[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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