Saturday, September 16, 2017

MAX7219 8 Digit Display Module Interference

While building the 50 Mhz receiver I notice after boxing it that there was a lot of noise, whistle type, in the audio overriding any possible reception.

Interference most of the time comes from supply line and I was pretty sure that was the source since the Arduino and the display module were previous connected to separate usb power without it being noticed. I knew the problem was in the display because the original one got "toasted" and interference was over when removed.

This week a new display module arrived and the interference started again after assembly.

...pesky little things, next time will go back to the usual LCD type.

First action was to kind of "shielding" cable from Arduino nodule to display, that made no difference, the cabling carries control signals plus power and I roled thin foil over it.

Second action was to add a small filter in-line with power supply. That placed the interference at a much lower level (not noticeable in operation). I also added a filtering capacitor of 220uF at the Bitx board vcc input. In terms of power I have the main line in the box to the Bitx board then Arduino, display and VFO are regulated downstream, one regulator for the Arduino and display module (module power is supplied by the Arduino) and the other regulator for the VFO.

Filter for the MAX7219 module that cured the interference is this one:

..worked fine first time, components values were eye balled, I have used similar ones to audio amp's (specially the LM386).
after installing and probing, looked like this:

The initial interference was like this at the speaker terminals:
and like this at the MAX7219 display module vcc input:
...this is AC coupled at the oscilloscope, on a perfect system should be a flat line, that is; no AC component on the DC line.
From the Arduino side at the supply line for the module,

looked like this:

...the VFO signal is all over the "shop".

Now with the filter and even more audio output the signal looks more of an proper audio one:

I like when it works first time! Now I can continue with testing and validating the receiver design (it's already fully boxed), when done I will post more details.

Bellow the current state in terms for from panel an inside:

Have a great weekend!

Update 2018-12-29:

Lex, PH2LB, had similar interference from the MAX7219, he sorted with inductors in low pass filtering and some shielding on the power lines, here a nice build from him along with description:


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