Well this one has been a long time coming. From the very first amp i built till now has been close on 5 years. Those first amps were rather scary contraptions dead bug on scrap pcb and had more than a few fets explode on me. And after the debacle that the 2×4 amp was, this time around i decided to cut it in half and make sure it is all working right and to have the mechanical aspects of the build right also, so if something does break and lets be honest something always breaks, that its easy to fix. I have not really started to push this yet, but atm with conservative biasing, drive and voltage i am getting a respectable 36 watts out. Its been key down for the last 15mins and other than the heatsink needing to be bigger to dissipate the heat its going well.
Here is the schematic.
Here is the PCB as laid out in my ecad of choice.
So rather than try and go the home made SMD route, i just went old school with the mechanical design. What i did was mount the fets loosely to the heat sink, then i snug them up, bent the legs with the pcb and then tightened them all down. It meant i could slide the PCB on and off the legs really easily until i was ready to solder them on permanently. If i fet dies, it should make replacing it a lot easier.
Setup for testing. I kind of just snuck up on it with voltage and drive. 13.8v and 200mw drive and I was getting about 7 watts out. Slowly and surely i brought the voltage up and the drive as well. In the end i was giving it 1 watt of drive at 24v and was getting 36w out. Its got way more to go, but its nice and stable.
A step in the chain, was about 25w here.
60v peak is 36 watts. Now i need to put some 200v caps in the low pass filter and make that look like a sine wave LOL
PDF of the schematic : irf510-2×2-amp
Gerber files for the pcb : REMOVED found an error when I ran 10A through it.
Here is the next part of my home brew CW transmitter. An Arduino and AD9850 are providing the DDS VFO signal. Q1 is a buffer to isolate the AD9850. I am not sure that it is needed, but its there to stop the gain stage from loading it up. Q3 is an amplifier that takes the 400mV P-P DDS signal and amplifies it to 8/9V P-P or there abouts into a 50ohm load. Thats about 200mW, enough drive to supply something like the QRP Labs 10w Linear. My plan is to design an amp to follow on from this that will provide 30 to 50w. More on that to come. Q2 is a PNP switch, pulling the base to ground turns lets power flow to Q3 thus keying the transmitter on and off. Simple but works.
Here is the schematic.
The PCB as laid out in Ecad.
Here is the board as built, if you are not colour blind and can read the colours on R5 you will notice i stuffed up here HAHAHA. Also Q1 silk screen is backwards.
Signal output into 50 ohm load at J2. This jumps up to about 10V P-P into the QRP Labs Linear amp. Not sure why yet.
Schematic PDF: CW-Buffer-Amp
Board Gerber Files: DDS-CW-Buffer-Amp
Ok, I have still been working at building stuff. Here is the latest piece of what will become a CW transmitter. These boards are 7th order Chebychev low pass filters.
This is the PCB as laid out in ecad. 4 boards per PCB and you can get 5 PCB from JLCpcb for $2 so that is 20 low pass filter boards. A life times supply.
My current project is a 40m CW transmitter, here is the LPF sitting on top of the TX/RX switch.
Filter response plotted with the NanoVNA.
If you want to have your own LPF boards made, here are the gerber files: LPF_Singles
There was a post over on EEVblog that got me thinking. What do you do when you are unable to, for various reasons not be able to procure iron core toriods. Well, obviously air core inductors are a thing and highly practical for the low values of inductance used at VHF and UHF. But, what about at HF?
Well it turns out that as long as you do not mind a little bit of size to the inductors, they are still a vary practical option. So what i did was grab out some 1mm enamelled wire and wind 9 turns on a bit if 22mm OD pipe i had laying about to see what sort if inductance such a coil would create. Turns out about 2uH. That is highly usable value in low pass and band pass filters in the lower HF range. And with some thought on the actual construction and mechanics of the coil, very usable inductors could be made this way if you are unable to buy iron core toriods for whatever reason.
Anyway, that is some food for thought. There is nothing magical about iron core toriods, air cores will work just as well and should have a reasonable Q equal to or better than a toriod. So if you are stuck, there is a solution.
Today I got all excited and thought about seeing what kind of range of usable range i could muster by connecting a small amp to the DMR hotspot and connecting it to the outdoors antenna. I was putting about 0.5w out into a slim jim about 3m off the ground. I could still hear the kerchunkers about 1km away. Nothing stellar but you really could not expect to much giving the setup.
So after that I got all excited and connected the Hackrf to the same little amp and again to the outdoor antenna and fired it up in FM CW beacon mode and got an ultimate range of about 3Km. Again nothing stellar but considering everything, not to bad either.
So the moral of the story here is I really need to get the antenna much higher into the air if I want to be able to turn this into something a little more usable without resorting to much higher power.