LNB Locking

[gu6efb]

Hi

I am looking for some help and advice.
I have an octgon dual LNB with a 27Mhz Xtal I have followed an article by EA5DOM http://www.df9np.de/Contents/TVSat_LNB.pdf on locking this LNB to a GPS locked signal source using a DF9NP 27Mhz LNB disciplined to 10MHz reference.

With the mod complete when I apply the 27Mhz lock reference the received signals all sound as if I listening to them via aurora when the reference is off the received signals are fine.

I have tried attenuating both the 10meg and 27meg signals but it made no difference. I have checked the 10 and 27 meg signals they are on frequency and look clean on the spectrum analyser.

I am lost at the moment so any pointers would help.

73 Keith GU6EFB

[G4IUG]

I looked hard at this. I have a 0.1ppm TCXO fitted in a 25Mhz LNB so have done a little work !

You mention 10Mhz ? Where is this ?

What is your new 27Mhz source looking like ?

Do you know the level that is needed to swamp the internal 27MHz oscillator and if you have that level ?

Dave

[g4eml]

That mod seems to leave the original crystal in circuit. So your external reference would have to override it. When I modded my Octagon I disconnected the internal crystal before feeding the external reference in.
You could try cutting the track to the crystal or remove the zero ohm resistors.

[DL9QJ]

Hi Keith,

did you solve this problem? I noticed the same using this setup. No difference if I left the crystal or cut the track to it. Same when using a different 10 Mhz reference.

The output signal was clean when I used a GPSDO instead.

73 Oliver DL9QJ

[G4WIM]

I had the same problem with my first attempts to lock a 25MHz LNB - ie the phase noise degraded drastically.

I was using a HP8648C as the reference source but no matter what I did the phase noise was rubbish.

Changing over to a home brew 25MHz vxo disciplined by a 10MHz GPS signal it all worked nicely.

I can only assume the phase noise of the HP sig gen is very lacking at 25MHz even though thee specs and measurements seem to imply it should be ok.

So you may want to try another ref source ?

Regards Tim

[g0mjw]

I would expect it is - isn't YIG based? Oh no - that's an earlier one. Still these Synths can have pretty high phase noise.