[hfsig] 20m WSPR shield for Raspberry Pi
Keith Wilson
keith.wilson.pcs at gmail.com
Mon Mar 25 20:35:36 EDT 2019
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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