<div dir="ltr">Hi Bruce, and thanks for the generous response!<div><br></div><div>My power measurement was made with a good 50 ohm load so maybe I have a nice hot MOSFET!</div><div><br></div><div>As mentioned in my previous email, the testing was with a USB power bank battery, and I still have the 120 Hz apparent mixing products. I tried a separate antenna (my SOTA antenna) so I was not attached to the station ground. (My station ground is bonded to the household power ground) So I shouldn't have any way for 60 Hz power to get into the Pi or USB battery. Still puzzled by the presence of these apparent mixing products. Because they are down 30 dB, I probably won't get double decodes by receiving stations.</div><div><br></div><div>Last question, TAPR is out of the 30 m boards. Are they still available?</div><div><br></div><div>73,</div><div>Keith - KE4TH</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Mar 22, 2019 at 9:14 PM Bruce Raymond <<a href="mailto:bruce@raymondtech.net">bruce@raymondtech.net</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">Hi Keith,<br>
<br>
I have to agree with Bryan (well, I suppose I don't *really* have to
agree with him, I just want to :-). The 20m transmitter is Zoltan's
design, but very similar to my 30m transmitter. The final is an MMBF170
powered by 5 volts.<br>
<br>
1. The power output is approximated by the formula P = V^2/2*RL. The
power supply is roughly 5 volts and assuming a 50 ohm load (RL), P =
5^2/(2 * 50) = 0.25 watts.<br>
<br>
It's reasonable to expect some losses and the safest way to list the
output power is to say you'll get at least 200 mW. Also, if the supply
voltage is higher than 5 volts then you'll get more power. It's unlikely
that it would be *that* much higher; it would take 6 volts to give 360
mW. Another possibility is that your antenna impedance is less than 50
ohms. If your antenna impedance is, say, 35 ohms, then P = 5^2/(2 * 35) =
360 mW. The last (and most probable) thought is that the MOSFET in your
transmitter is hotter than typical and gets driven harder, producing
more output. I've played with this on the 30m transmitter and have
gotten power outputs in this range by biasing the MOSFET on more. The
threshold voltage for a MMBF170 MOSFET is between 0.8 and 3.0 volts with
2.1 volts being a typical value. The 20m transmitter has a voltage
divider putting 2.3 volts on the gate. If your MOSFET is fairly hot then
it would be biased on more and likely put out more output. The end
result is *yes* the output is real. => Watch for the MOSFET getting
hot. If it does, you might want to add a heat sink or change the gate
bias resistor (R2) from 1.2K to something larger, say 1.5K. <=<br>
<br>
2. Power supplies - in the words of Socrates, suffering an learning are
two names for the same experience (I don't know that Socrates actually
said that, but I like to say he did). I have learned through hard
experience that inadequate power supplies cause a whole bunch of
problems, and they're usually very difficult to troubleshoot because the
problems are either intermittent or just not something I'd normally
suspect of a power supply. The power supplies normally used for the
Raspberry Pi are usually marginally adequate. I'm very impressed with
the job the designers of the Pi did, but they cheaped out on the power
supply filter on the board (electrolytic capacitor).<br>
<br>
Now we compound the problem with trying to run a transmitter off of the
same power supply in addition to running the Pi. This doesn't help
things. In the beginning I bought a bunch of cheap 5V/2A power supplies
from China that worked with my Pi/30m transmitters. I had a bunch of
weird problems, such as the software getting corrupted during normal
operation. At first I thought the problem was cheap SD cards or some
problem with the operating system/software. I now believe the problem
was power supply glitches causing the Pi to get confused and do bad
stuff. I switched to bigger power supplies and my problems disappeared.<br>
<br>
My recommendation is to get a 5V/3A power supply and make your
measurements again. 120 Hz sidebands sounds like AC bleeding through the
power supply, even if it seems that the power coming off the supply is
clean. It could also be some sort of interaction between your antenna
ground and your power supply ground. You might try a different power
supply and/or an isolation transformer for a test. This might be similar
to hum problems direct conversion receivers have that are associated
with grounding.<br>
<br>
73 Bruce Raymond/ND8I<br>
<br>
<br>
<br>
<span>Bryan Corkran wrote on 3/22/2019 4:22 PM:</span><br>
<blockquote type="cite">
<div dir="ltr"><span></span></div>
<div dir="ltr">I had a lot of trouble with power, in the end I bought
the “official” 2.5 amp adapter and had no trouble after that.<div>Keith
is right the shield is designed for the V1 board hence the little slot
in the middle for the display port. I had problems with the shield
fouling on the heat sink I’d added on a 3b board so I used a GPIO
extender to raise it a small amount.</div><div><br></div><div>Bryan,
VK3KEZ<br><div dir="ltr"><br>On 23 Mar 2019, at 5:36 am, Keith Wilson
<<a href="mailto:keith.wilson.pcs@gmail.com" target="_blank">keith.wilson.pcs@gmail.com</a>>
wrote:<br><br></div><blockquote type="cite"><div dir="ltr"><div dir="ltr">I have the 20m WSPR shield working on a new Raspberry Pi 3
B+. I see apparent mixing products in the output, 120 Hz away from
fundamental, when using a USB power bank to power the Pi. Since these
are not coming from a switching power supply, where are they coming
from? These products start at about 30 dB below the fundamental.<div><br></div><div>Also,
with a scope I measure the voltage output at 12V peak to peak into
quality 50 ohm dummy load. This is 0.36W, higher than the 20dBm (0.10W)
specified. Is this too good to be true?</div><div><br></div><div>Note
the shield was not designed for the Raspberry Pi 3 B+ so it can't be
fully inserted on the 40 pin GPIO plug, but seems stable enough
partially inserted. Getting WSPR reports from across the USA and
occasional overseas reports too.</div><div><br></div><div>Keith - KE4TH</div></div><div id="gmail-m_2816230274241412704DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2"><br>
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