Using Linrad on microwaves... it can help

0. Introduction.

1. Description.

2. Findings.

3. Time machine.

10/20/06 7:35 PM
Leif Asbrink

Have a look at the schematic diagram here:
http://www.expandedspectrumsystems.com/prod2.html

In early versions C8 and C11 were far too small (0.1 uF I think)
Check that they are big electrolytic capacitors.

If I were responsible for the design I would break the DC
connection between the output ground and the power supply
as well as between the output ground and the RF input ground.

One way would be to connect J3, pin1 with wires to all of the 
ground points of the AF circuitry so all of them would be
separated from the ground points of J1 and J5. 

It means the groundings of U1,U2, R5, R7, C10, C22, R4,R13
and C11. Then connect a 1k resistor from the J3 ground to the
J1/J5 ground. 

There will be a need to connect the U1 and U2 grounds to the 
J1/J5 ground with decoupling capacitors to leave the RF current 
path unchanged around the mixers. The U3 and U4 ground points 
have to stay connected to the J1/J5 ground since they carry
DC currents.

It may be a good idea to reduce the coupling and decoupling 
capacitors C14, C20, C25, C26 and C12 to 10nF and use the same
value for the new decouplers at U1 and U2. The sum of all these
capacitors should have a high impedance at the frequency of
the LF noise.

                       ---   o   ---

Rather than modifying the unit you might try to operate it from
a battery with nothing connected to the RF input (or any other 
point on the unit).

This is an experiment: Just a single cable must be allowed. 
>From the Time Machine to the soundcard with a battery to supply 
DC. You should then not see any LF noise:-) If you do, the reason
has to be pick-up of magnetic field or perhaps, but less likely
pick-up of electric field. Wrap the board plus battery in plastic
foil and then wrap household aluminium foil around and connect 
the aluminium foil to a wire that goes to any ground point of the 
Time machine. This will eliminate electric fields completely.
If something is left it is magnetic. Then move the unit around
and you will soon know the source of the magnetic field......

If you can not move the source of the magnetic field away you
can get rid of it with thick copper (3 mm or more) and/or 
transformer steel. (Junk a big transformer to get the material)

                       ---   o   ---

I would guess that you will not see any pick-up of either electric
or magnetic fields. There will just be a nice-looking noise floor
when you operate from a battery without any input signal.

If you then connect the 8012A you will most probably get the
LF noise back. You can then connect the 8012A through an RF
transformer and do the calibration. Maybe you will want
to use a transformer always..... (Easier than rebuilding the
unit)

Finally when the unit is calibrated and operates without AF
noise in your system from a battery you might find that the
12 V power supply can not be used without getting the AF noise 
back.

There are two different mechanisms that both can be cured fairly 
easily: 

1) Ground currents. Make sure that the 12V supply is floating
with respect to every other ground point. Then route the 12V
DC line to the PC and connect its ground to the ground of the 
soundcard (use a d-sub screw) Then route the 12 line parallel 
to the audio cable (twisted is fine) up to the Time Machine.

2) Inadequate filtering. Just put a big electrolytic capacitor
on the output of the supply. 10000uF perhaps.


All the above assumes that the Time Machine has its own 12V supply.
Surely you can use a common supply, but then you have to think 
carefully where the ground loops are and what current they carry.

With the re-design suggested at the beginning ground loops and
AF noise carried by them would not enter the soundcard. 

WARNING. I have not tested the modification (I have no
Time Machine) but I do think it would work fine. If you
destroy a board testing it without sucess I take no 
responsibility.


By the way, I think R7 and R5 should be connected in series
with C19 and C21 and not to ground. Then there should be
capacitors parallel to R13 and R4. This way the RF components
would still see 150 ohms to ground (50 ohms would be better)
while the audio output would be loaded by 500 ohms. The advantage
would be that the MAX410 would not be loaded with very strong
RF signals (at RF+LO) It can produce no more than 4.5 V/us
at the output so RF signals easily cause intermodulation
that appears in the audio output.