EMRFD Message Archive 9104

Message Date From Subject
9104 2013-09-09 13:25:42 Ashhar Farhan star topology in radios
i am using arduino microcontroller to control a new radio. the arduino generates an audio buzz each time it writes to the LCD. this buzz is coming from the digital lines (if i remove the LCD, the buzz is still there. If i turn off writing to the LCD, then the noise goes away).

i guess i will have to move to star topology. so how does one interface an  ugly built radio (all components on a ground place) with a digital controller like arduino built on a veroboard?

- farhan
9105 2013-09-09 19:57:08 bob_ledoux Re: star topology in radios
I place the digital display and operating controller in a tight RF case made of circuit board.  It might also require some bypassing of voltage and signal lines as these pass into the display case.


If the star topology works, I'd like to read about it.

9106 2013-09-09 20:20:06 Ashhar Farhan Re: star topology in radios
If I use two separate power supplies, then buzz seems to go away... i suppose this means that the buzz is coming through the +ve line? I have already connected the digital board and the analog board's power points together in a 'star' topology. the receiver i am working on has very high audio gain. the buzz is being picked up by the W7EL audio pre-amp. If i power down everything except the audio chain, the buzz is still there.

- farhan

9108 2013-09-09 22:12:22 victor Re: star topology in radios
1) Try connecting the power supply of the Arduino (positive AND negative connections) across a large electrolytic capacitor right at the power supply input to the receiver.
2) Add resistors (~10K to 100K if it still works) in series with all output logic lines connected to the Arduino, right next to the Arduino output pins.
Victor - 4Z4ME

9113 2013-09-10 19:14:12 iam74@rocketmail.... Re: star topology in radios
9114 2013-09-10 20:58:38 Bill Higgs Re: star topology in radios

Sadly, microprocessor circuitry is basically an RFI generator, in my opinion, and calls for more shielding and bypassing to keep everything playing nice. A couple of other things to try, however...

I recall a few years back listening to a presentation on audio preamplifiers for broadcasting, where the presenter said "all amplifiers are differential amplifiers." It's stuck with me. The point is, any wiggle on the ground line is equal to a wiggle on the signal line.

He went on to discuss star grounding, as you suggest, but also emphasized the importance of where the ground return is located on the circuit board. The idea was to place the power supply ground on a multiple-circuit board as close to the ground point of the highest gain or lowest level stage. That way any "wiggles" on the supply requirements of other stages would not affect the preamp. Also, the heaviest connection you can use to the negative supply rail to avoid voltage drops being reflected into the preamp.

Granted, this was in the ancient days before microprocessors were used in the same unit as audio circuitry, but it may be worth a try to play with the attachment point of the negative supply line.


Bill, NT4C
9115 2013-09-10 21:27:32 AD7ZU Re: star topology in radios
I would offer a few suggestions to try before re-doing a layout.
I dont know what display interface you are using .. maybe its SPI or I2C?
one step that may help is to use a ribbon cable.
then alternate active signals and grounds in the cable...
like so...
pin 1  gnd
pin 1   gnd
pin 2  dclk  spi clock
pin 3  gnd
pin 4  din  spi data in
pin 5  gnd
pin 6  sdo  spi data out
pin 7  gnd
ribbon cable connectors are cheap, easy to install
i dont know if the display is remote from the arduino in your design but this might be an approach if it is.
another alternative is to mount the display directly to the arduino board and isolate everything digital from the analog. 
typical commercial designs often mount all the display, knobs, switches, and a ucontroller on a single board that sits parallel to the front panel and is isolated from all high gain analog circuitry.
the addition of some low value resistors in series with the SPI connections will help damp the ringing and reduce noise. ... say 10 - 20 ohms.
you can increase or decrease the clock and data frequency (SPI or I2C clock rate) from the arduino to see if that makes a difference
scope the power supply lines to see if they are clean.. if not some bypassing is in order.  at the ucontroller, display,  and again at any analog devices that share a p/s .  there are usually manufactures guidelines available that describe the bypass requirements.  digital devices require bypassing very close to the device.
some combination of any or all of the above will probably fix your noise problems.
finally are you using the PWM outputs???  if so have you checked if disabling PWM makes a difference?
a few ideas..

9116 2013-09-10 23:52:44 Ashhar Farhan Re: star topology in radios
I traced the culprit to LCD. If I disabled LCD writes (it uses a 4-bits-plus-clock kind of an interface). I was foolishly updating the LCD on every frequency jump. I rewrote the display routine to refresh the display at 100 Hz jump and the noise went away.

Yes, I should have put resistors in the LCD interface lines. But that is too late now, I have ordered 10 PCBs for our Lamakaan Radio Club.

I have put together this controller board that can be used to control almost any homebrew radio rig. It's derived from Arduino: it has a 16x2 (or 16x1 if you prefer) LCD panel on the top, I2C for Si570, SPI for AD985x boards, some analog lines brought out and six general I/O lines for T/R, bandswitching, etc. I am cleaning up the code and I will push it to github. It forms the basis of my sweeper (called Sweeperino) and a new transceiver that is still on the bench. I will post both, real soon now...

- farhan

9118 2013-09-11 06:28:12 Russ Ramirez Re: star topology in radios
Henry Ott's book "Noise Reduction in Electronic Systems" describes the issue you are having. Since it is a common, logical practice to combine power supply feeds when like voltages are required, this sort of noise issue crossing over from the digital to the analog circuits is almost to be expected. The obvious recommendation is to do exactly what you already observed; use a different supply. However, there are several less expensive techniques for dealing with this issue. I suggest having a look at this: http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf for some additional ideas.