Micro908 KIT


A collection of construction Tips & Techniques, and various Errata for the kit.


The Notes listed on this page may seem like a lot, but for the most part they are just extra useful guidance to builders of the Micro908.  Let me know if you have any questions about this information and I'll gladly help you out.  
~George N2APB n2apb@amqrp.org 

COMMON ISSUES (items apply to all kits)
Max Supply Voltage is 12V <== NEW Battery Matters Using older HC908 Cards Test Points
Loading New Software Fitting Battery Holder in Enclosure P5 Programming Port Soldering Assistance
C33 & C34 Polarity on PCB U1 Mounting Hole Correction Cutting the Encoder Shaft Aux Connector J13 or J14?
Calibrate before Use PCB-to-Enclosure Alignment The MON Jumper Folding the LCD jumper "W1"
Sleep Pushbutton Use? Little black Boxes on LCD U6 Mod for DSPx card use Small standoff holes for DDS card
USB adapter for serial control Ticking from speaker C6 orientation on DDS Amp End Panel Overlay Label Error

FIFTH-RUN KIT ISSUES  (kits shipped ~ Nov-Dec 2006)  
    (This run shipped with the DDS-60.  Also, pcb mods for mounting, C33 & C34, and U6 were done at the factory.)
DDS Chip Alert <== NEW

FOURTH-RUN KIT ISSUES  (kits shipped ~ Feb-Mar 2006)  
    (This run shipped with DDS Amp grandaughter card in DDS card bag)
C33 & C34 Polarity on PCB 12.1K resistor not needed

THIRD-RUN KIT ISSUES  (kits shipped ~ Aug-Oct 2005)  
    (This run shipped with DDS Amp grandaughter card in DDS card bag)
C33 & C34 Polarity on PCB 12.1K resistor not needed

SECOND-RUN KIT ISSUES  (kits shipped ~ Feb 2005)
   (This run shipped with a MAV-11 RF amp in the DDS Daughtercard and 'Rev A2' marked on the pcb and had the side
    mounting holes in the pcb already enlarged.)
C33 & C34 Polarity on PCB
10K Trim Pot for DDS Card DDS Card Upgrade Parts Orientation for the MAV-11

FIRST-RUN KIT ISSUES (kits shipped ~ Sept 2004)
   (This run shipped with the MAR-1 or MAR-4 or MAR-6 MMIC amp in the DDS Daughtercard bag)
C33 & C34 Polarity on PCB Filing the PCB to Fit Enclosure Using older HC908 Cards Test Points
10K Trim Pot for DDS Card DDS Card Upgrade Parts Two R49 Resistors "C25" vs. "C35"

DDS Alert -- If you recently obtained a free sample of the AD9850 DDS chip from Analog Deveices for use in your Micro908 Kit containing the DDS-60 card, you need to read this.

Max Supply Voltage is 12V -- We had improperly stated that the supply voltage for the DDS-60 card could reach as high as 16V, making it convenient for use with power supplies on the workbench and within the Micro908. However, we recently discovered that the maximum voltage stated in the spec sheet for the AD8008 amplifier used on the DDS-60 card is 12.6V, thus we strongly recommend not using any higher voltage than this for your application using the DDS-60. We have not heard of any AD8008 failures when powered with a higher-than-spec voltage, but it is definitely recommended to ensure operation of the IC within its specified supply voltage range. We apologize for any inconveniences. [In an earlier design of the "DDS Amp" circuitry, the designers had used a similar op amp that was indeed spec'd for a higher supply voltage, and they neglected to catch this difference voltage specification when changing to use of the similar AD8008 op amp.] 
     So what this means for Micro908 owners is that you should only use up to 12V DC as your external power supply for your Micro908.  If you use rechargable batteries in your unit, you will notice a lower charge current (if this is monitored on your bench supply) and it may take longer to achieve a maximum change level.  Further, that maximum charge level may be lower than you were previously accustomed to, but still sufficient for longer use in the field.
     To possibly save yourself the concern of powering the Micro908 with >12V, or for example if you have a bench supply that is fixed at 13.6V, you could use a 3-terminal regulator to limit the voltage being supplied to the instrument, as illustrated below ...

