Update: now with the colours matching in all graphs:
Time-series generated with colorednoise (following Kasdin&Walter), power-spectral-densities and Allan deviations computed with allantools, and compared to theoretical predictions in IEEE1139-2008.
The PSD lines and MDEV lines seem spot-on, but are the ADEV lines systematically a bit low?
Code here: example_noise_slopes.py
Following on from modified total deviation mtotdev() the Hadamard total deviation htotdev() algorithm is very similar but instead of phase data takes frequency data. Now included in allantools.
Here's a comarison against Stable32 and the NIST SP 1065 table values.
There's no bias-correction for now, and apparently it is customary not to use htotdev(m=1) at an averaging-factor of 1, but instead use ohdev(m=1). This is why the 'raw' value from allantools at tau=1 in the plot below is a factor of 2 too low.
Both mtotdev() and htotdev() are very slow algorithms - help with making them faster would be appreciated!
I wrote a first rough (and very slow!) implementation of Modified Total Deviation mtotdev() for allantools.
mtotdev() combines the features of modified Allan deviation mdev() (being able to distinguish between white and flicker phase modulation) and Total deviation totdev() (better confidence intervals at large tau).
Here are some results. The Time Total Deviation ttotdev() follows trivially from this work also, since it is only a scaled version of mtotdev(). I used the "NBS14" 1000-point frequency dataset and compared my results against Stable32 and those in NIST SP 1065 (Table 31, page 108).
The figures show mtotdev() and ttotdev() from Stable32 runs and allantools. I've also added mdev() and tdev() traces to compare against. At first sight this looks strange, but mtotdev() is a biased estimator, and Stable32 applies a bias correction which explains the results.
Somewhat surprisingly Stable32 applies a bias-correction only when run in the "all-tau" mode. These numbers agree with those from the NIST SP 1065 table. For this dataset Stable32 is undecided on what power-law the dataset follows at large tau, which results in deviations that jump up and down (because a different bias-correction is applied at different tau, red datapoints).
When Stable32 is run in "octave-tau" or "decade-tau" mode no bias correction is applied. These numbers agree with the ones from allantools.mtotdev().
This figure shows the ratio of variances between allantools.mtotdev() (no bias correction) and a Stable32 all-tau run (bias correction applied). The lines correspond to the bias-correction values for different noise processes.
There also seems to be a misprint in NIST SP 1065 equation (28) page 26, where it says ttotdev() is scaled by tau-cubed (when tau-squared is correct). Who reads these things anyway - not many it seems 😉
I gave a talk at the White Rabbit Workshop in Amsterdam about our long link, ideas for fiber asymmetry calibration, and recent stability measurements. The slides are online.
I updated my earlier picture with the noise-floor of three different phase-noise measurement setups. The picture now includes a typical good OCXO trace and the CLK2 10MHz output of a GM-locked White Rabbit Switch (v4.2 firmware).
A look inside the symmetricom 6502 frequency distribution amplifier.
Input stage is based on LMH6702 http://www.ti.com/lit/ds/symlink/lmh6702.pdf.
Output stage is based on LMH6609 http://www.ti.com/lit/ds/symlink/lmh6609.pdf
PDF of sketched schematic, based on the photos: 6502B_schematic_v2
I measured the phase-noise at 10MHz earlier. This amplifier has a far-out phase-noise floor of around -163 dBc/Hz.
View from the bottom, looking up towards the H-bulb (or dissociator?) in one of our H-masers at work. Any ideas on what the spiral is?
There's a cross-section of a typical H-maser at USNO: http://tycho.usno.navy.mil/maser.html
Update 2016-01-14: Added noise floor of Symmetricom 5115A:
The question of using a general purpose Spectrum Analyzer vs. a dedicated phase noise test set for measuring phase noise came up on the EEVBlog forum. Here are two noise floors for 10 MHz phase noise measurements measured recently. One with a R&S FSW8 spectrum analyzer and the other with a Microsemi 3120A phase noise probe. The 3120A noise floor improves by maybe 10 dB in a longer duration measurement (e.g. hours instead of minutes). On the other hand the SA can measure phase noise out to at least 1MHz offset frequency and at any (I think?) carrier frequency up to several GHz.
Update 2015-12-18: Things improved quite a lot by simply wrapping the board in aluminium foil!
The amplifier phase noise floor is now at around -156 dBc/Hz while the 6502 is at -163 dBc/Hz. The AM noise numbers are similar.
Original post 2015-12-17: I put together a first prototype (only one output channel) of my TADD-1 inspired frequency distribution amplifier. Preliminary schematic here.
I compared the prototype board to two commercial distribution amplifiers: an SRS FS710 (quite awful) and a Symmetricom 6502 (very good). I also compared my new data with John Ackermann's measurements from 2007.
The new board showed ugly spurs at 50 Hz and harmonics using an el-cheapo wall-wart 12 VDC SMPS, so I also tried it with an "ultra-low noise DC-source" a.k.a 12 V lead-acid battery.
Here's a peek inside a Meinberg Lantime M600 PTP/NTP server. It follows the same modular design as the Lantime M300 with the GPS receiver on the right and the computer-board to the left. The new thing is a Time Stamping Unit (TSU) in the middle.
The TSU (by Toradex) seems to be built in a memory-stick form factor (SODIMM?) around a Marvell 88ap270m chip (PXA270 processor?). Maybe it's a Toradex Colibri PXZ270?
We need a number of frequency distribution amplifiers in the lab. Let's not reinvent the wheel but rather do a face-lift for the TADD-1. John Ackermann has phase noise measurements on the TADD-1 and TvB has temperature coefficient results on the TADD-1.
Here's a draft design for an SMD version of the TADD-1 frequency distribution amplifier. The plan is for a 2-sided 180 mm x 110 mm PCB. Two of these could be mounted side by side in a 1U 19" enclosure to give 2x8=16 outputs on the front panel. If a companion pulse distribution (1-PPS) board is built, the output at the back of this board can be used to drive a PICDIV on the pulse-distribution board. This gives 8 frequency outputs and 8 pulse outputs on a 1U 19" panel. The 2-pin connector at the DC-input can be used to power the other board in the same enclosure.
Comments? Suggestions?
PDF files:
