[hfsig] 20m WSPR shield for Raspberry Pi

Bruce Raymond bruce at raymondtech.net
Fri Mar 22 23:13:02 EDT 2019

Hi Keith,

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.

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.

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. <=

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).

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.

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.

73 Bruce Raymond/ND8I

Bryan Corkran wrote on 3/22/2019 4:22 PM:
> 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.
> 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.
> Bryan, VK3KEZ
> On 23 Mar 2019, at 5:36 am, Keith Wilson <keith.wilson.pcs at gmail.com 
> <mailto:keith.wilson.pcs at gmail.com>> wrote:
>> 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.
>> 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?
>> 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.
>> Keith - KE4TH
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