[hfsig] 20m WSPR shield for Raspberry Pi
Keith Wilson
keith.wilson.pcs at gmail.com
Mon Apr 15 17:20:31 EDT 2019
Gentlemen - an update to my power output measurements on the 20m Raspberry
Pi board:
The (impedance unknown, alligator clips to BNC) cable between the board and
the dummy load was affecting the dummy load impedance. The SWR was showing
about 1.55 with a RigExpert!
I tried an alternate method using the Flex panadapter and sampling with an
Alpha Delta coax switch, attenuation about 90 dB. The WSPR board signal
was down about 15 dB vs a 5 W signal. So that gives an output of 22 dBm
which looks pretty good. I also tried a 0.5 Watt signal and the Flex
panadapter looked pretty accurate i.e. down 9-10 dB from the 5 W signal. I
think this is a better test!
Keith - KE4TH
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On Tue, Mar 26, 2019 at 4:10 AM Bryan Corkran <bcorky at gmail.com> wrote:
> It certainly works the other way round .
> That is ,Bruces software works fine on the 20m board.
> Of course the web page setting are now 20m and 20 for the power.
>
> From a WSPR point of view I think the advantage of Bruces 30m board that
> is no monitor or keyboard is needed after its configured by its own web
> page at the initial setup.
>
> Bryan, VK3KEZ
>
> On 26 Mar 2019, at 12:44 pm, Bruce Raymond <bruce at raymondtech.net> wrote:
>
> Hi Keith,
>
> Thanks for the antenna analyzer feedback. I'm pleased at the feedback I've
> gotten. The display is only a 2x16 LCD, it's not graphical. I hope that's
> not a deal breaker.
>
> I think the 30m WSPR board should work with the same software that the 20m
> version uses. I haven't tried it, but the 20m and 30m transmitters are
> doing the same thing (driving the Pi clock to generate WSPR symbols).
>
> I discovered something strange today. I have the 30m WSPR board hooked up
> to a Pi 3 with the experimental software loaded. I hooked my scope up to
> the output (FFT on scope) and I'm seeing a whole bunch of spectral garbage
> several MHz wide below the carrier. It's ~-30 dBc. It goes away when the
> transmitter shuts off and comes back when the transmitter turns on.
>
> I don't recall seeing this before, so I'm wondering if it's the new
> software load causing the problem? I'll go back to the previous software
> load and see if it's doing the same thing. If it is the software I'm
> suspecting the new version of WsprryPi is the culprit.
>
> I bought a Pi 3B+ and it showed up today. I'll do some testing on it, too.
>
> 73 Bruce
>
>
>
>
> Keith Wilson wrote on 3/25/2019 7:35 PM:
>
> Bruce, I think there would be interest in an antenna analyzer kit at that
> price point, especially if it comes with its own display. I use a
> RigExpert model that has been very handy, but not very cheap!
>
> For the 20m WSPR board testing, I'm using a Flex radio / PC as my signal
> analyzer. I have a sampling arrangement that reduces the signal by 80-90
> dB.
>
> Will the 30m WSPR board work with the same WsprryPi software the 20m
> version uses?
>
> 73,
> Keith - KE4TH
>
>
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> On Sat, Mar 23, 2019 at 1:30 PM John, W9DDD <w9ddd at tapr.org> wrote:
>
>> 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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