Posted by
Kok Chen on
Dec 29, 2010; 7:32pm
URL: http://elecraft.85.s1.nabble.com/TCXO-0-5PPM-tp5872529p5875311.html
On Dec 29, 2010, at 12/29 10:12 AM, radioshoppe wrote:
> As mentioned in my email about the best I can do with Method 2 is
> about +/20 hz.
Do you have an RTTY program that has a crossed-ellipse display
(affectionately called "crossed bananas" by RTTY nuts)? If so, that
might get you closer than 20 Hz by using your sight instead of your
hearing.
A steady Mark tone should appear as a single oval that is perfectly
vertical or perfectly horizontal. You tune your rig by the amount of
the mark tone that you have set -- if you can set the mark tone to
some "nice" tone, like 2000 Hz instead of 2125 Hz, you won't have to
do much mental gymnastics to figure out the proper VFO offset.
Not all software have a narrow cross ellipse display, so you may need
to find one that is satisfactory. Basically, with the standard RTTY
shift, the cross-ellipse will go through 90 degrees of rotation with
170 Hz worth of offset.
If you can find software which you can change the FSK shift, you can
get even more accurate readouts. For example, setting the shift
(i.e., the difference between the Mark and Space tone) to 50 Hz will
move the peak of the oval by 90 degrees with just a 50 Hz VFO offset.
I just tried WWV in cocoaModem's "wideband RTTY" display (whose oval
is narrower than for other RTTY modes in cocoaModem). By using an FSK
Shift of 10 Hz, I can easily eyeball a 1 Hz change.
If your RTTY software does not allow such a narrow shift, it might
have MSK (Minimal Shift Keyed) mode, where the shift is 22.5 Hz.
(Note: MSK, not MFSK; they are different animals.)
If you are using WWV, be sure to use a narrow filter so the
modulations don't come through, or wait for the sidetones and
subcarriers to stop. Else you will get an oval that shakes around :-).
Alternately, you can look for iPhone apps for tuning musical
instruments. Set your VFO offset for middle C or something :-).
73
Chen, W7AY
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