Need help soldering all those SMT components? -- We've lined up a great resource: Mike WA6OUW, at “KitBuilders” (email: wa6ouw@aol.com) to assist in soldering all SMT components onto the DDS Daughtercard, or onto the Micro908 motherboard, or onto the IQ-VFO pcb, or onto any other project pcb you might need help on.  In fact, he will assemble any kit for you for just a modest fee.  Just contact Mike by email in advance to find out what he will charge for your job and then write a check or M.O. for that amount payable to “KitBuilders” and send it with the kit components to: KitBuilders, 6361 Berrybush Ct., Gilroy, CA  95020 .  He will promptly assemble all components and  return your kit by mail. Mike doesn't test the kit when done, but it's an absolutely fabulous soldering job ... which says a lot for all the surface mount components on the boards these days. The AmQRP uses KitBuilders for assembly of the HC908 Daughtercard product, so we know the quality is there!     

Orientation for the MAV-11 Amp on the DDS Daughtercard -- The photo of the MAV-11 on the DDS Upgrade sheet shows the proper orientation for the device, with it being on the lower end "output pin" of the device.  The pcb silkscreen is wrong - it was intended for the older MAR devices (and the newer ERA devices), which have the dot on the input pin.  The schematic is also wrong, showing the dot on the input pin instead of on the output pin.  

Proper orientation for the MAV-11 amp on the DDS Daughtercard

Loading New Software into the Micro908 -- There are three ways to get software updates into your Micro908. Any one can be used, but people have been finding the best luck in the order shown here ...

Method 1) Select "Software Load" in the Config menu -- With TeraTerm running on your PC, and with the
                Micro908 connected to the serial port of your PC ...
  a) select the "Software Load" item in the Micro908 Config menu
  b) see the HCmon prompt on the PC screen
  c) clear the HC908 memory (type 'C' <enter>)
  d) load new program (type 'L' <enter> and see  'waiting ...')
  e) select TeraTerm's 'Send file ...' item and navigate to the place where the S19 file is that you wish to
      load, and then select it
  f) see line-after-line of text data being sent to the Micro908. When done, the HCmon prompt will be
     displayed again;
  g) cycle the power on your Micro908 to restart the program and see the newer version number displayed
      on the LCD.
  h) immediately select "Default Settings" in Config.

Method 2) Select "Debug Monitor" in the Config Menu -- This method transfers control to the Debug
                 Monitor that talks to the PC over the serial port.  So with TeraTerm or HyperTerminal running
                 on your PC, you can manualyy clear memory and load a new program ... just follow steps 
                 b-to-h above.

Method 3) Install the P3 MON shunt and enter the low-level Debug Monitor when Micro908 is turned on. 
                 Follow the same steps (b)-(h) above, and remember to remove the MON jumper before
                 restarting your Micro908. 

Little Black Boxes on LCD -- When running the AA908 software for the very first time on the HC908 Daughtercard, you will experience a 15 second delay after turning on power, during which time the LCD shows nothing but "little black boxes" in the top line.  Then, the sign-on display will show. This delay occurs because the software checks the EEPROM during initialization and if it sees that the on-chip memory is blank, it will initialize it with the default settings before proceeding, which takes about 15 seconds.  Subsequent power-ons will see the EEPROM memory as being already written, and this delay will not be incurred.  In the next software version I will change the software to only load the essential default settings, which takes less time, as well as display an "Initializing" message.  So the moral of this story is ... upon the first-ever power-up of the HC908 card with the AA908 software, it could show "little black boxes" for up to 15 seconds before the normal display shows.  (By the way, another symptom of these "little black boxes" being displayed is that you indvertently left the MON jumper on P3, directing the software to be talking to the serial port instead of to the LCD.  Just remove the MON jumper and restart the Micro908.)

Small holes in PCB for DDS card standoffs --  If the builder wishes to use one (or both) nylon standoffs for the DDS Daughtercard, the standoff holes in the pcb need to be slightly enlargened for the nylon screw to go through. Be careful not to cut any nearby traces in the process. Same too for the other mounting hole near the "Vf" test point, although I normally leave that one off, with just the one on the right being sufficient to ensure that the DDS card doesn't go down against the motherboard.

Battery Matters  -- Here are some notes all related to this topic ...

1) Higher-value should be used for R47 -- The value of R47 (12 ohms) is too low to charge batteries with elevated supply voltages.  A conservative value would be 47 ohms (1/2 W) for use with an external supply voltage of 13V.  With discharged batteries the charging current would be about 100ma, dropping to about 66 ma with the batteries fully charged.  This is a pretty good trickle charge. R47 must be increased if the external supply is higher than 13V.

