Hand/Body Capacitance

[emmanuelgoldstn]

Hi all - built one of these recently: https://www.qrp-labs.com/ultimate3/u3s.html

Such a cool kit. One problem I seem to have is when i connect my GPS 1PPS output to the kit (which allows for time synchronization and frequency calibration), i have problems based upon where i stand in relation to the kit. Putting my hand close by seems to affect whether or not the PPS signal is processed properly by the kit. You can see a little heartbeat icon that pulses in time with the GPS pulse. Sometimes it just hangs, sometimes it goes too fast, sometimes it works right - depending on where I am. So I am assuming its some kind of body/hand capacitance thing, but I am a super newb and don't know how to validate my hypothesis or to stop it. Do i need to ground the setup? Right now I just have it connected to a battery, a random wire antenna, and the GPS (which is plugged into the wall). Will placing it in a metal enclosure help?

Thanks!

[uncle_bob]

Hi

You are over-thinking this.

1 pps is 1 Hz. Unless your hand is several thousand miles wide, 1 Hz does not care about your hand.

Even at a few feet, a 1.5 GHz circuit board does not care about your hand or where you stand in the room.

What *does* matter as you move around is the way the GPS signal gets into the receiver. Are you trying to do this all with an indoor antenna? You really should have an outdoor antenna that can see at least 6 GPS stats at all times. A monitoring program is very helpful for working this out.

Bob

[mikro]

Is your ground connected properly between the Kit and the GPS module? What kind of signal comes from your GPS-device? Does the PPS-signal have pull-up resistor, because it seems that the pps connection is connected straight to MCU of the kit. There might be internal pull-up resistor enabled tho.

[emmanuelgoldstn]

Is your ground connected properly between the Kit and the GPS module? What kind of signal comes from your GPS-device? Does the PPS-signal have pull-up resistor, because it seems that the pps connection is connected straight to MCU of the kit. There might be internal pull-up resistor enabled tho.

Thanks mikro- I'm not connecting the GPS and the power source of the kit to a common ground at the moment. Just powering the kit from a battery, and the gps from a wall wart. That's where I was thinking I might be having problems. I'm not sure how to describe what kind of signal comes from the GPS. It's basically just the 1Hz pulse that looks like this:

I haven't included any kind of pull-up, though the kit or the gps might have one? I wish I knew enough to tell you for sure.

Hi

You are over-thinking this.

1 pps is 1 Hz. Unless your hand is several thousand miles wide, 1 Hz does not care about your hand.

Even at a few feet, a 1.5 GHz circuit board does not care about your hand or where you stand in the room.

What *does* matter as you move around is the way the GPS signal gets into the receiver. Are you trying to do this all with an indoor antenna? You really should have an outdoor antenna that can see at least 6 GPS stats at all times. A monitoring program is very helpful for working this out.

Bob

Thanks for your thoughts on this uncle_bob - I was thinking maybe that I was introducing extra capacitance into the circuit somewhere else, since it is very noticable immediately when I move my hand closer to the circuit. I am using one of these: https://buy.garmin.com/en-US/US/oem/sensors-and-boards/gps-18x-oem/prod27594.html, and the pulse comes in consistently - nice and strong - once the signal is acquired. I've used this GPS device for some very sensitive Time Difference of Arrival calculations, so I'm pretty sure it's functioning fine.

Thanks to both of you for your ideas!

[mikro]

Just to make sure, do you have the GND-pin connected between the GPS-receiver and the kit?

[uncle_bob]

Hi

Do you have a cell phone in your pocket? That looks a lot like a phone checking in with the nearest tower.

Bob

[djacobow]

I have the same kit, and the pps signal had similar noise on it. I don't know if it is coupling of the serial data signal to the pps signal in the cable, or if gps vcc is insufficiently decoupled.

However, my kit works without issue. Are you using Hans's GPS (QLG1) or another unit?

It does sound like maybe a floating ground might be at issue. Does the device report the serial gps days reliably?

[emmanuelgoldstn]

I think it might be a floating ground. I am using a garmin GPS, and I don't have the ground connected between the kit and the gps. I suspected the ground might be the case. I'll give that a shot and see if it helps. Thanks to all!

[djacobow]

Good luck. By the way, I hooked up the "PA" Vcc to my kit to 12V and have three transistors in place. By my crude measurements, it's putting out between 300 - 500 mW.

I'm in Northern California, and in WSPR mode, my unit has been heard all over the US, also Alaska, Japan, Korea, Australia, NZ, even a few random reports in England and one from Antarctica. Too bad they're not  QSOs.

It's a fun kit.

Best,
Dave
WE6EE

[emmanuelgoldstn]

Very fun kit to build. Got it up and running haven't gotten back to it since I was having troubles with the GPS signal. Seems to work fine without it, but I haven't hooked it up to a real antenna yet. Do you just have it hooked up to a 12V bench power supply?

[djacobow]

I powered it at first from a lab supply, but now it's just powered by the first 12V supply I grabbed from my bin of abandoned wall warts. For the 5V to power the rest of the kit, I just put a 7805 and some caps on a small piece of breadboard, so the whole thing powered by just the 12V. The regulator gets hot; I may replace it with a switcher, but it's fine. The whole kit only pulls a watt or two at the wall outlet, including GPS.

It had been awhile since I did a through hole board, so that was fun. I also liked winding my own inductors, which I had not done before but am much more likely to do going forward.