[aprssig] NMEA ISSUE HELP

[Stephen H. Smith]

On 2/5/2011 4:55 PM, Andrew Rich wrote:
> If you want the GSP-18 to put out "real" RS232 you need to invert the data 
> and put it through some like a MAX232 chip, that goes TTL to true RS232
>

Or you can use ordinary op-amps in non-inverting mode.   Power off a small 
split +/- 5 to +/-12 VDC power supply.       Connect the inverting input 
through to the center point of a voltage divider make from a 100K and a 150K 
resistor.  Connect the 100K to the positive supply and the 120K to the negative 
supply.   (This biases the inverting input slightly positive with respect to 
the PS center point/ground, thus making the output of the op-amp rest at -volts 
with no signal on the non-inverting pin.     Connect the GPS-18 signal to the 
non-inverting input of as many op-amps in parallel as you need for the number 
of outputs.   Connect the simple voltage divider to the inverting pin of all 
op-amps in parallel.    I use LM124/224/324 dual op-amps for this application. 
   About USD .50 each.

Essentially, the op-amps running open-loop (no feedback) are acting as voltage 
comparators.  At the data rate of GPS (4800 baud) the op-amps have no trouble 
reproducing a clean squarewave.  The limited slew rate and ;bandwidth is 
actually a bonus in this application, as it acts as a free low-pass filter to 
clean high-frequency noise off the incoming waveform.   (The MAX-232 type 
devices have almost too much bandwidth and will pass noise readily.)

In my mobile installation, the input to the non-inverting pins consists of a 
4.7K resistor to neg power, and an NPN opto-isolator's output to plus power. 
The input of the opto is driven directly by the GPS-18 signal.   This scheme 
can provide complete ground isolation between the GPS and multiple loads.

For the split power supplies, I use tiny low-power isolated switching DC-DC 
converters made by Astrodyne.   These devices are about the size of a standard 
16-pin .3-inch-wide through-hole IC, and solder to a PC board.   The particular 
model I usually use is here:

<http://www.astrodyne.com/ecatalog/usa/dc-converter/DUP75-12D12>

Or browse the full range of offerings here:

<http://www.astrodyne.com/ecatalog/usa/dc-converter>

Some of the versions of these devices can produce a constant +/- 5 or +/- 12 
VDC on an input voltage ranging from 9 to 18 VDC, making them ideal for mobile 
radio applications.


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--

Stephen H. Smith    wa8lmf (at) aol.com
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