Re: #ubitx-help Strong Audio Tones in and outside audio receive passband #ubitx-help

Jerry Gaffke

If you have a v4 with the socketed Nano, probably easiest to just buy a bunch of Nano's.
Find one that does not have the processor clock at the wrong frequency.

Be sure to buy Nano's that don't already have the pins soldered to the Nano,
since the pins must be added to the back side of the Nano.

The Nano's use a 16mhz ceramic resonator to determine the CPU clock frequency,
and that can vary by 50khz or more between units.
Apparently, harmonics of the 16mhz oscillator beat with harmonics of the 12mhz BFO, creating the tones.

If you hear the tones while using a v3 uBitx, try putting a finger on the (very dinky, about 2x3 mm)
ceramic resonator near the ATMega328P processor, on processor pins 7 and 8.
You will have to remove the LCD first to get your finger in there, but the radio will still work.
Since on the v3 uBitx the Nano is soldered down to the Raduino, it will be much tougher to
swap the Nano out.  Might be easier to add small capacitors (10pf) to ground from one or
both ends of the resonator.  But that resonator and ATMega328P are quite small, will be
a difficult solder job.  Might work to put insulating tape over the resonator, then grounded copper
foil over the top of that, effectively duplicating what happens when you put a finger on it.

There is also a 12mhz resonator associated with the Nano clone's ch340 USB chip.
This resonator is not active unless there is a USB connection to a host computer for a firmware download,
so interference from the 12mhz oscillator should not normally be an issue.


On Wed, Jul 4, 2018 at 06:19 PM, <groupsio@...> wrote:
Is replacing the Arduino still the easiest way to fix the 13kHz tone in the headphones?

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