Conversion of the Maspro SCE-975 LNB for 10GHz receive use.
The unit itself
The Maspro SCE-975 LNB, was originally used for the FSS (fixed
satellite service) band use, block-down-converting 10.950-11.750
GHz. It is a very small squat waveguide-terminated LNB, that uses
HEMT (high electron mobility Field Effect Transistor) technology
to produce an LNB with (for the time in which it was in
production) a low noise figure, around the 1.3dB mark, possibly
better.
Inside the unit, the modular construction of this unit is
apparent, when the screening covers are removed from the top and
bottom of the unit. On the top, a three stage HEMT amplifier,
followed by an interdigitated filter, with a single schottky
junction mixer (which is located under a smaller screening
cover). On the underside, a MIC (Microwave IC) IF amplifier,
followed by a discrete stage, with the power supply circuitry,
and required regulation stages. The two boards are held down with
around 8 screws per side, and the two boards are linked with a
small piece of etched p.c.b, supplying DC, and returning the IF
signal from the mixer.
Visible from the IF board side is the local oscillator capsule.
The adjuster screw is the only visible part of this module,
however at this stage, we need to get everything separated if we
are going to convert the unit for 10 GHz operation.
The conversion
SEZME! - static electricity zaps microwave electronics. So make
sure you have provided all the necessary earthing precautions
before you even touch this unit. HEMT transistors are
particularly vulnerable, and you may not even realise you've
damaged them (a poor noise figure is sometimes the only
indication of damage).
The conversion is fairly simple, but due to the nature of the
construction of the LNB, some care is required to get it right.
Start by using desolder braid of a good quality (chemtronics
chem-wik lite is very good) to desolder the linking p.c.b. from
the IF board side of the LNB. Once you are satisfied the boards
are separated (wiggle the linkage p.c.b gently to make sure),
turn the LNB over, and on the RF side, using the desolder braid
again, desolder the probe from the waveguide aperture at the
(waveguide) end of the unit. Try not to move the probe up or
down. Watch out for static especially here. Once you're sure that
the probe is separated, remove all the screws holding the RF
board down, and gently pull the RF board up and out of the LNB
casting by holding a small pair of pliers on the twist-tag that
held the screening cover in place. Hopefully the board will
separate without any problems. If however the waveguide probe is
still attached gently heat this whilst pulling up on the board.
The RF board should now be free.
Put the rest of the LNB to one side, and turn the RF board over.
Underneath is the LO (Local Oscillator) capsule. This is a
self-contained 10 GHz oscillator. We want to modify this to run
at 9 GHz so that the 10 GHz band tunes-out on a satellite
receiver at 1000-1500 MHz.
To remove the LO from the RF board, we have to first desolder the
two coupling pins that link through to the RF board. These carry
around 4V in to the oscillator, and the 10 GHz signal out of the
oscillator. Desolder them with braid, and make sure you've got
all the solder off. The RF board uses plated through holes for
these connections, and we don't really want to damage them, even
though the striplines on the top of the board are the only part
of the p.c.b that really matters.
Using desolder braid again, desolder the sides of the tin can
from the RF p.c.b. The can is only soldered at two ends, once the
bulk of the solder has been removed, gently heat and pry up the
can from the RF p.c.b until they separate. Now you should have
the module separate, and that means we can put the RF p.c.b to
one side safely in an antistatic bag whilst we work on the LO.
To get inside the LO requires considerable care. Enough heat is
required to melt the solder on the can and free the p.c.b, but
not so much that the surface mount devices inside will also come
undone and move around! Probably the best way to start is to
attempt to desolder around the underside seam where the can and
p.c.b meet, and then to heat the can gently, with a small gas
torch and pry each side of the p.c.b out using a pin. The pin can
be easily inserted in the hole at each of the four corners of the
p.c.b. With the right amount of heat, and with most of the solder
removed, however, the board will probably come out first time.
Once you have the board out, look it over. It should resemble the
layout as shown here. The dielectric resonator is the yellow
cylinder perched on the ceramic pillar. Remove this and put it in
your spares box. Look at the point labelled #1. At this point use
a sharp knife (scalpel is great) to cut the source stripline off
just after the source lead on the transistor. Make two cuts close
together so that the stripline is no longer connected to the
source, and remove the p.c.b trace inbetween the cuts.
Now look at the point labelled #2. At this point you need some
copper or brass foil (0.003 to 0.005 shim stock) to make up
the length of the stripline to the length of the second tuning
marker (you should see two tuning makers there on the board). If
you can't see the tabs on the board, make the stripline as long
again as it is wide. Solder two wires onto the unserside of the
p.c.b. One goes to the groundplane on the underside, and the
other goes to the 4V input pin, located in the stripline you
modified at point #2.
Put the board to one side for a moment, and using some desolder
braid, clean all the excess solder from the inside of the can's
soldered seams. This makes things easier to do the alignment
with. The can should ideally be a loose-ish fit over the
oscillator p.c.b now.
For the fortunate few, there are spectrum analysers, these will
instantly tell its operator when something is working and when
something is not, what frequency it is on, and whether the signal
is pure or not. For the dabblers however, there is a less
elegant, but viable alternative
I use a satellite receiver with an FSS LNB to look for signals
around 9 GHz. This makes a cheap 9 GHz monitor for close-up work.
