M620 - DDS/Controller Board



PART 2 – VFO Controller Board Construction

The concept of the controller board is to provide a simple and stable local oscillator source controller as well as providing a user interface and some metering capability. Typically these bands use a frequency doubled VFO, which are usually touchy, or Crystal controlled which limit the frequency. A DDS probably not the most cost effective way to go, but it is certainly accurate and flexible. The design also incorporates A/D inputs that form a voltmeter to be used to sample the battery voltage as well as a signal in and power out for the display.  The first prototypes simply controlled a DDS-60 daughter card because the DDS was hard to prototype due to the fine SMT package pitch pin put of these devices.  Provisions were made to incorporate the DDS circuitry on the board as well; just not as elaborate as the DDS-60’s.

The cornerstone of this design would be the 8x2 LCD display – HDM08216H-3-S00S - which fits in the proposed case.  [C], [D] The reason I specified this exact part is because the LCD attaches directly to the controller board which is just slightly larger than the display, so the footprint needs to correctly line up. And it was also the “thinnest” one available.  The 16-pin connector actually does a pretty good job of supporting the display, but as a percaution, an insolating layer made out of an old business card was trimmed and tacked to the bottom of the LCD display with a couple dabs of glue as there are some metal tabs that come perilously close to the circuits of the controller board.   I later added a couple of screws and standoffs for a little more mechanical stability.  

The other main components that the proto design uses are a SMT PIC 16F88 [A] and the AD9851 DDS [B] plus a rather curious combination of other thru-hole and SMT parts.  The main reasoning for some parts which could only be found in SMT and some are simply what I had on had to build up several boards.

 Building the board using a technique using fine wire solder on each IC pin, followed by a solder wick to clean up any shorts between the pins worked quite well for soldering the ICs.   Once the board is built, it is programmed via the provided programming header [G]. Install the LCD directly to the controller board after some perliminary testing.