So i got busy today and wired up the uBitx. Turned it on and tuned around a little to find some signals, well there was not much going on anywhere but I did manage to find this one ZL on an other wise dead band. Its good to know that the receiver works, now i just have to wire up a mic and check its working on transmit. But that is going to be another day.
So I recorded a little video from tonight’s 7.130 dx net. I have been giving the receiver a good workout to see how I like it. So far, to be honest, I think i prefer the Bitx40 over the uBitx. Though, it might be I just need to play with the IF and BFO settings to move the bandpass around a little to where I think it sounds better.
So, it been a rather big week at work and I did not want to start something that will take me weeks to build and I thought I would get this done today in an hour or so and be testing it. Well one thing lead to another and I still need to make the low pass filter for it. Well that looks like a job for tomorrow. Nothing extraordinary here, 100mhz oscillator and ADE-1 mixer and a 5v power rail for the oscillator.
Well i have it all built, now i need to dig out a DDS LO and give her some power and see if it is a skookum choocher or if the pixies turn to smoke and leave the building just like Elvis. I did not enjoy building this all that much, the form factor is tiny and everything is stuffed in there as tight as it can be. But, if all goes well, tonight we might hear something coming out of a speaker.
While QRP Labs makes some wonderful kits, they really could do with adding 20% more board area so that old sausage fingers can got in there and do some Da Finger Poken. I get that some things really need to be packed in tight, but others do not and I think the filter boards can be much bigger and contain a 2nd set of holes for the wires rather than trying to solder them to the tops of the header pins. Anyway that’s enough moaning from me. The kit is going together well all tests have been successful and I am confident that it will eventually power on and work.
I got reading glasses a few months back, but I can tell you I am really noticing how hard some things are to do, to solder that one smd park i had to use both my glasses and the huge magnifying glass to get the job done 🙂
I love it when a plan comes together, needed 3.1uh on the 2 inductors, and guess what I got, 3.1uh HAHAHA.
I still have the polyphase board to build. Then i need to start working out what bits i am going to add to it to make it complete. Arduino and SI5351A obviously and some kind of AF amp should do the trick. More on this to come.
So i got some more boards made in China. Board one has a PLL Synth DDS VFO, Tx Rx switching and an AF Amp. Board two is a 3 band Bandpass or Lowpass filter board and Board three is a shield, this boards are designed to be stacked together using double height headers with SMA patch leads joining blocks together.
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.
So i got it all built, for some reason the gain adjust is not working and I need to investigate that to see why but other than that, it seems to be working and the gain is about what was predicted in the simulation as can be seen by the scope output below. So I will fix the few bits that need fixing and then move onto working on the IF filter. I think this will do the job required quite nicely.
Oh and I have worked out whats going on with the variable gain, Q2 is a current source, so it supplies more current to the amp above, not less, so what I need to do is redesign the amp to have the MIN amount of gain required for my circuit, then use Q2 as a gain boost. Now that I understand what is going on, I can redesign the circuit to have 3 to 12db gain swing. More than enough for the kinds of girls I go out with.