RE: [linrad] Mistake: frontend LO frequency stability

[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 am not sure of this - if there is anyone
who really knows I think it would be interesting to have a posting
here:)

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
> 
> 
> 
> 

LINRADDARNIL