Another project i have had going on for sometime an arduino weather station. Got some of it now working on the breadboard and still have some modules to go including RF transmission of data back to an indoors station monitor. More on this to come. Right now, I am just making sure that the libraries are all working and integrating the code together into the one program.
I was reminded by a video by VK3YE about a PA chain he is working on using BD139’s that I had sometime ago thought about using them in a class c amp. I had seen something similar in a schematic for a circuit by Diz at Kits and parts. When you can get a good 5w from a pair of transistors that cost 20 cents and a couple tv baluns, why settle for less in your qrp rigs.
So i was looking for a way to make a negative voltage. Did a bunch of google and came up with a number of different ways. One that was kind of catchy was to use a 555 timer as an arsetable vibrator with a couple of caps and diodes to invert a signal voltage.
So in the above schematic, everything on the right of the 555 timer just sets up the chip to make a square wave which is then fed into C3. C3 and D1 invert the signal and D2 and C4 act as a filter.
So i built this on the breadboard as it gets very little love in this house. The cap on the right is the input and the output is taken from the junction of D2 and C2.
Using my signal generator rather than a 555 timer, I fed an 8v 50khz signal into the circuit. Yes my RedPotato scope has loss from the input protection diodes on its input and i am not using any compensation. So the yellow trace in the scope output is the input signal and the 2nd probe was used to measure the output voltage, 8v in -7v out, which is about right accounting for all the diode losses on the scope input and in the circuit under test. All in all i learned something out of this, which i am happy about and now actually understand what its doing and how it works.
On ebay there are these audio meter kits for like a buck delivered. I plan on using one of these as an S-Meter in receiver I working on. I know its hard to see in the image, but the 5th LED from the left is lit, the 10 turn pot makes adjusting the signal range a rather trivial matter. Just trim for the max audio voltage from the AF stage and it will be in the ball park. Its certainly no calibrated S-Meter, but it should give a fair indication of the strength. I am just feeding it with a few volts 600hz sinewave from my signal gen.
So the HABIB (HB1B) got himself a matching tuner. I also have a Sotabeams 10m pole, so the plan is to run these 3 things together with a 1/4 wave 40m vertical with a couple of radials, using the tuner to match the impedance difference. Won’t be the greatest for locals, but should improve things with DX with a low angle take off of the vertical.
Ok, so i could not get the variable gain to work as desired the last time I updated this. Now it worked, I was getting Oh i dont know -3db of gain attenuation, but that was not anywhere close to where I wanted to be, IE to have 0 to -20db attenuation. So I have given up the ghost on that idea and put it back in the sometime when i know more basket.
Ok, so I still need to be able to vary the gain of the RF Preamp, well, rather than vary the gain, lets attenuate the crap out of it. So after a lot of googling about trying to find solutions, i came across a circuit that is a variable attenuator. It should attenuate things by a minimum of -3 to -20db, so that’s in the ball park, its just a handful of resistors, caps and diodes, and by varying a supply voltage with a pot, you can change how much signal passes through the circuit.
Its simple enough, that even I can do it. About the only thing it requires are PIN Diodes, which if you are a popcorn kind of guy like me you wont have, but i have plenty of signal diodes, so off to LT Spice to build and evaluate a the circuit to see if it will do the job I require and then, if it works ok, build a prototype on the breadboard to make sure its going to work for reals.
UPDATE: Well I am thinking that LT Spice might not be the best tool for this job, by their nature diodes are non linear and a pin diode is linear and it is a non trivial matter to do linear artistry in LT Spice. The outputs i am seeing are quite wrong the higher the attenuation becomes as can be seen by the following image, however, when i ramp up the voltage, the output trace does track in input trace perfectly, so i think i might need to just breadboard this one and see for reals what is going on.
Will update again after I have breadboarded the circuit.