[hfsig] 20m WSPR shield for Raspberry Pi

John, W9DDD w9ddd at tapr.org
Sat Mar 23 14:18:22 EDT 2019

Please check again, there were some links that didn't get updated to the 
new version of the 30M WSPR.

John, W9DDD

On 3/23/2019 1:02 PM, Keith Wilson wrote:
> 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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