Yet further misuse of my HP8753B !

• ok, let's start with a disclaimer: any ideas from my website you apply at your own risk !
• time and frequency accuracy are the basis for pretty much everything in todays society
• Finnish hams had great fun in 2008 with our (RATS) national HF Frequency Accuracy Measurement Competion (in Finnish, but check images)
• being the happy owner of an HP8405A, I often use this for HF antenna alignment, microwave device group delay measurement and frequency stability measurements
• using an A/D converter with this Vector Voltmeter, the phase output can be handled by a PC, otherwise a regular pen recorder is just fine too
• I finally wanted to make the frequency error measurement even more flexible, so I turned to my old faithful HP8753B, that I have many a time misused before too...
• with the firmware installed on my particular model is v.02.01, June 27th. 1988 there seems to be no problem with direct phase analysis
• there are several real advantages of using a VNA:
  - variable detection bandwidth
  - averaging
  - smoothing
  - arbitrary center frequency
  - simple frequency alignment
  - direct display
  - electronic documentation
  - instability issues detected easily
  - etc.
  
• so far I have used the VNA for comparing stability of 10 MHz and 300 MHz sources, but it will work at any frequency within the analyser coverage
• initially I used my Trimble Thunderbolt as the "trusted reference", after at least a couple of weeks ontime
• the KO4BB Trimble Thunderbolt GPS Disciplined Oscillator Display does a fine job of showing critical data without a PC, mine is built on a piece of veroboard
• first I did some stability analysis of the system over periods of 10 seconds to 1 hour as a manner of calibration (the analyser allows 24 hour logging)
• phase variation is less than 40 millidegrees over 60 seconds without any particular filtering, smoothing or averageing
• with the stabilized GPS secondary reference I made a test with my Ball Efratom Rubidium reference against the Trimble
• here is a 60 minute plot of the phase drift after the rubidium standard has been powered up for 4 hours (it took 41 minutes and 42 secods for a full 360 degree swing)
• this measurement shows a Δf/f of 2.8*10^-11, but the same ballpark figure can be achieved in just a few minutes as the short term stability of the Rubidium reference is not too bad
• alignment of an oscillator under test is fun, as you can see the results immediately on the VNA screen, but this is an iterative process taking a while due to restabilisation effects
• here is a plot of a typical OCXO after alignment: the short-term stability is excellent and the phase drift is -154.4 degrees in 50 seconds yielding a Δf/f of 8.6*10^-10 (downwards)
• an example of alignment of the above oscillator as a 60 second plot: initial error in divisions 7-8, and after a couple of tweaks the error is visible in divisions 3 to 6
• the OCXO will be very accurate for short periods of time, but will drift back and forth, remaining however in the order of 10^-9
• so the ghastly bit of this story is the horrendously poor short term stability of my Meinberg 166 with 10 MHz reference option LQ as in Low Quality :-(
• what can I say ?
• OK, so the above measured QK OCXO is going to replace the personal computer grade 10 MHz reference in the Meinberg, pretty damn soon too
• some fun shots: warmup of the Rubidium reference, warmup of another OCXO that is 300 Hz too high, typical frequency change in the OCXO a minute or two after alignment
• Here is a simplified process for the measurement:
  - make a coarse comparison between your references using a regular dual channel oscilloscope
  - you can either use the Y/T Mode or the X/Y Mode on your scope for Lissajous diagrams
  - if necessary, make coarse alingment with the scope (this VNA method is for high accuracy calibration !)
  - disconnect your directional device(s) from the HP8753
  - set the VNA to CW mode and set the center frequency to be the same as your reference and DUT
  - connect your reference (e.g. 10 MHz) directly to the Reference input channel (observe damage levels etc.)
  - the RF input maximum level on my unit is +20 dBm, 25 VDC, much less RF is actually necessary
  - your reference drive level should be appropriate to extinguish the Phase Lock Alarm
  - connect your DUT to channel A (similarly observing damage levels etc.)
  - set the analyser to A/R measurement
  - set the display format to Phase
  - set the scale/div to 36 degrees/div
  - set the scale reference position to 5 DIV and the reference value to 0 degrees
  - I prefer to use 1601 sample points
  - set Trigger to Continuous
  - start with a Sweep Time of 60 s (6 seconds/div)
  - you may use Averaging/Smoothing/IF BW reduction to deal with noise and/or short term instability, but do it smartly !
  - using the above may hide potential problems if you are not careful !
  - if you want to use really narrow IF bandwidths, you may need to synchronise the VNA to your (e.g. 10 MHz) source in case the VNA timebase is off calibration...
  - this system is sensitive enough to detect changes in your reference frequencies due to e.g. magnetic and graviational orientation, barometric pressure...
  - it is possible to analyse down to the millidegree level with some care, but at least I do not have any MASER's, yet anyway ;-)
  
• OM Ramppa, OH2LIY has independently verified this method by using it for calibration of his Rubidium reference source
• after 1 hour warmup and about half an hour of alignment, 1 hour of measurement on the VNA gave: plot 1, plot 2
• for comparison he used his Trimble Thunderbolt GPS secondary reference at 10 MHz
• My colleague Juha Kiili, OH2LKV, also tested the Rohde & Schwarz ZVA vector network analyser and this also seems to work perfectly for time correlation analysis.
• on the ZVA, the maximum sweep time is 127500 secods, so it should be possible to analyse really small difference in time/phase.
• OM Hubert, DB7ME confirmed this method works fine also on the HP8753C ! (15.10.2014)