[600MRG] double reporting of strong wspr signals

[Hans Summers]

Hi John

> The U3 board is very susceptable to producing doubles without
> good solid DC and common sense bypassing.  The creator of
> the U3 was not a believer in bypass caps.

Err... Ahem... The creator of the U3 believes in bypass caps, ground
planes, and shielding. In general by default, believing that the more of
all, the better. So a few comments seem warranted.

Firstly I'm very happy to consider any improvements to the U3, to
incorporate in the next PCB revision when I get the next batch made. So if
you have any suggestions for improvements and measurements showing the
value the let's discuss it!

Secondly I think you're using the wrong terminology slightly. According to
my understanding the term "bypass capacitor" (a.k.a. "decoupling
capacitor") is applied to the relatively small value capacitors distributed
throughout a circuit, close to consumer components of significant sudden
transients, in order to provide quick response to those transients and
minimise their effects on the local and wider circuit. Their application is
the reduction of noise in the range from some hundreds of KHz upwards to
some MHz, that kind of ballpark.

The ripple you are referring to is 120Hz power supply ripple. This is
reduced by power supply filtering, which in the simplest case typically
means a relatively large value electrolytic "smoothing capacitor". The
energy stored in this large capacitor is supplied to the load circuit to
sustain the supply voltage between the 120Hz peaks.

Now, to what the U3 kit is, and what it is not. It is designed as a
minimalist low cost QRSS and WSPR modes low power LF and HF transmitter. It
does NOT include power supply voltage regulators or smoothing. This is in
order to maintain the minimalist low cost approach. So the kit requires a
5V supply for the microcontroller and DDS, and a supply for the PA, which
can be tied to the 5V supply or a higher PA voltage can be provided. People
are using up to 12V form the PA supply. It is left to the builder to
provide these supplies and ensure they are robust and ripple free. People
use many different ways to power their U3's and the U3 doesn't claim to do
anything to improve the quality of its supply voltage.

The U3 assembly manual states at the top of page 2: "Best results will be
obtained with a well-regulated and well-smoothed 5V supply".

Regarding bypassing capacitors in the U3. The circuit contains a 0.01uF
bypass capacitor at the ATmega328 microcontroller supply pins, and another
0.01uF bypass capacitor in the PA circuit. I suspect the 120Hz sidebands
are likely to arise in the signal generation process, which is to say, the
pre-assembled AD9850 DDS module. The DDS module contains five 0.1uF bypass
capacitors across its supply and one 10uF capacitor.

So altogether the U3 contains eight capacitors across its supply. But I
still think 120Hz ripple causes sidebands and needs to be removed at the
supply side, external to the U3.

In my setup here I can often get 100Hz sidebands (50Hz mains power here,
not 60Hz) produced in the receiver with very strong signals too. I'm not
sure of the mechanism.

Anyway - feel free to modify and measure the U3! I'm sure it isn't perfect.
Anything you find that can measurably improve the performance in a valuable
way and is practical to include in the design, I can add to the next PCB
revision! And in the meantime note the improvement modification on my U3
webpage http://www.hanssummers.com/ultimate3

73 Hans G0UPL, creator of the U3 :-)
Http://www.hanssummers.com