2) The NiMH shunt -- This jumper is used when you wish to provide a trickle charge to some NiMH cells you might have in the battery holder.  The shunt could stay in place longer term, as long as you keep an eye on the rechargable batteries such that they don't overheat due to overcharging when connected permanently to the external DC supply on the bench.  My practice is to plug the instrument into the external bench supply overnight to charge the batteries and always use the instrument using batteries (i.e., disconnected from the external bench supply).  This always leaves me with topped-off batteries, it "exercises" the batteries to keep them in good condition, and provides one less "teather" for the unit (i.e., the external power cord is not connected when I'm using the instrument.)

3) Can the unit operate with the NiMH jumper off? -- Yes, the unit will work just fine without the jumper in place - the batteries will just not be trickle-charged.  Also, with the jumper open,  it's no problem using alkalines batteries.  They won't last too long considering the several hundred ma drain, but there will be no interaction between the external voltage source and the batteries with the NiMH jumper open.

4) Can I use Nickle-Cadmium (NiCd) rechargable batteries instead of NiMH? -- NiMH batteries are fine since most have a mA-H rating of greater than 1000. NiCds with at least this rating should be ok as well, but the more common 450-600 mA-H types will see a somewhat higher than usual charge current. That being said, they can probably be used if you limit charging times to no more than 5 or 6 hours to minimize long term overheating.

5) How low can the batteries (or external source) go before the unit stops operating? -- The Micro908 can operate on a power source (batteries or external) as low as 9V-or-so.  Below this point, the 8V voltage regulator on the DDS Daughtercard will start to be affected, the DDS-generated RF signal will degrade, and the erroneous/erratic readings will be seen..

6) Maximum Supply Voltage -- The highest external voltage source you should use is 16V.  Some walwarts (or AC adaptors in general) produce a much higher voltage that could cause problems longer-term with your Micro908.  A quick and easy check is to power the unit with your AC adaptor and measure the terminal voltage with the Micro908 turned on.  If the DC voltage is below 16V, you should be just fine.

7) Batteries App Note -- See this brilliant treatise on rechargable batteries, as provided by our very own Joe Everhart, N2CX.

U1 Mounting Hole Correction -- The mounting hole for the 7805 regular (U1) was mis-position in the pcb and will result in the heatsing interfering with some other components.  The solution is easy ... 

1) Bend the legs on the regulator per normal and insert them into the pads with the heatsink loose between the regulator and the board. The heatsink becomes the "spacer", if you will.

2) Hold the regulator tight to the heatsink and board and solder the leads. 

3) Slide the heatsink out, and using the hole of the regulator tab as a guide, drill a hole in the pcb. 

4) Slide the heatsink back in and attach with the nut and bolt.

Don't Forget to Calibrate Before Use -- When first powering up the Micro908 with the AA908 software in it, the display actually read 0 +j 49 with a 50-ohm resistor attached to the RF jack!  I said "Somethings whacko here" and tried another board ... same results.  Then I recalled that these were newly-programmed HC908 cards and the Micro908 was UNCALIBRATED.  Moral of this story is ... don't forget that a Calibration is first needed before valid results can be seen.  I will see about putting default calibration data in place during initialization, but that will not replace the need to calibrate.

End Panel Overlay Label Error -- The end panel overlay has the labels reversed for "Aux" and "Keybd". Funny, you don't know how many times we all looked at that end panel!  The Antenna Analyst software doesn't use either of these connectors, and the connectors are different so the plugs wouldn't fit into the other's jack. We'll over-order on the next round and send out replacement overlay labels when ready.

Fitting Battery into the Enclosure -- We forgot to mention that you need to remove the lower two shorter plastic standoffs   on the inside of the top shell.  Doing this will give the battery holder enough room to fit inside the enclosure when it's all buttoned up.  This is easy to do - just take a pair of side cutters (preferably heavy duty ones) and nip off those little standoffs close to the inside surface of the enclosure shell, as shown in the photo below.

Sleep Pushbutton Use? -- The "go to sleep" feature was not implemented in this first version of the PCB because putting the processor to sleep only saves about 1/100th of the power consumed by the instrument.  The other 99% of the power usage comes from the DDS Daughtercard, which is not software turn-off-able. We do have a splendid little "working app note" describing how to add auto power-off capabilities to the board.  We'll be introducing that after the dust settles. So for now, the Sleep pushbutton serves no purpose ... unless we can find a scathingly brilliant way to use it ... any ideas? ;-)

The MON Jumper -- This is the little black "shunt" that fits over the 2-position, 0.1"-spaced pinheader P3 located just to the left of the HC908 Daughtercard on the main pcb.  Under normal operation this MON jumper should be left off.  It is only used when one needs to gain access to the low-level Debug Monitor when the board is powered up.  It's also used as Method #2 of loading software updates intot he Micro908.  (See the Note above concerning this method.) 

Using an older HC908 Daughtercard -- You can indeed use a previously-purchased HC908 Daughtercard that you might have around the shack. You'll need to load the Antenna Analyst software onto it as described in Appendix G of the manual.  (Note: You'll need to use the "alternate" method described there.) This alternate/manual loading works just great though - I use it all the time myself!)

Test Points -- The holes are small but all you need to do is bend in half a small scrap length of wire (like a clipped-off component lead) and insert it into the two holes, leaving a small loop sticking up from the board.  Solder it in place and clip of the bottom two ends sticking out the bottom side of the board.  The loop on the top side will allow you to easily hook up your scope probe, ground clip, or DVM probe.

10K trim pot -- This part is included for you to add to your DDS Daughtercard.  It's part of the mods described in Appendix H on page 39.

P5 Programming Port -- This is an unused connector the part is not provided in the kit.  I added a capability for a hardware programming pod to connect to certain signals that would enable a custom software developer to re-flash (i.e., reprogram) the bootstrap loader software on the HC908 Daughtercard. Hardly anyone will be doing this, so the connector was not provided.  Just leave the P5 set of pads empty.  (Note:  If anyone is interested, the hardware programming pod we use is the "MON08 Multi-Link USB" from P&E Microsystems.)

DDS Upgrade Parts -- For the 19 people who ordered a Basic Micro908 Kit *without* a DDS Daughtercard option, we forgot to include the 5 parts needed for you to upgrade your existing DDS card.  (This DDS upgrade produces a greater and constant RF output signal level in the face of changing supply voltage.)   We'll get these 5 parts out to you in about a week, free of charge because that was what was originally intended.  In the meantime, you can still build up your Micro908 Kit and test out most of the functions, as directed in "Section 6: Power Up and Test".  You could also use your existing DDS Daughtercard (with the lower output) to get some useful signal level at the low end (~1-10MHz), but above that point the RF signal from the original DDS card falls below usable levels for the reflectometer and you'll need to make the mods to your DDS Daughtercard.  Sorry for the omission ... but we'll get you set up properly right away!

"C25 vs C35" -- There is indeed a C35 ... it's in the lowest right corner of the board. The note under the "Capacitor & Diode Card" on page 4 of the manual should read: "C25 is taped to the outside of the bag." The Parts List on page 27, under the "Capacitor & Diode Card" section, for the 0.1 uF cap, should have a count 19 (instead of 18) and should have C25 added to the group.

"Polarity markings for C33 and C34 on PCB"  (for kit runs 1 & 2) -- The silkscreened '+' polarity sign for the SMT electrolytic capacitors C33 & C34 are on the wrong pad of the pcb. The positive side of the caps should be going to the Test Points labelled as Aud-R and Aud-L, respectively.  These two 1 uF SMT electrolytic caps (small silver cans) have through-hole mounting on the pc board instead of surface mount pads. You are still able to use them but you will need to bend one of their small tabs down for soldering to the board.  Also, the pads for the negative sides of C33 and C34 are grounded (this is an error!) and you will not use this pad at all.  You will need to cut the two traces close to the 'pos' holes - these are the traces that go between the positive pads of the caps over to their respective test point (Aud-R and Aud-L).  Then you can install the caps from that positive pad to the trace going to the respective test point, essentially bridging each cut just made.  When installing these two caps, use an Exacto knife or razor blade to bend the negative lead DOWN for insertion into the positive hole, and solder the positive lead to the trace that goes to the respective test point."  See the photos below for guidance.

Cut traces close to each 'pos' pad      Bend down 'neg' lead of each cap.         Insert neg lead into 'pos' hole,           
and scrape away mask from trace.    (Neg lead on side with  black mark.)      solder lead to trace, and then             
Tin each trace with solder.                                                                                         solder neg lead in the pad on bottom.
(Click any photo above for higher-res view)

"Polarity markings for C33 and C34 on PCB"  (for kit run #3) -- The silkscreened '+' polarity sign for the SMT electrolytic capacitors C33 & C34 are on the wrong pad of the pcb. The positive side of the caps should be going to the Test Points labelled as Aud-R and Aud-L, respectively.  
We shipped axial leaded electrolytic caps for C33 and C34 in this 3rd run of the kit.  The pads on the pcb for the negative sides of C33 and C34 are grounded (this is an error!) and you will not use this pad at all.  You will need to cut the two traces close to the 'pos' holes - these are the traces that go between the positive pads of the caps over to their respective test point (Aud-R and Aud-L).  Then you can install the caps from that positive pad to the trace going to the respective test point, essentially bridging each cut just made.  Just solder the negative lead (side of the cap with the black band) into the + hole and the other lead into the hole for the respective test point.

Two R49 resistors supplied -- We mentioned this in another note on page 4 of the manual.  You will only use the 47K resistor, as shown on the schematic.  You can add the extra resistor to your junk box!

U6 Mod -- U6 is the tiny "flea-sized" level shifting IC needed for the HC908 Daughtercard to communicate with the DSP daughtercard. The receive and transmit data traces to the DSP were inadvertently swapped and you need to cut the traces and solder wires instead. See the AF908 user manual page 24: http://www.amqrp.org/kits/micro908/AF908/AF908%20User%20Manual%20v1.2a.pdf

Cutting the Encoder Shaft --  The note in the manual about cutting the encoder shaft was a typo ... should've been "cut off about half of the shaft".  Cutting off about 1/4" and leaving 1/4" will be just fine.

AUX connector "J13 or J14"? -- The schematic designator of the AUX connector is also a typo ... should be marked as "J13" on the schematic.

Filing the PCB fit enclosure -- The production pcb is actually cut more carefully than the prototype boards we used, so no filing is required around the perimeter of the board, despite what is stated in the manual.

PCB-to-Enclosure alignment -- There is a slight mis-match of the mounting holes for the pcb-to-case, but you'll find that the compliance in the holes and the diameter of the screws still allows things to mate up pretty well.

12.1K Resistor Not Needed -- The 12.1K resistor that may be located on the large SMT sheet used in the 3rd kitting round is no longer needed. You will find a 5.6K resistor in the DDS sub-kit that will be used for R1 on the DDS Daughtercard.

Ticking from speaker -- a faint ticking is normal when the volume control is turned up.  It's an artifact of the way the sound is created in the instrument. If it bothers you, just turn the volume control down.

Polarized C6 orientation on DDS Amp --  C6 on the DDS Amp granddaughter card is a polarized tantalum electrolytic, chosen for its very low ESR and hence its good filtering characteristic.  The polarity marking for the POS side is the banded end of this SMT capacitor, and it should be mounted toweard the inside of the little DDS Amp card, as shown at http://www.amqrp.org/kits/DDSamp/index.html

USB Adapter for use with Micro908 -- Expert Micro908 kitter and homebrewer Nancy Feeny, NJ8B, reports that "the [USB adapter] is all that I use because my laptop doesn't have an RS-232 port. Not only do I use it to re-program the Micro908, I also used it with TeraTerm to program all of the HC908 Daughtercards that we do. I have never had a problem using the USB-to-RS232 adapter which I got from sewelldev.com (http://www.sewelldev.com/usbtoserial.asp)"

Folding the LCD Jumper "W1" -- The W1 jumper electrically connects the LCD to the main pcb. If you fold it "under" as described in theAssy Manual, there's a chance that the "stubs" from the clipped-off P2 connector pins may stick up and into the W1 jumper, thus shorting one or more of the LCD signals to ground (or to something else on P2) and making the LCD display of characters quite unusual.  For that reason, I've taken to folding the W1 jumper connection for the LCD on the OUTSIDE, instead of underneath.  See the photo below showing this.

W1 jumper for LCD should be "folded out" (instead of inside/under the LCD) 
to prevent the occasion for shorts to the snipped-off P2 pins on the connector beneath the LCD.

Dummy Loads for Testing -- Both the Assembly Manual and the Technical Reference Manual describe the construction, use and benefit of mounting resistors (and other components) on the end of a BNC plug for use in calibrating the Micro908 and testing the AA908 software.  But I've never shown a close-up photo of the connectors I've been using on a regular basis during development of the Micro908.

This photo shows my array of four connectors used in calibration and checkout of the instrument.  
Shown left-to-right: 50-ohm load, 100-ohm load, 270-ohm load and a complex impedance load (series RLC with trimmer cap across the fixed C). Notice how these are all mounted as close as reasonably possible to the connector ... no long wires are used, thus preventing stray RF pickup and/or introduction of unintended and unwanted reactive readings. Hope this gives you guys some ideas for your own "cal & test fixtures".

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Page Last Updated:  Dec 17, 2006