reference oscilator is a Datum FTS 1050.  I've used both a HP 5370B and my Tek 2252 scope as time
interval counter and looked at the data files with Plotter and Time Lab.  The results are all more or less the same for

With regards to the sqrt(2) removal I'm not familiar with that phrase but the measurements I was making were for ADEV.
Using Plotter I also removed the small ammount of drift but the ADEV values didn't change much.

My HP105B has the older style (non 10811) oscilator.   It does out perform all the 10811's that I own (especially with
regards to longer term stability.)

Regards
Mark Spencer

--- On Tue, 8/30/11, Tom Van Baak <tvb at LeapSecond.com> wrote:

> From: Tom Van Baak <tvb at LeapSecond.com>
> Subject: Re: [time-nuts] Question for any HP105 owners
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Received: Tuesday, August 30, 2011, 2:09 AM
> Hi Mark,
>
> What reference oscillator and measurement system did you
> use?
> Is that with or without a sqrt(2) removed? Is your HP105
> the older
> style with the 00105 brick oscillator or a newer one?
>
> The modern HP105 is essentially a well engineered wrapper
> around
> a 10811 oscillator so the performance you see will be that
> of the
> 10811. Your 3e-12 measurement beats the spec for a 10811
> (1e-11
> at tau 100 s) by a wide margin.
>
> It is possible that the 10811 used in HP105 met some higher
> spec
> or were hand selected. I don't know. Perhaps someone from
> HP
> (Rick?) could clarify this for us. We do know there are a
> number
> of different grades of 10811.
>
> One lesson you learn if you test many 10811 is that they
> can vary
> by quite a lot (an order of magnitude or more), even those
> with the
> same part number in the same instrument model.
>
> For example, see the tau 100 second point of a dozen
> 10811's:
> Notice they all are under the 1e-11 at 100s spec, but some
> are
> much better than others. It's the luck of the draw. And one
> reason
> why some of us troll eBay for old or cheap 10811 hoping to
> find
> one that's better than the previous best.
>
> /tvb
>
> ----- Original Message ----- From: "Mark Spencer" <mspencer12345 at yahoo.ca>
> To: <time-nuts at febo.com>
> Sent: Saturday, August 27, 2011 4:53 PM
> Subject: [time-nuts] Question for any HP105 owners
>
>
> > Just curious what a typical ADEV figure is for the
> HP105 ? Mine seems to be approx 3X10-12 at 100
> seconds. Just curious what results any other HP105
> owners have seen in practice.
> >
> > Mark VE7AFZ
>
>
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