Micro908 Kit - Service Pack

   Over the course of the first round of QRPers assembling and using the Micro908 "Antenna Analyst" Kit, we've determined several optimizations that substantially improve usability and accuracy of the instrument. These optimizations consist of a small handful of replacement components that owners should install on their circuit boards, and a new software release that takes advantage of the optimized hardware.

   On Nov 19th, a "Service Pack" was being mailed to each of the 200 original first-round customers of the Micro908 Kit. Most owners will soon receive the 4"x8" padded manila envelope. If your address has changed since receiving the Micro908 Kit itself, or if you've passed the kit on to a new owner, please let us know of the address correction.

Benefits of installing the Service Pack:

   1) Instrument accuracy is significantly improved. Best-case results of 1% accuracy have been seen some trial units, although +/- 5% is typical.

   2) Calibration has been dramatically improved. Only a single adjustment is required, using just two resistors. (No more clicking through 10 adjustments on each of three load resistors!)

   3) Current consumption has been reduced. No more burned fingers on the DDS card's MMIC or its bias resistor; and the batteries last longer.

   4) Harmonics from the DDS-generated RF test signal are down more than 30 dB from the fundamental due to better gain balancing in the system (DDS signal levels through the ERA-3 amplifier and into the reflectometer and buffer amps.)

   5) New software features. In addition to supporting the new/improved calibration steps, the latest "AA908" software (version      4.0) also provides .

       a. Calibration parameters are stored separately from the user- configurable settings in EEPROM, thus speeding up the "Saving to EEPROM" steps.

       b. The sign of the reactance is now determined and displayed as +jX (inductive reactance) or -jX (for capacitive reactance).

       c. Three new Modes have been added beneath the Mode pushbutton:
            Inductance measurement,
            Capacitance Measurement, and
            VFO signal generation.

       d. Several smaller bug fixes.

Hardware:
   The Service Pack contains the following hardware components that replace existing ones on the Micro908 base pc board (and several on the DDS Daughtercard).

   1) ERA-3 MMIC amp - This change in the RF amplifier on the DDS Daughtercard provides a more constant signal level across the HF spectrum, as compared to the MAV device we previously used, thus making instrument calibration more easily achieved.

   2) 12.1 K resistor - This change for R1 on the DDS Daughtercard programs a lower signal from the DDS for presentation to the new ERA-3 MMIC amplifier.

   3) 130-ohm resistor - This new value for R4 on the DDS Daughtercard biases the ERA-3 amplifier at 35 ma, providing for much cooler operation.

   4) 71.5K resistors - These lower values for R51, R19, R24 and R29 on the Micro908 pcb provide lower gain to accommodate the different RF signal level coming from the ERA-3 amplifier.

   5) 220 pF caps - C17, C19, C21 and C41 are used in the reflector as the "integrating capacitors" on the Micro908 board, providing faster response in the signal detection process, as compared to the 0.1 uF values originally used.

   6) 49.9-ohm and 274-ohm resistors - These 1% parts are provided so you can perform calibration without scrounging your junk box for appropriate valued resistors.

Installation:
   Just remove the original components, clean off the pads with the Easy Braid solder wick, and solder in the new components. Be careful about component value (double check markings in the Contents list provided in the Service Pack) and the orientation of the ERA-3 amplifier (the dot goes "up").

Software: AA908 v4.0
   This latest software will be posted this weekend to the project website for all to download. Once the Service Pack hardware changes are made, you can load version 4.0 into your Micro908 and experience the benefits described here.

Thanks & 73,

~George N2APB n2apb@amqrp.org
& Joe N2CX n2cx@amqrp.org
Micro908 Kit details and ordering is at http://www.amqrp.org/kits/micro908

PS: The next round of Micro908 Kits being prepared seamlessly include these improvements. Those who buy a new kit will not need to make the Service Pack updates.

Material and concepts presented on the AmQRP website is Copyright 2005 by the American QRP Club, Inc.
These pages are designed and maintained by George Heron, N2APB (n2apb_at_amqrp.org)
Page last updated: March 19, 2005