Hi Bob,Can you tell me what you mean by "just put the two inputs in quadrature,
attach to a sound card and you have all you need for phase noise."? What
inputs do you assume I have? I don't think I can attach two 10MHz inputs to a
soundcard and expect anything useful.
Bob
From: Bob Camp <[email protected]>
To: Bob Stewart <[email protected]>; Discussion of precise time and frequency
measurement <[email protected]>
Sent: Saturday, October 1, 2016 6:24 PM
Subject: Re: [time-nuts] Measuring phase with an HP 3456A?
HI
DMTD = Dual Mixer Time Difference
Single Mixer = what is commonly used for most things.
If you have a single mixer setup, just put the two inputs in quadrature, attach
to a sound card and you have all you need for phase noise.
Bob
> On Oct 1, 2016, at 4:30 PM, Bob Stewart <[email protected]> wrote:
>
> Hi Bob,
> I don't have a DMTD breadboarded up for testing. This was just a test of the
> new LPF using only a single Mini Circuits ZLW-1H DBM, and things kind of
> progressed from looking at the output of the LPF on the scope to "I wonder
> what I would see on the 3456A?" sort of thing. I'm running a
> holdover/recovery test on the code and hardware changes to get a reliable
> 1PPS from my GPSDO, so there is some very slow movement over the range of 0
> to 100ns.
>
> Bob
>
> From: Bob Camp <[email protected]>
> To: Bob Stewart <[email protected]>; Discussion of precise time and frequency
> measurement <[email protected]>
> Sent: Saturday, October 1, 2016 3:14 PM
> Subject: Re: [time-nuts] Measuring phase with an HP 3456A?
>
> Hi
>
> What is the beat note coming out of the DMTD?
>
> Put another way:
>
> DMTD involves three oscillators. Two are on roughly the same frequency and
> the third is
> offset from the other two. The difference frequency is typically something
> like 10 Hz.
> It does not *have* to be 10 Hz, but that is one way to do it.
>
> So, moving on using 10 Hz (which may be wrong):
>
> If you are at (say) 10 Hz, you get a 1x10^6 “error multiplication” on the
> output. One cycle
> at 10 MHz gives you one cycle at 10 Hz. The one cycle is 10% of 10 MHz, it’s
> 0.1 ppm
> of 10 MHz. You get a 10 degree phase change at 10 Hz for each 10 degree phase
> change
> at 10 MHz.
>
> The 10 Hz offset limits your phase noise process. The upper (or lower)
> sideband wraps around
> at 10 Hz and then starts dumping back into the other sideband’s data. You
> also need to have a
> signal processing chain that will tolerate the carrier being “in band”.
> Between the two … not
> such a great way to do it.
>
> It’s *much* easier to simply hook up a single mixer (half of what you have
> already) and look at
> the two sources in quadrature. Then the sidebands line up. The carrier is
> gone. The dynamic
> range can be *much* less.
>
> Bob
>
>
>> On Oct 1, 2016, at 2:32 PM, Bob Stewart <[email protected]> wrote:
>>
>> I've been spending a small amount of my time looking into making a sort of
>> hybrid DMTD with a pair of DBMs up front feeding the stereo input to a sound
>> card. So, I got the 100KHz LPF back from Oshpark and hooked it up to my
>> scope for verification - an obvious step. Then I hooked it up to my 3456A
>> just for grins. (The two DBM inputs are 10MHz outputs from two different
>> GPSDOs). So, as I watch this, I think the obvious question: can this
>> measure phase angle better than the 5370A? I guess I need to send it
>> through a full 100ns of phase change to get a calibration value. So, who's
>> been down this road and what did you discover?
>> Bob
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>
>
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