Embedded Files
This project is the WA7NBF Wideband Low Voltage Offset Attenuator. Other active offset attenuators use a 9 volt battery for power. The WA7NBF approach is to use a 1.5 volt AAA size cell. This is made possible by the use of the ALD110900 E-pad dual MOSFET which is designed for zero threshold. Data sheet for the ALD110900 is here. The WA7NBF design also uses a standard 2 pin crystal. The crystal can be socketed so that 1MHz, 2MHz, or 4 MHz crystals can be selected.
A Wideband Low Voltage Offset Attenuator
by Neil Robin, WA7NBF Revised: April 7, 2010
Schematic for WA7NBF Low Voltage Offset Attenuator
All the circuitry below R2 potentiometer is for the oscillator and power. R2 used to be available from Radio Shack and was inexpensive. D1 is the mixer diode with the LO injected via R1. The return path is through R3. As the wiper of R2 travels more negative a small DC bias is placed on the diode to help its conductivity. When the wiper is in its low sensitivity position, the bias is removed but a strong RF signal coming in will bias the diode with C1 holding the offset voltage. Generally, its preferred to keep the oscillator circuit ground separate from the chassis but you can try it either way to minimize leakage of the LO.
Parts List and Estimated Cost
The following parts list is NOT an exact bill of materials NOR is it intended to be used to place an order from parts suppliers. It is strictly used to develop an approximate cost estimate for the group build so that initial funds can be collected. Additional work is needed to develop the final bill of materials. Building a prototype, first, should minimize the amount of bill of material errors.
Hammond Flanged Aluminum Die Cast Box Drawing: http://www.hammondmfg.com/pdf/1590AFL.pdf
Dual N-Channel Zero Threshold™ EPAD® Precision Matched Pair MOSFET Array ALD110900 Datasheet: http://www.aldinc.com/pdf/ALD110800.pdf
Perf board and printed circuit board Fritzing cad files
An initial board layout for has been created for the 1.5 volt active offset attenuator. Fritzing is used for the layout program because it has schematic capture, breadboard, perfboard, and printed circuit board capabilities.
Install Fritzing to view / edit the cad files for the 1.5 volt active offset attenuator. Download Fritzing from here: Click to download Fritzing from http://fritzing.org/download/
A Rough Draft of perf board and printed circuit board layout files for the 1.5 volt active offset attenuator follow below. DO NOT use the following layout files to create a functioning circuit without first performing a thorough review. The layout files provide an example of how Fritzing may be used for this project and HAVE NOT been checked for correctness. For example, the board layout is not correctly sized to fit in the Hammond Flanged Aluminum Die Cast Box mentioned above. Also, the circuit may NOT function if the layout files are used as is. Building a prototype, first, should minimize the amount of layout errors. The files are provided "As Is" and "As Available". Warren County Amateur Radio Association (WCARA) does not make any express or implied warranties, endorsements or representations whatsoever as to the operation of the 1.5 volt active offset attenuator if the following files are downloaded or used. Download Fritzing files for the 1.5 volt active offset attenuator by clicking on this text.
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