Whoops…error
on FT8 drift in my just sent email…
?
I
quote from W3SZ’s paper 2/25/2019:
?
For
example, for FT8 with it’s 12.6 second receive cycle and
its tone spacing of 6.25 Hz, the drift of a received
tone must be less than 6.25/12.6 = 0.5 Hz/ second in
order to maintain channel integrity and obtain
satisfactory decoding performance.
?
Unquote.
?
Sri
for confusion
?
Bill
??W2RMA
?
?
Greg:
?
Your
logic is fine.? Rather, the issue here is the
expectation of accuracy as well as current practice on
the microwave bands.? I am a 10 band rover, with rigs
50-10368 mhz.? For 902 up, I use an external xvtr as
well as for 222.? (Maybe the FT817 will someday have
222.)
?
From
a frequency ‘accuracy’ perspective, good practice these
days is such that stations can easily be within < 200
hz at 10368 and better, say, at 1296..? A digiLO or
similar unit can create microwave LO signals with
reasonable phase noise using a 10mhz OCXO, GPS locked
oscillator or Rubidium oscillator.? I use a Morion
double oven 10mhz ?OCXO and can run the bands and be on?
.100 (or whatever) all the way up.
?
From
a ‘stability’ perspective, the FT817 is ok for FT8 on
50/144 in a controlled environment (as well as 40M, the
only low band I’ve used it on) and sometimes on 432.?
However, sun on the FT817 in the field can cause issues
as can whether you run output power at ‘1 bar’ or ‘3
bars’.? Moving from an AC house to outside can cause
drift.? Any change in ‘heat’ generated inside the FT817
can cause drift.? Recall that the ?allowed drift for FT8
is about 12hz in the 15 second over (per W3SZ’s paper)
and that includes your rig’s IF’s LO + xvtr’s LO and is
additive to the other guy’s drift.? I know guys who have
struggled with stability at 222 with a K3.? ?
?
FWIW,
I wrote an article on using the digiLO to create the
FT817 oscillator frequency.? I built a prototype, but
the leakage and therefore cleanliness of the signal
weren’t good enough.? TBC.
?
73
?
Bill?
W2RMA? EN90xh
?
?
?
?
?
?
?
?
From:
[email protected] [mailto:[email protected]] On Behalf
Of nz8r@...
Sent: Friday, November 08, 2019 7:24 PM
To: [email protected]
Subject: Re: [ft817] GPS lock the FT-817 for
transverter
?
Pardon the inquiry.
I am interested in learning a bit more about upper
microwave transverters and their operation.? ?
Do these typically mix the radio (say at 144mhz) as an IF
to a multiplied oscillator??
If this is true, the errors would be? something like
below:
FT-817 error? ? ? ? ? ? ?=? Ferror
Base oscillator error = Oerror
MultError? ? ? ? ? ? ? ? ?= Oerror * N
Total Error? ? ? ? ? ? ? ? =Terr
Osc's can self start pretty reliably up to about
100mhz....
Therefore N needs to be 100 to 150,? such a large N means
Oerror needs to be really small--probably as good as
technology will permit.
In the end this is Terr =? Ferror + MultError
It seems the Ferror is really tiny and MultError is huge.?
?
In other words, it seems to me the FT-817 errors are not
relevant.
What am I missing as to spending money on fixing the FT817
error,? when 100 times the impact is in the Oscillator
errors on the Transverter?
Thanks in advance,
Greg
nz8r
