Filter Capacitors and Voltage Rating

As i explained in a previous post, I am building an amp that is going to be capable of doing more than 100w. No biggy for most people but when everything you have built previously has been qrp there becomes a whole new set of challenges. With qrp you can just she will be right mate and do what ever you like “Almost” and it will work to some degree. So with this in mind, i set forth on working out what voltage rating i would need for the capacitors in the low pass filter.

What i really wanted to know, was the WHY behind the decisions others had made in their design choices and I started to ask others WHY as well. Often the reply was just use Mica Caps. Did not matter what power level, the answer some gave was just use 500v Mica and you will be right. No explanation, just this is what I do.

So that just never sat well with me, the answer cannot always be use $60 worth of 500v caps in your low pass filter. It took a while but i finally came across some information that seems credible and more importantly reliable and my gut feeling was right, Mica while good, might often be over kill, but still there are some caveats that are important to understand.

Firstly the voltage that a capacitor in a LPF sees is the peak to peak voltage of the AC wave form. Might be obvious to some, but its not information that is easy to find on the internet. So knowing that we can then use the power formula to find out what voltage might be present in the filter. Power(watts) = Voltage^2 / Load(Ohms) where in this case, the voltage of the signal is RMS Volts and once you have solved for it, you need to convert RMS to Peak to Peak by multiplying by 1.4.

So in my case 100w = 70.7vrms/50ohms :: 70.7*1.4 = 100v peak to peak. Now obviously there can always be an impedance mismatch between the low pass filter and the antenna and with that a change in the signal voltage. In my case i also looked at 100w at 25ohms and 100ohms load to get a better picture of what might be expected of the capacitors in the low pass filter. Being that I am actually designing the amp deck to be capable of 400w even though i am only running it at 100w, it means the capacitors need to be able to withstand a peak voltage of about 350V.

So onto the next point, I wont be using Mica Caps, for the very simple reason that $50 worth of caps is a lot of money to throw at something that might be a dismal failure. I will be using SMD ceramics and here is something i learned just recently, ceramic caps derate in capacitance the closer you get to the maximum voltage rating. According to a TI white paper i was reading, you should probbaly derate ceramics in filters by at least 1/3. So I need at least 360V rating, that means using 1Kv or 2Kv ceramic caps should be perfectly fine. The good thing is, Kv rated SMD caps are cheap, in the sub 50 cent range in 1 of qualities on mouser and they are NP0 and 5% tollerance. So i have solved this problem now and can move onto other things like output transformer and core sizes i need to use for the power level.

More fun.

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Clickety Clack of the Relays

Slowly integrating all these bits together in software and currently all the relays are clicking in sequence when I change bands on the control board. If you look closely on the upper right, you will see an updated version of the Band Pass Filter board that uses different inductors and filter topology. I need to populate and test one filter to ensure its right and then build the entire board.

Something that I did not notice was that when I changed Ecad software my mounting hole pattern has changed slightly, so neither of the old filter board designs will stack with the other boards. So i have to redo the low pass filter board and I am going to redo the rf frond end board as well and move things around to make it a little more usable and to make sure the 3.3v regulator has a heatsink on it. With bugger all current running though it, it still gets burn your finger hot and that is not right. I am starting to wonder if these ebay parts are actually dodgy or something.

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Rig Control by VK4HAT

So i am on the 4th iteration of this board and I have just about got it right. Kind of sad that its taken me so many goes to get this right, but what can i say. LOL. There is still one bodge wire on the backside of the board because I somehow left off a trace joining one row of push buttons off the 5v line. I am also not sure about the 5V regulator, its running very hot and it should not because its should only have about 300ma running thought it and there is no short to ground anywhere i can find, to explain it running so hot. By hot im guessing its hitting over 60 Deg C as its painful to touch.

Anyway, back to the point of this post. I have gotten back into coding things up and am making inroads. It mostly works now. Pressing the mode button will cycle through LSB, USB and CW. The band button will cycle through 80, 40, 20 and 15m the 4 bands that this radio will support, selecting the appropriate filter banks for both Low Pass and Band Pass filter boards as well as change the IF filter to a narrow CW filter in that mode.

The logger button will put it into logging mode, which along with a keyboard will make log entries and save them to SD Card in ADIF format. The logger is loosely based on my old arduino logger code which can be found in the menu bar above. Though, the code that is going into this is much more refined and polished and fixes most of the clunkyness that was in the earlier versions. I am a much improved programmer now than i was a few years ago when i wrote that code.

The VFO button will exit the logger and put the radio back into VFO mode. I still need to add in a couple more push buttons, one to change the Attenuation setting and the final one to allow changing the TX RX offset. At the moment it is hard coded for a 600hz offset for CW operation, but, i am going to make that variable for real split operation in SSB as well.

Anyway, that is where I am at with this at the moment. When i get things closer to being done, I will probably make a video of all the functions in action to put on youtube. But that is likely to be a couple of weeks away yet. Lots of work to be done in the mean time.

 

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WPX and 3 Endorsements

WPX Award Confirmations

Its taken me 4 and 1/2 years to qualify for Worked All Prefixes. Mostly this has been achieved using low power. I wouldn’t even call it mostly QRP. Maybe 50% at most. However its all been done with low power. And by low power, im talking less than 50 watts.

For 4 years i was license restricted to 10w SSB and CW and made over 1/2 the contacts. But since upgrading and getting FT8 and other digital modes, mostly I have been running between 20 and 30w.

FT8 has been a game changer for me. I have really upped the contact rate. Like close to 1000 contacts in 6 months and that is not being very active on the bands.

WPX Endorcements

So along with the main WPX award, I qualify for 3 endorsements. WPX Digital Asia, well given that I have been operating almost exclusively on FT8 that one is no surprise. The other 2 are also low hanging fruit , Asia and Oceania. If you cant work everyone in your region there is really something wrong no? LOL.

LOTW Fees

The only disappointment here is that LOTW wants $50 Usd to process the award. Add in the 3 endorsements and cost will be $100 Usd. Converting to local currency that is $150. That is an extraordinary amount of money to have an award processed. Obviously I am not going to pay that. You would have to be mental in the head to spend that kind of money to have someone rubber stamp your participation award, so that the award manager can rubber stamp your award.

Applying for the award is $12 + postage Usd. Not bad, and reasonable. But, if i wanted the endorsements then that is 4 time $12 + postage. Say about $100 Australian. So in total, I could spend $250 or 1/4 the average weekly wage to get a participation certificate and 3 add on stickers

And they say ham radio is not a rich white mans folly. I mean, come on, that is an extraordinary amount of money for a piece of paper that only has some meaning to me that I have passed some arbitrary number of contacts. Horses arse comes to mind for wasting that much money on a piece of paper.

LOTW Horses Arse Award

In a follow up to this, you actually do not have to apply for the award and get extorted by LOTW. You can fill out a spreadsheet and apply directly to CQ Mag and bypass the middle man. Now they might ask you to confirm some certain contacts just to be sure to be sure, but then you are only paying LOTW for a couple of confirmations and thats gotta be better than spending a small fortune on nothing.

So I will probably fill out the spreadsheet and apply for the main award. Being a paper chaser that i am. HEHE

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