DosaidSoft {recent}

I am back after a little hiatus to give a quick update on the transmitter. The new V2 transmitter board has proven to be flawed after some testing. It has been tested and shown that the long clock traces are rounding the waveforms a bit too much. I am going to update this post with the new V2.1 which will correct the issue.

In addition to the clock trace problem, the new NCP 8-SOIC surface mount drivers are a bit more demanding in their heat dissipation requirements than I thought. The new V2.1 board will require some small heastinks on the drivers to prevent overheating. The drive voltage has also been decreased to 15V instead of 18V because it has been proven to be just as efficient operating at 15V and the drivers get a little less hot.

So the changes in V2.1 will be:

• Shorter clock traces
• Heatsinks required on drivers
• Voltage changed to 13.7V or 15V on the drivers
• Inverter chip removed (integrated inverted inputs on drivers used instead)

Stay tuned for an update. And please post a comment if you have anything to offer, I’d like to get an idea if anyone is still reading this blog. I will continue to update it regardless, however.

Update:

I am sending the V2.1 to fabrication and they should be done in a few weeks; I will then begin the testing. I believe they will work much better. I still think there may be an efficiency problem with the output match. I might need to change to using coax in the balun instead of wire. I am not sure how this is done though, so I will do some research.

Here is a photo of the V2.1 board:

The latest version of the transmitter board has just finished production and will be arriving soon.

The new version has several improvements over the last. It is now using the NCP drivers and using one of those per MOSFET. The layout is a bit different as well. In order to reduce trace length as much as possible, the MOSFETs are now mounted below the board at a right angle. The new heatsink will be a square tube AKA cooling aggregate. I have not completely decided on this part yet, but fischerelektronik https://www.fischerelektronik.de/web_fischer/en_GB/heatsinks/D/Cooling%20aggregates/index.xhtml

. . .has some very nice heatsinks for this purpose. This will go right under the center of the board and the MOSFETs will mount upside down onto it.

I will also be using a PWM to modulate from now on. RF2017 (eBay) from Greece has now supplied me with an updated PWM module that is capable to modulate 500W carrier.

I have collected some more datasheets and created an excel spreadsheet for the parts list (which used to be in a text file and was limited). Sorry, the new PCB file has been removed from the archive. If you would like the PCB, please contact me or Tech Ingredients to order. You can still find the old PCB file and Gerber files in the archive if you want to send to fab yourself. You may modify the file using Sprint Layout.

You can download the updated archive here: https://drive.google.com/open?id=1C16XIHy0UGuOIszfQyuvUErVxDF4z_b3

You may notice that there are some datasheets in the archive containing shortwave receiver ICs. I am also building a receiver around the KT0915, but that project is on the back burner. The chip is interesting though because it can receive all the way from 150KHz to 110MHz with FM or AM demodulation and no gaps. Making this project into a transceiver may be possible if there is some type of RF relay, but I know very little on the subject and will have to do more research. Either way, the chip has the best sensitivity out of all the SW receiver all-in-one chips I could find. I want to keep this simple, so I do not plan on constructing a receiver from scratch. The chip is very easy to use. You just need a microcontroller (like an Arduino or PIC) and the chip itself. The chip will output a line level signal with no external components. A PCB is in order and will be experimented with when my main project is complete.

OK, so the project is finally completed and I’ve done some tests. Of course, this project will never be 100% complete. The transmitter does work and at a high efficiency of 95%, but only below 5MHz. It should work great for the 80 meter and 160 meter ham bands, but my goal of 40 meters (7MHz) has not yet been met.

I need to do more work to determine the weakest link. It is likely that it has to do with the drivers. I need to take some measurements with a scope to determine the problem. I will keep this post updated with the changes I make to get it working on 40m.

So far, there is one notable change I made that differs from the schematic I posted in my earlier post. I ended up needing 18V into the drivers rather than 8V. So I upgraded the power supply to 24V and used a 13.7V reg for the control board and then the two 18V regs for the drivers.

My efficiency is now as follows:

Update 02/27/19

I’ve done more testing and have concluded that it is not possible to do more than 5 MHz (5.5 MHz tops) with this PCB. The TC4452 drivers are just not going to do it and I can’t change them out for something better as the ‘better’ ones use a different pinout. Looking on a scope I can see the rise time on the DDS module to be about 7ns regardless of frequency (3 – 8 MHz). However, looking at the output of the TC4452(s), it is about 20ns rise and 25ns fall time. I think this is too large for it to be efficient at 6 to 8 MHz. Rise time is critical for these types of amplifiers because the MOSFETs operate at the highest efficiency when they are fully on or off. If they are part way on (during the rise time), they generate a lot more heat.

Another thing I tested to try to improve the efficiency was changing the duty cycle to 40%/60% rather than the normal 50%/50%. This apparently reduces the chances of cross conduction (the time when both sets of MOSFETs are on at the same time). We don’t want both sides on at the same time because this is a push pull amplifier. It will result in poor efficiency and possibly popping a FET if there is too much cross conduction. After changing the duty cycle to 40/60 (ie 40% on time and 60% off time for each side respectively), the efficiency of the amp went up to 80% from the original 74% shown above. That is not nearly enough of a change to call it ‘working’ at 7 MHz though.

In order to get good efficiency at 7 – 8 MHz, I am going to have to redesign the board. Two alternative drivers that should work better at these frequencies are the IXDD614 and the NCP81074A. In addition to changing the drivers, I will use one driver per FET. The NCP only comes in a surface mount package, so I am a bit concerned with heatsinking, but I have seen this one used in a picture. I think it will work well.

So stay tuned. It will probably be a while until the next update here because I have to design and order the new PCB, but I’ll be sure to update on how that one is working at the higher frequencies.