radio tx and logging to SD card. Problems with circuit isolation?

Hi All,

I have a working sketch that keys a radio module to transmit GPS data, I can successfully receive and decode the data using my radio setup.
The radio tx only needs to happen every 30 seconds or so, and as I’d like to have a recorded log with a higher frequency I am also spitting out the data sentence over serial at 1hz. Again this works fine when I look at it in the serial monitor, and the radio transmission is still solid.

The problem starts when I try to log this 1hz data sentence to SD card. Initially I tried with an SD card shield and found that the radio keying was pulsing when the card was being written to, rendering the receive un-decodable . I put this down to the buffer writing to the card interfering with the interrupt that’s running the radio keying. Then I thought about offloading the SD writing to an external logger.

I now have a Sparkfun Openlog which is a little ATmega328 and sd card slot, hooked up to the hardware serial on my Arduino Uno (it will just log anything tx’d to it over serial). It works, and the logging is working fine, however, i’m still seeing the radio keying being messed up when the card is written to. Now I’m guessing this is something to do with the power being used when the SD card is being written. If I disconnect the Openlog module it returns to normal.

I have another Uno that I tried to power the radio module with seperately, but i still see the interference. I wonder if the digital pin doing the radio keying (high, low) is also effected by the current draw of the logger.

I’m fairly new to the electronics side, and wondered if I need to use some kind of isolation circuit or another power supply with a voltage regulator. I’m sort of stumped at this point.

Thank. I’ve found this forum really helpful so far.

GPEE: another power supply with a voltage regulator.

Quite likely, but not likely to be anything more than a wall-wart.

You don't say how long either the radio transmission or the SD storage take, but would it be feasible just to schedule them so that they don't interfere with each other?

I tried a separate power source (9v battery this time)for the logger which has its own voltage regulator. It seems to fix the problem. I'm going to investigate throwing in some capacitors around the power to see if that helps so that I can run it all on one battery as the current draw for the GPS and radio are very low. I'm guessing at the decoupling capacitor stuff though.

I would be interes5ed in knowing a little more about your setup. Are you using proper radio moeems as the tx rx? Or have you been able to bodge something? I tried this with morse code and pmr radios last year but the pop between letters stopped it decoding well