Failure is always THE option.

Failure is always an option as the mythbusters were wont to say, but for me it seems more like failure is always the option. Built every module and tested each individually and then bringing it all together as a whole some of it refuses to play nice together. So, the idea of a multiband receiver might just get canned and i go back to making a single band receiver.

So what is wrong with it? Well, the rf preamp did not really work right. The bandpass filters were always crap, but that is an easy fix, the LO needs buffer amps to drive the mixers properly and the AF amps are kind of ok but i think need a little more gain in there and the attenuator, im not sure its even working. It makes noise but its nothing to write home about.

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Dummy Load Characteristics

I am currently in the process of building a dummy load capable of 500 watts. With the amp project capable of potentially a couple hundred watts, being able to dissipate a lot of power for extended periods of time is kind of needed. Yeah i want to run full key down for like an hour into a dummy load to really stress test the whole amp design.

The dummy load will use 25 100 watt 50ohm resistors in a 5 in series 5 in parallel configuration, mounted on a huge heat sink with fans. Hopefully this will dissipate the amount of heat that will be generated. Anyway, this got me thinking about the dummy load that i built many years ago using 20 5w ebay resistors in parallel and how well it actually performed. Back then i did not have the test gear to actually characterise it. But i have had a VNA for a while now, well, not really a VNA more a glorified S11 meter that measures via vectors. 🙂

So i plugged it in and gave it a look and well, to my surprise it was not all that bad. 1.05 to 1 from 1Mhz to 60Mhz, which is the limit of measurement for my VNA. I am only interested in HF so more than good enough for the kinds of builds I make. I think it would be usable into the 2m band, but from there on parasitic’s would really take over and make it more or less unusable.

After doing this measurement, i kind of decided to order and better VNA and be able to do 2 port measurements and have extended frequency range. I have some other things i want to build like rf taps etc that would really benefit from having a full characterisation done. Oh and filters.

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Audio Amp and Filters

This audio amp is going to be the death of me. I am on like the 4th iteration of this board and still its not working right. Each time, something that worked now does not and this time it was the preamp. Everything else is working great, the SSB AF filter works perfectly, the AF amp works perfectly, i still have to build the CW filter and test it, but that I think should be ok. So i bypassed the preamp and things are great. Not sure i actually need the extra gain.

So now i have all the individual bits of the receiver working to the point where I can join them all together and see if I actually have a working receiver. If i get time tonight, that is exactly what I will be doing. Its getting close now to actually making noise.

 

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Power Amp Project, An Update

I got the heat spreader bar drilled, machined and tapped this time without breaking anything. Now i just have to order a dummyload and swr/power meter to be able to test it, as it should be all together today. I have the missing caps and inductors here to complete the job.

 

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IF Amp Board

I have built and tested the IF amp board for my receiver project. Its all system goes. The amps themselves are termination insensitive amps “TIA” by Wes Hayward. No reinventing the wheel when these things work so well. Though the 2nd IF amp might eventually become a Hycas so there is some AGC happening there.

 

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100w? + Amp Part 2

Schematic: vk4hat_amp

Ok I have made wholesale changes to the schematic. Changed the voltage divider biasing to a more conventional biasing schema, adding in some feedback, which i am a bit unsure of, its probably not totally necessary for 40 and 20m band amplifiers, but for higher using IRF510’s its needed, I can always not populate those footprints when i build it. The resistors i had from source to ground were removed, those were meant to be swamp resistors on the gate to tame the lower bands, but they are probably not really needed and finally the biggest change has been to the output transformer, in which the single turn winding is going to be copper tubes with VCC to the drains delivered via the center tap on the single turn winding.

I have also been thinking about ground paths and return loops and all that other stuff that is over my head. But, I think something i am going to do is change the SMD bias components to through hole and move the bias traces to the top of the board, this will give me an almost unbroken ground on the bottom layer. Which can only been a good thing I guess. And I also need to do some research on the rf power handling of trace widths. Yet another thing i really don’t have much of a clue about. So i need to unroute the whole board almost and kind of start again. But for now, its off to find some credible source on trace widths and rf power levels.

 

Lots has changed since i posted this, this morning. Pretty much changed everything in the board layout. Here is a 3d render of where its at. Yes, i changed back to through hole parts because it would allow me to jump a trace using resistors and keep the bottom copper layer almost completely unbroken. So its a perfect ground plane to stick the broken top copper to.

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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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