[hfsig] 20m WSPR shield for Raspberry Pi

Bruce Raymond bruce at raymondtech.net
Sat Mar 23 15:23:45 EDT 2019

Hi Keith,

You've obviously picked the ultimate in terms of getting rid of hum - 
using a battery (I didn't read the original post carefully enough). 
You've hit on all of the fixes I can think of.

120 Hz implies full wave rectification of 60 Hz. Unfortunately, 60 Hz is 
all around us, so that doesn't help much. Another relatively obvious 
thing is to question the test equipment, although that's a real long 
shot. I don't see how you get mixing products from a spec an or a scope. 
I suppose an easy test for that would be to look at a known clean 
signal, like from a signal generator, and look for mixing products. This 
whole direction seems like a real long shot.

You've got more power coming out of the transmitter than we would 
expect. That probably means that the MOSFET bias voltage is fairly high 
relative to its threshold voltage, so the bias current and gain are 
high. It's possible that you're generating much stronger harmonic 
components from the MOSFET than you otherwise would, raising the 
harmonic content of the output.

OTOH, 30 dB down means ~3% of the amplitude relative to the carrier. I 
doubt this would have any impact on a WSPR receiver's ability to decode 
the signal.

I originally put together a 30m WSPR transmitter kit. I sent ~45 of the 
kits to John/TAPR. These are sold out. I redesigned the unit to use 
surface mount parts and had 200 units assembled and tested, and 
delivered to John. There should be plenty of these units available.

73 Bruce

P.S. I'm working on an idea for an automatic antenna analyzer kit for 
HF. It will scan a range of frequencies and find the resonant point and 
VSWR at resonance (actually min VSWR, not necessarily resonance), and 
2:1 VSWR bandwidth. This will be a kit with no surface mount parts and 
no case, but will have mounting holes so you can mount it in your choice 
of cases. It will probably sell in the ~70 range. Does this sound like a 
worthwhile project?

Keith Wilson wrote on 3/23/2019 1:02 PM:
> Hi Bruce, and thanks for the generous response!
> My power measurement was made with a good 50 ohm load so maybe I have 
> a nice hot MOSFET!
> 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.
> Last question, TAPR is out of the 30 m boards.  Are they still available?
> 73,
> Keith - KE4TH
> On Fri, Mar 22, 2019 at 9:14 PM Bruce Raymond <bruce at raymondtech.net 
> <mailto:bruce at raymondtech.net>> wrote:
>     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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