Increasing the Hughes LNB oscillator range

[Llew]

This mod gives some adjustment to extend the Hughes LNB oscillator frequency well into Band C.

Although as Vipersan and myself have found, the Hughes is perfectly capable of encroaching into the lower region of that band, the fixed frequency of the DRO oscillator (no adjustment screw) negates further investigation of that Band.

Access to to the DRO can be achieved by opening up the metal screening can. It's not too difficult to do, it's quite thin - I simply scored round the small holes around the top of the can with a Stanley knife! You will blunt the blade after several runs around the holes, but I managed after replacing the blade a couple of times. I left one side, so that I just needed to bend it back (see pic) in case I needed to replace it as was ( if mod failed).

After drilling a hole in the cover, an M4 nut was Araldited onto the cover to take a 12mm M4 screw. This size screw can be screwed in fully without touching the DRO (essential).

Conveniently, the Neoprene strip on the underside of the cover gives a good seal to prevent water ingress through the screw hole.

In practice, careful screw adjustment now seems to give about 400 GHz extra reach on top of the maximum frequency I was previously able to lock (20792V 12.5W).

The feed on 5.2W 20880 is now centred between 20800 and 21000 using 19035 as the oscillator frequency.

 

[zorbua]

Very nice Llew quite a precise modification, especially gaining that extra 400 MHz.

 

[Captain Jack]

I apologise if I appear dumb but did you replace the DRO with an adjustable one or is the one in there adjustable already? Since you mentioned that there's no adjustment screw at the start. Or did you mean no easy access to it?

 

[Vipersan]

I apologise if I appear dumb but did you replace the DRO with an adjustable one or is the one in there adjustable already? Since you mentioned that there's no adjustment screw at the start. Or did you mean no easy access to it?

The Hughes local osc is fixed ..in a small metal box with no screw ..so no adjustment..
..
The dro puck is inside a closed metal box ..
Llew has made a hole in this box and fitted a screw thus making it adjustable.
rgds
VS

 

[Captain Jack]

Thanks VS. It still didn't make sense until I read how DROs work. Understand it perfectly now.

 

[Llew]

Yes, I expect some LNB manufacturers are confident enough to know their local oscillator DRO's are spot on frequency to be able to enclose the puck in a screening can without any subsequent frequency drift, but most seem to have adjuster screws for final tune-up, certainly most KA and KU ones.
I believe an earlier Hughes LNB LO was adjustable.

 

[Terryl]

The small can is not so much to keep it on freq, but to keep the DRO from emitting or going into some nasty spurious oscillations.

If the DRO unit has a spur on it's output it could cause some nasty interference at some other frequency outside of what your looking at.

A sign of a spurious oscillation would be a drop in the output level, or someone from a local frequency co-ordination office showing up at your door with a quit and decease order.

It is best to check with a spectrum analyzer for any problems when pushing the range of a microwave oscillator outside of is design.

Back in the old days we had lots of problems with the oscillators just going out of spec by them selves, you would note a drop in the RSSI, or the FCC knocking at your door.

I had a 7 GHz TV microwave act up due to a spur and cause a big problem on a 21 GHz military channel, needless to say, SAC was not happy.

 

[zorbua]

Most professional equipment had their Osc embedded in a temperature controlled environment to keep it drift free.

 

[Llew]

The small can is not so much to keep it on freq, but to keep the DRO from emitting or going into some nasty spurious oscillations.

If the DRO unit has a spur on it's output it could cause some nasty interference at some other frequency outside of what your looking at.

A sign of a spurious oscillation would be a drop in the output level, or someone from a local frequency co-ordination office showing up at your door with a quit and decease order.

It is best to check with a spectrum analyzer for any problems when pushing the range of a microwave oscillator outside of is design.

Very true Terry. Needless to say, any mods and adjustments I do to LNBs are checked for spurious signals with my Promax.

 

[Vipersan]

Hmm ..
seriously doubt its just shielding ..
Basic principles of DRO operation..require a resonance chamber in which the puck is located ...or it simply wont oscillate correctly..
Particularly at such high frequencies.

4. Mechanical Consideration

In a DRO, the electrical layout is only one aspect of the oscillator design. Mechanical interests also highly influence the local oscillator (LO)’s performance. The cavity’s size and height have loading effects on the LO which can reduce the phase noise performance and create an unwanted frequency drift over temperature. Under best conditions, the DR would be free to resonate in free space, but because of obvious real estate consideration, the LO needs to be constrained within a shielded cavity. The rules-of-thumb dictates that in order for the cavity to have a reasonable thermal and loading effects, the cavity should be at least three pucks high and three puck’s diameter wide. This height requirement is one reason most DRO designers prefer to set their DR on a standoff, so that the housing or PCB on which the DR usually rests does not affect the resonator’s performance. The PCB material’s mechanical integrity also needs careful consideration because of LO drift over temperature and long term aging effects, especially if the cavity is resting on the PCB.

Finally, the fine tuning and adjustment of the DRO will be set through a tuning screw that will increase the DR’s resonant frequency as it closes the electrical field above the puck. This should provide as much 80 MHz of tuning range. However, it is important to notice that tuning the frequency with a tuning screw is achieved at the cost of reduction in both unloaded Q and temperature stability. This worsen of temperature stability is due to the increasing slope of the tuning curve as the metal plate gets closer to the DR surface.

 

[Llew]

A good overview VS. The very small tuning range quoted implies they're probably referring to an oscillator in the much lower GHz range than we're using, but the principle still applies.

On the question of screw-adjustable DRO's, it's still not obvious why this isn't used on the Hughes LNB, as I would have thought given the fact that the DRO frequency has to be doubled before the mixing stage, any small frequency drift will be x2.

I notice there are adjustable resistors in the HL/VR circuits, but these appear to be after the mixer stages, so I guess it's not to do with the oscillator. Who knows though?