Solution to Power Supply and Operational Instability

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john.chartkoff
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Joined: Tue May 26, 2020 7:13 am

Solution to Power Supply and Operational Instability

Post by john.chartkoff » Tue May 26, 2020 9:37 am

Hello to the group from Columbus Ohio, USA.

I just successfully configured and interfaced a 4G RPi4 B to a new second gen Kerberos SDR 4 channel unit.

The Kerberos SDR system is clearly a very well engineered product. Congratulations to the engineering team.

The following notes are provided in hopes they may lend assistance to others.

I found that several of my micro-usb supplies were operating at below 4.6 VDC. Everything lit up as expected, but the system was very unstable.

I used a simple inline USB voltage and current meter to measure power supply performance, which was surprisingly substandard and wildly variable between units.

Here is a similar meter on Amazon:
"Charger Doctor Digital Multimeter USB Power Meter Tester Voltage Monitor Current Detector with mAh Capacity Readout for Chargers, Solar Panels, Cables, Power Banks"

I had numerous stability and operational issues with the software until I got the power issue stabilized.

Everything worked fine once the DC Voltage to the KSDR box was a clean 5.0VDC.

To accomplish this with reliability in a vehicular mobile application, I modified an old style externally powered USB2 hub. With testing, I found the output voltage was around 4.3 volts, due to the voltage drop from the power supply diodes.

To solve the problem, I purchased an adjustable switch mode constant voltage power supply from Amazon, installed it inside the USB hub, and hard wired it in.

I used the following device:
"eBoot 12 Pack Mini MP1584EN DC-DC Buck Converter Adjustable Power Step Down Module 24V to 12V 9V 5V 3V (12 Pack)." Amazon is out of stock of these exact units at the moment, but any adjustable supply that makes 5V at 3A should work.

Once installed, I set the output voltage on the supply so that the inline USB meter read 5.2VDC at the output of the hub when connected and everything is powered up. In operation, the actual output voltage of the internal power supply in my configuration is 5.9VDC (due to the diodes), to produce 5.2VDC at the output of the USB hub, and then 5.0 VDC at the input of the KSDR (due to the voltage drop of the USB cable). In practice, the voltage drop of an 18 inch 28 gauge USB cable is approximately 0.2 volts at 1 amp.

Power was connected with a USB-A to USB-C cable to the RPi4 and a USB-A to micro USB connector to the KSDR.

Data was connected by a USB-A to micro USB between the RPi and the KSDR.

Incidentally, I found that the RPi does seemingly power the KSDR box alone, from just the data cable, but the power is apparently insufficient to operate the KSDR box. The secondary power cable was necessary for stability.

With this configuration, I can reliably power the entire arrangement off the unregulated 12 volt main battery buss of the vehicle.

The KSDR box alone draws just about 1 AMP at 5VDC when it is operational, and the RPi 4B uses about 0.6 AMPS at 5VDC.

I found that you can not use the presence of the blue and white lights inside the KSDR box as an indicator that it is sufficiently powered. The lights will come on, even if there is an under-volt condition, but the system will be unstable. You may get it working, but then it could lock up, or otherwise not work properly.

Providing stable power appears to be the only challenge.

I would like to pose several questions - more precisely as requests:

1: Could the KSDR box could be revised to operate on 3.3V to reduce heat generation - without too much re-engineering?

2: Are there are any plans to port the Android app to run headless on the RPi, in order that DOA lines and position solution rings could be exposed to an interface for use with other equipment?

3: Are there any plans to use the 40 pin GPIO connector for I2C to communication, as well as power to the RPi, by way of a ribbon cable - thus simplifying power and data communications?

4: Will a future hardware revision include an internal power supply?

Again, congratulations on a very nice job of hardware and software engineering!

Kind regards,

John

rtlsdrblog
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Re: Solution to Power Supply and Operational Instability

Post by rtlsdrblog » Fri May 29, 2020 9:27 am

Thank you for the notes! Yes a lot of the trouble posted on this forum comes down to simply a poor quality power supply being used. It seems that there are A LOT of bad ones out there. Answers to your questions below:

1: Could the KSDR box could be revised to operate on 3.3V to reduce heat generation - without too much re-engineering?

Probably not. The most common supply is 5V. Even if it was designed to take a 3.3V supply the majority of heat is not generated by the LDO. Most of the heat comes from the R820T tuner which is known to be a hot operating chip.

2: Are there are any plans to port the Android app to run headless on the RPi, in order that DOA lines and position solution rings could be exposed to an interface for use with other equipment?

Do you mean run the code on the Pi and expose say a web interface for the map? THe problem is that the KSDR DSP code is quite CPU intensive. And at the same time, the code to calcualte the intersection points of every bearing also becomes a bit CPU intensive. There may be ways to optimize the code, but we really need the Pi to be free to do the DSP code in real time as if it falls behind too much the KSDR may need to be resync'd.

3: Are there any plans to use the 40 pin GPIO connector for I2C to communication, as well as power to the RPi, by way of a ribbon cable - thus simplifying power and data communications?

I2C communication won't do much for us. We still need the USB cable for the data transfer. I2C just doesn't have the bandwidth like USB,

4: Will a future hardware revision include an internal power supply?

As in an internal power supply that you can just plug into the wall? Unlikely. We need to support all countries, so plug ends would be an issue. Easiest solution that we're probably adding is an optional 12V jack alongside the USB power connectors.

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