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Re: [linrad] Mistake: frontend LO frequency stability
Thanks Leif, for your dedicated explanation.
> From: "Leif Åsbrink" <leif.asbrink@xxxxxxxxxxxxxxxx>
> Hi Peter,
> > Leif, I presume obviously that your correction underneath does not only
> > concern your WSE 144 c.s., but any 144 MHz converting frontend ?
> > Be it DC ( to be home built ) or not ( as your WSE chain ) ?
> I really do not know. At a bin width of 0.5Hz and with a dot length of
> a minute or so one will get an impressive sensitivity. Pushing it a
> further will not give much since it is so much easier to get a 3dB more
> power than to lock the rig to a frequency standard
> Many EME-ers are stable enough already today to run QRSS at a bin
> bandwidth of 1Hz. Most stations drift too much, some even too much
> for JT44 but modern rigs can be good enough as they are.
> Knowing the frequency within a few kHz is enough to locate an
> extremely weak signal. With 5Hz/pixel and 50% interleave a 1024
> screen can display 2.5kHz with full sensitivity. This should be
> at least 3dB more sensitive than JT44. Most rigs have the stability
> With 0.5Hz/pixel one would see only 250 Hz. One would gain 5dB.
> Here a TCXO will be perfectly adequate, the GPS is overkill.
> I think some stations already have the stability needed when
> the rig is well warmed up. The WSE converters are at this level
> if they are allowed proper warming up.
> For a bin width of 0.1Hz a standard TCXO is adequate. I do not know
> where the limit is, at what bin width GPS becomes an advantage,
> probably it is a disadvantage because the GPS will introduce phase
> noise. I think the GPS will ensure a correct center frequency but it
> will give a wider signal.
I don't think so : filtering out the GPS jitter could give a rest jitter of
a few parts x 10^-11 only, which will not widen the signal for bin widths
much less than 0.1 Hz : http://www.rt66.com/~shera/index_fs.htm ( last pic
but one : click on it for jitter enlargement : watch it ! you see the
d.steering voltage, converted by W5OJM to frequency, to keep the oscillator
locked to GPS )
( shortly I do hope to see this result myself after having finished the GPS
locked ovenised HP frequency standard for my coming Linrad frontend ).
>I am not sure of this - if there is anyone
> who really knows I think it would be interesting to have a posting
Yes, indeed, I am very curious.
Has anyone ever made Linrad tests with 144 MHz EME long dots ( > 60
sec./dot ) , really stable oscillators ( Tx also ) ( better than 10^-10 )
and FFT bins < 0.1 Hz ?
Or : where ( as from how small a FFT bin ) does a LO stability of better
than 10^-10 begin to pay off ?
At this moment only curiosity drives me to ask.
Lateron maybe I'll try to find another aberant :-) to make this kind of
Linrad experiments with.
> I have two 10MHz references. They differ by 9.8Hz at 144 MHz, I
> watched them for a few days and I can not see any change:)
> (I use them as references for my HP signal generators)
> Would be fun to lock Linrad to one of the references and monitor
> the stability of the other one. I guess the result to expect is
> well known - but not to me
> Leif / SM5BSZ
> > From: "Leif Åsbrink" <leif.asbrink@xxxxxxxxxxxxxxxx>
> > > Hi Peter and All,
> > >
> > > In a previous posting I wrote:
> > > > The low noise local oscillators in the WSE high level
> > >>converters that I
> > > > currently work with are stable enough if they are kept at
> > > > constant temperature.
> > > > An FFT that spans 40 seconds with a bin bandwidth of about 0.05Hz
> > > > (50% interleaved transforms with a sine squared window) will have
> > > > a bandwidth
> > > > below 0.1Hz most of the times when averaged over 2 minutes.
> > >> It may go up
> > > > to 0.15 Hz occasionally but as I have them, lying on a table
> > > > indoors, there is nothing to gain with a better LO for 144 MHz QRSS
> > > > except for absolute frequency accuracy.
> > > The above is NOT correct. My brains slipped, the scale was in Hz and
> > > in 0.1Hz per division. I was just careless......
> > >
> > > When I discovered this mistake I decided to have a closer look.
> > >
> > > The conclusion is that the WSE converters have to locked to a
> > > frequency reference if bin widths below about 0.5 Hz are used.
> > > Provision is made to allow this, all LO frequencies are multiples
> > > of 100kHz. It is nothing I will give priority, but may be some
> > > 73
> > > Leif / SM5BSZ
73, Peter PE1ECM