Loop noise

[Jųan]

Greetings Picbasicans, my 16F877 is connected to a radio. Some of my functions have loops, eg:



Loop:
Display time on LCD
Display frequency
Some other command
pause 25
BUTTON Pin1,Down,Delay,Rate,BVar,Action,Label1
BUTTON Pin2,Down,Delay,Rate,BVar,Action,Label2
BUTTON Pin3,Down,Delay,Rate,BVar,Action,Label3
'' " "
goto loop

The thing is that I can hear this loop through the speakers, eg, I can hear a put put put put every 25 cycles. Changing the pause time will only change the frequency of the pulses. Any suggestions how I can get rid of this. I have tried using while wend command but I need to refresh other commands (eg: diplay time). Any comments are appreciated.

Regards

Juan

[Dave]

Jųan, What you have is an RF generator. If you had a spectrum analyzer you would be surprised at how many different frequency's you circuitry is transmitting. What you need to do is to place your circuitry into a metal enclosure and place either PI filters on the output wires from the enclosure if any or, low pass filters on the output wires from the enclosure if any, and ground the circuitry to the enclosure.

Dave Purola,
N8NTA

[Jųan]

Thanks for your reply Dave, I accept what you say. Makes sense :-)
My radio cosists of two parts. The receiver and the controller board. The receiver is in a metal box already. I will enclose the control board as well. I still wonder why I hear the put put put only in certain parts of the program.


Juan

[Dave]

Jųan, As you code is executing you may be reading eeprom or sitting in a tight loop looking at an input port or writing data to a lcd display for update. These all change the electrical path of the circuit. Infact you have more than 1 source of oscillator in your circuit if you are using an LCD or VFD for display. If you mount the display inside an enclosure the RF emissions from you circuitry are comming out of the opening for the display. Some times if they are strong enough you may be required to put RF screening in front of the display. Better pieces of test equipment that use displays use this method to cut down on the emissions.

Dave Purola,
N8NTA

[Jųan]

After disconnecting my I2C LCD, the pulse noise seems to go away. You're absolutely right. Just gotta figure out how I can go about putting some sort of filter or RF screen. Any suggestions?


Juan

[Dave]

Jųan, There are some quick and dirty things you can try. Place a piece of aluminum window screen in front of the display before mounting it in the enclosure. They make different grades and gages of screen. The rule of thumb is the diagonal dimension of the openings should be the wavelength of your noise divided by 3 or 3 times smaller then the wavelength of intrest. Make sure the screen is grounded to the enclosure. Use this rule of thumb for any openings you might have as they are all places for emisions to escape. Being a amateur radio operator and equipment builder this rule of thumb has come in handy in the past years.

Dave Purola,
N8NTA