NimbleSig III Construction Tips
have decided to build up another NS3 module to try out some different
output transformers and would like to relate some construction tips
based on what I notice as I build this one up. I hope this information
is found helpful. I plan to build on this page as I get feedback from
others. I will give contribution credit for any received tips
that I add to this list.Mechanical Layout
SMB Connector Mounting
- I do the mechanical layout and chassis work first before the mounting of any parts on the PC board as shown here: Chassis Preparation. I use the blank PC board as a template for marking holes for drilling the chassis. If your plan is to mount the PC board with #2 machine screw standoff
hardware then the board corners should be drilled first. The
positioning of these holes is critical. Using my drill press I
first drill a 1/16" pilot hole followed by 5/64" and then the final
3/32" hole which is slightly over sized for #2 hardware. Using a square
needle file I then file notches in the connector board edge to mate
with the coaxial connectors. The notch depths should be filed up to the
edge of the center conductor connection pads to fully remove the narrow
ground strip along the board edge. Otherwise the center conductor may
become shorted to ground when soldered as the filing disrupts the
solder mask protection. A photo of a board I prepared for a die-cast
aluminum box can be seen here: PCB Prepared for Mounting.
Parts Placement Reference
three SMB connectors used for the DataIN, DataOut and RF_In are too
close together for the connector flange hex ridges to clear. This
spacing was predefined and dictated by the surplus enclosure design.
This may not be an issue if you plan to use an alternative enclosure as
the data lines, which are not particularly critical, can be wired out
to an alternative connector mounted away from the RF coaxial
connectors. If using the same layout as dictated by the surplus
enclosures install the two outside connectors first and then use a #8
lock washer as a spacer for the center connector. This will allow the
flange nut ridges to clear. You will need to spread the lock washer
open a little to clear the #10 SMB connector threads. Refer to these photos: SMB Connector Mounting Photo Gallery
New SMD Parts - Installation Sequence
- For parts placement reference make a color print out of the parts placement pictorial at: NimbleSig
III PCB Parts Layout Pictorial
. It is relatively easy to read this blowup pictorial
compared to the PC board silkscreen and there will be less chance of an
error if you use them both.
- Note that the IC pin
"ones" are identified with a black dot not to be confused with the vias
which are purple on the pictorial.
is quite easy to orient the RF detector chip and EEPROM the wrong way.
Viewing the board with the silkscreen text the right way up the pin
ones for these two chips are at the bottom right corners.
With the experience of building up a half dozen or more of NS3 modules under my belt I have settled on the sequence described below. Note that I use conventional soldering techniques with a very fine tipped temperature controlled iron along with very fine (0.015") solder. I use a good quality 7X dissecting type stereo microscope along with good florescent (daylight color temperature) lighting. This information will be of limited value to those set up for more advanced techniques. The sequence that works best for me is as follows:
- build the low pass filters up with the tiny 0402 components
- build and test the power supply sections. (Note: If
you are soldering by hand it is easier to install the relatively low
profile power regulator chips, resistors and ferrite beads that are
located close to tantalum capacitors prior to soldering in the higher
profile tantalums. Otherwise the tantalums tend to get in the way of
the tweezers. Similarly do not install the JTAG connector until after all the small passive components have been installed.)
- install the remaining passive components, re-check the power supply buses
- install the EEPROM and AD8307, re-check the power supply buses
- install the MPU, complete continuity tests for adjacent pin shorts and re-check the power supply buses
last and probably most critical step is to install the AD9958. I have
written a more detailed procedure for this step which can be found
SMD Parts Management
Low Pass Filters
most of the SMD components cannot be identified as they are not marked
and many have values that cannot be readily measured parts organization
and placement management is critically important. If the parts get
mixed up it is a nightmare.
- As shown in the linked photo I use labeled pill dispenser organizers for
the components. These two pill cases can hold up to 56 unique values. I
don't bother keeping the common 0.1uF bypass capacitors here as I buy
them in 100 lots and just leave them on a long tape which is well
- When I receive the parts shipment I mark the back side of each tape with the value with either a pen as shown here or by using my trusty scissors I cut the value out of the spare label provided with the part and stick it on to the back of the short section of tape.
I am installing parts I only take out one value at a time and place
them into a staging area for pickup. I found the inside of the lid for the Bud enclosure
with a small sheet of plain, white paper placed within the ridged
perimeter works nicely for me. Once I am done with one value I put any
remaining back into the appropriately labeled pill organizer
compartment and then (and only then) move on to the next value.
JTAG SIP Connector
- The inside rows of 0402 components consisting of the parallel resonant circuits should be installed first as shown here. Then the bypass capacitors can be installed from the outside edges last.
install these components by tinning one of the pads first whilst
leaving the second bare. Then while viewing with magnification the 0402
component can be slid into place with the tweezers in the left hand
whilst the solder is reheated with the iron in the right hand (or visa
versa). Notice the center pads have be pre-tinned with solder in preparation of the installation of the bypass capacitors as shown here.
MPU Idle Loop Execution Pin Voltage Reference
JTAG SIP connector should be the last component installed on the board
as it is relatively high profile and tends to get in the way.
- I suggest this component be soldered in manually even if a soldering oven is used for the rest of the board.
- Prior to soldering I cut the pins off flush to the bottom of the board as shown in the sip connector install photos.
- For troubleshooting reference the MPU pin voltages measured while NS3 was running within the idle loop are available via this link.