[hfsig] 20m WSPR shield for Raspberry Pi

John, W9DDD w9ddd at tapr.org
Sat Mar 23 15:29:53 EDT 2019

I want one of those antenna analyzer kits.

John, W9DDD

On 3/23/2019 2:23 PM, Bruce Raymond wrote:
> 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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