Re: [linrad] Mistake: frontend LO frequency stability

["Peter van Daalen" <vandaalen@xxxxxxxxxxxxx]

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
little
 > 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
 > needed.
 >
 > 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
 > here:)


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

 > 73
 > 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
not
 > > > 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
day....
 > >>
 > > > 73
 > >>
 > > > Leif / SM5BSZ

73, Peter PE1ECM





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