In an ordinary (FSS) LNB the wanted sideband to be converted is
in the 10.950-11.750 GHz band. This is the upper sideband that is
block-converted. The lower sideband however is in the 9.050-8.250
GHz range. Although filtering inside the LNB normally removes
this unwanted LSB artefact to improve both the noise figure and
rejection, when there is a few mW of 9GHz around, it still
manages to get though the tuned amplifiers and filtering to
appear as a signal on the satellite receiver. This lower
sideband, however, tunes in the opposite direction to normal, and
so the tuning of the satellite receiver is reversed, which means
the higher in frequency we tune up on the satellite receiver, the
lower the received signal is. However, at 9GHz it's simple 10 GHz
- 9 GHz = 1 GHz, so expect to see the 9 GHz signal around the
1000 MHz mark on the satellite receiver.
That's the theory. Typically you could be looking around +/- 80
MHz of the 1 GHz figure, so you have to scan around with the
satellite receiver when you're looking for oscillation. Also the
LNB is pretty deaf at 9 GHz, so more than a foot away, and you've
lost the signal altogether. However these methods, although
time-consuming are my only practical means at present, and they
work for me - this is how this unit and a few more were done.
Using a 9 GHz DR, or a 9.75 GHz one especially
sandwiched* for the purpose, place the DR slightly
offset from its original position, without a spacer, more over
the stripline on the gate of the oscillator transistor. Apply 4V
d.c to the oscillator p.c.b, and bring a metal object over and
above the DR. (I use the handle of a scalpel knife). If you don't
see any sign of oscillation (not untypical) then experiment with
the positioning of the DR by moving it around with a pin ( I use
the sharp edge of the scalpel). Should the slightest whiff of
oscillation be seen whilst this is done, stop, and bring the
metal object back over the top of the DR, moving it up and down
over the DR until oscillation is seen. If you see this, then you
have the DR in almost the right place. It is hard to describe
this practical work in an easy way, as seeing is the only way of
learning such techniques. However I did learn them out of my own
instincts. If you don't see oscillation, repeat the whole
procedure, but instead bring the can of the unit above the DR,
and move it up and down to look for oscillation.
Once oscillation is established, make a mental note of the
position of the DR, and sparingly apply some slow setting
adhesive to the underside of it (cheap nail varnish is ideal, I
prefer the clear type, as coloured varieties may have different
dielectric properties). Look for oscillation again, and when
satisfied, try to get the unit to oscillate nearly on frequency
with the can nearly on. This is a certain sign that you've nearly
done it.
If you can get oscillation when the can is on, at or around the
desired frequency then leave the DR to set, otherwise just keep
moving the DR around by small amounts and keep checking until you
can get the oscillator working. You have plenty of time as the
varnish takes a while to set.
Once the oscillator is working, check it will work repeatedly, by
turning off and on the 4V d.c supply to make sure that the
oscillator will start up every time. This is very important, as
we don't want the LNB to become tempremental. If all is OK then
leave the whole thing to set for a day.
When the adhesive has had time to set, power up the oscillator
and check that it still works like when you left it. If it
doesn't then you will have to pry off the DR by softening the
adhesive (nail varnish remover), and then repeat the whole tuning
process again. If it does, however, you can proceed to put the
whole LNB back together again.
Solder the four corners of the p.c.b to the can using the minimum
amount of solder. Re-check that the oscillator is still working.
Bear in mind that it will change frequency a little after the can
is soldered. If all is still OK then solder the p.c.b back to the
can using the minimum amount of solder and heat to make a good
soldered seam. You have now successfully modified the local
oscillator to 9GHz and deserve to pick up some good DX on 10 GHz
for your efforts!
Put the oscillator block back onto the RF p.c.b, and solder the
LO module to the RF p.c.b first, at each end, like before. Make
sure the module sits right down on the RF p.c.b as there is not
much clearance inside the LNB. Then carefully re-solder the two
feedthrough pins on the top side of the RF p.c.b, with just
enough solder. Carefully clean up the waveguide probe hole, and
the probe itself with desolder braid, then again carefully put
the RF p.c.b back into the LNB enclosure, making sure that the
linkage p.c.b goes through to the IF p.c.b correctly, and that
the waveguide probe pin goes through the hole on the RF p.c.b
correctly also. Fix all the screws you removed from the RF p.c.b
and solder first the IF p.c.b linkage side, and lastly the
waveguide probe back - look out for that static again!
Now put the LNB on your favourite satellite receiver, and tune in
a workshop generated 10 GHz signal. You should find it just where
it should be. If not, then track your steps back and check
through everything as before. If all is OK then you've got
yourself a good sensitive and very small 10 GHz LNB.
At this moment in time, I haven't re-tuned the amplifier stages
or re-aligned the interdigitated filter for 10 GHz, but they seem
to work pretty well as they are, certainly better than other
similar types of LNB in this class (I.e Grundig / Uniden types).
Although this conversion is more involved mechanically than most
other units, it seems worthwhile to do. My motive was to get a
physically small waveguide type LNB for use in a confined space.,
as these LNBs are easily and cheaply acquired at present, it
seems a shame to waste a useful unit. With newer LNBs having only
a feedhorn input, and the increasing number of digital services
appearing in the UK you have to make hay whilst the sun shines,
and get hold of these waveguide ones before they're all gone!
A.M October 1998