Got back into building this receiver tonight, 8 resistors and a pot to go and its done. Well that’s a job for tomorrow now, I am off to bed.
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.
So i have merged the separate simulations together into the one file now to get a better understanding of how they will work together.
And the simulation looks promising, they seem to be happy working together.
So now I have built the RF Preamp onto the perf board with the bandpass filter. The only testing I have done so far has been checking for continuaty as i go along to make sure A goes to B and does not short to ground hehe. I still have to add in the pot for setting the amount of gain in the amp and to wind the 200 ohm to 50 ohm transformer that goes on the output of Q1. Its actually nice to see this moving along, lets just hope it works. HIHI.
RF Pre Amp
So I am back feeling a little better and it is time to move this project alone a little. Some say that for a 40m band receiver an RF Preamp is overkill. In some places it probably is, but where I live most of my contacts are over 1000km away, are weak at the best of times so I think having a little gain ahead of the mixer is a good thing, but it needs to be variable.
My initial thought was to replace an emitter resistor with a potentiometer but that would mean having rf running though cables to the front panel and the chances of things going bonkers increases when you do things like that. Emitter degeneration was still going to be the way to do this, but by using a second transistor as current controller, control the current and control the gain and that can be done with voltage and a pot. So no RF being shipped about the place.
So now for the actual circuit, this was one given to me by Pete N6QW homebrewing legend and all round good guy. Its one of his GOTO circuits, nothing fancy just a BJT amp optimized for bandwidth and as an IF Amp. So I took that and modified it slightly to fit my needs as a 7mhz rf preamp. And like batman off to the batcave, we hit the simulator in LT Spice to confirm that things are close to spec and things will work well within our design parameters.
Ultimate gain is about 9db, and with 3v on the base of Q2 the gain is like 3db or there abouts. Should be good enough for what I require. Now I just have to build it and see if the simulation meets up with reality. Will post an update when I have it built.
Now i also should mention that I was going to compare and evaluate a number of designs for each stage. I did pull some designs out of different schematics on the web and put them through their paces, mostly they were crap. There were some that were better, but they were not popcorn enough for this project. A very notable mention is the front end of the simple ceiver with its dual gate mosfet made from a pair of J310. While i have J310 here in the crap box I am saving them for another project. If I can keep this one to BJT only i will be happy. Anyways, time to melt some solder and see if it works.
I was digging about in my pile of crap the other day and came across these IF filters i bought from MiniKits. 455khz IF with 2k -6db bandwidth. So seeing I have not been well for the last couple of weeks, I decided to knock up a schematic using these parts and a bunch of other bits i have sitting here, with the idea of eventually building it.
The audio stages are what I will use in the CW Superhet project, so it was not a total waste of time, same with the RF preamp. Anyway here is the schematic, its unbuilt and untested, so it might be a big pile of crap. You be the judge.
So i rewound the inductors, after lots of stuffing about I got them ball park.
Ran the filter up in the bode analyzer and well the filter part looks close enough, but it still needs more inductance, maybe another 2 turns per toriod to get the center frequency up to where I want it. Now, that second hump in the plot at 21mhz, I got no idea why that is there, when you look at the next plot where i inject 20mhz into the filter and look at the output on the scope, you will see it does not exist in reality, but shows in the bode analyzer.
Filter output at design frequency.
Frequency output where that hump is on the bode plot. Its obviously an aberration in the analyzer and for now I am not sure why its there.