Jerry,
That's all well and good. But the fact that the frequency measurements made
on my Orion and several others mentioned earlier in this thread are so
repeatable says there's something wrong in the analysis. I still say the
frequency has to average within a few cycles of true in a 10 second period.
I tune in WWV in AM mode on one receiver and put the test receiver in SSB.
I then listen to the beat note produced in the test receiver for the tone
generated by WWV. You can easily hear 1 cycle beat errors this way. The
test receiver is then tuned for the lowest beat frequency. The frequency on
the test receiver does not move more than a cycle or two over a three minute
period and is very reproducible each night. This is at 10 mhz. I still
say, the proprogation efects have to average out over short periods. If
they didn't where are the extra or lost cycles going? There's no way you
can be off 30 hertz in the same direction for a long period of time
(minutes).
Carl Moreschi N4PY
Franklinton, NC
----- Original Message -----
From: "Dr. Gerald N. Johnson, electrical engineer" <geraldj@isunet.net>
To: "Gary Hoffman" <ghoffman@spacetech.com>
Cc: <tentec@contesting.com>
Sent: Tuesday, June 03, 2003 12:33 AM
Subject: Re: [TenTec] Some Observations on Orion Stability/Calibration
> I read a compendium on frequency measurement, probably put out by the
> IRE or IEEE, likely 25 years ago. I think its still in a certain pile
> but I don't want to disturb that pile at the moment. Its not propagation
> delay alone, its the variation in propagation delays. Think of it this
> way, if you were 1000 miles from WWV, a path length change of 1 part per
> million is 0.001 mile, or 52.8 feet. To hold frequency via the
> ionosphere to even 1 part per million the reflection layer AND
> reflection point (which can be quite diffuse) has to hold within 52.8
> feet of the initial point. As I recall the article comparing HF to VLF
> propagation of standard frequencies, one would be required (having a
> local standard with stability say a part in 1000 million) to measure the
> receive frequency every day about the same time (ignoring the days with
> truly upset propagation the result of solar flares or sporadic E events)
> for a month averaging the results. Then the frequency transferred would
> only be precise to about 1 part per million. It is not possible to
> predict the error, nor to estimate it.
>
> N4PY claims it should average in a few minutes. Not so, for half of each
> day the ionosphere tends to rise and for half a day it tends to fall,
> but has a lot of variations including multiple paths that are only
> observable, they are not predictable or removable. While N4PY and WWV
> are not moving relative to one another, the RF propagation paths between
> them are constantly in flux. Sometimes one path, sometimes multiple
> paths, sometimes a single path with a bounce near one end, sometimes
> multiple hops, sometimes the "optical" path bounce point has moved from
> one end to the other end as the ionospheric layer responsible for
> reflection (really a volumetric refraction instead of a mirror like
> reflection and that refraction adds to the uncertainty) undulates under
> the influence of the solar wind and solar flux.
>
> The ionosphere refraction layer has NOTHING to hold it to a specific
> position. It can vary from a few tens of miles to a couple hundred miles
> of elevation and does that daily. It only takes 52.8 feet of path change
> to make a part per million change.
>
> According to one ARRL Frequency Measuring test, decades ago, I achieved
> better than 1 ppm precision using the 1 KHz divisions of the dial of my
> 75S3B. I did do a careful calibration of the receiver using uncommon
> calibration points, but I also must have made a VERY good guess getting
> within 7 Hz (1/140th of a dial division)...
>
> If one adjusts the TT TCXO to match the frequency readout of radios
> without TCXO, one is probably using the wrong reference to set frequency
> and those rice boxes setting the net frequency were probably never
> exactly on frequency (unless they drifted past one day) and are still
> drifting in the very fine precision.
>
> Crystals do drift with age, tending to drift upward as damaged molecules
> from grinding rattle loose reducing the mass of the crystal slab. The
> best crystals are cut for a few MHz, 3rd or 5th overtone operation and
> are operated in a carefully temperature controlled environment. My two
> standards use a simple outside oven over a thermos type container with a
> proportionally controlled oven for the inside oven. And they hold 1 to 5
> ppb (parts per billion) per month compared to WWVB. Terrible compared to
> an atomic standard, not great compared to a really good crystal
> standard, but fine enough for my needs.
>
> When doing short term frequency checks against WWVB I see random motions
> of time of arrival of 10 to 50 nanoseconds. Some of that is randomness
> in my local standard, some is randomness at VLF of the propagation path,
> and I suspect some is wind induced motion of the suspended vertical
> antenna wire at Ft. Collins. Averaged over hours or days, which is the
> standard technique, traveling clocks have proved that 20 and 60 KHz
> propagation is pretty good. Better than my local standards, for sure.
>
> I find it is important to be sure to know that there are no flare
> products approaching the earth, that there are no disturbances to
> propagation, and that the weather is consistent between here and Ft.
> Collins before I try to do a short term VLF frequency calibration. Then
> I still have to ignore times when the receive signal is spread over 100
> ns or more due to noise and undocumented short term path variations.
>
> 73, Jerry, K0CQ
>
> --
> Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
> Reproduction by permission only.
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