So I am sitting here this morning drinking a massive cup of espresso coffee and thought i really should post up where I am at with the receiver project. Its getting closer now to final testing and making noise. In fact, the final board will arrive today from china and as long as it does what its meant to, i might only be days away from hearing beeps
So anyway, onto the IF amps and mixers. I find the whole amp design, input and output impedance matching and how one can effect the other to be slightly confusing. So I went with an IF amp design that eliminated that confusion for me, the TIA amps. Termination Insensitive Amplifier by Wes Hayward
The mixers will be ADE-1 Mini Circuits jobs as I have a bunch of them here doing nothing and figured why not. I do have a board here I am designing with a number of different mixer topologies on it that I will eventually have made and test out homebrew mixer designs more fully, but for now I will use the commercial product.
I built the 2nd IF amp first, so I can have a direct conversion receiver happening in short order once I have a working audio board.
Well it works as designed and does the business obviously. There is a missing trace on the board, so I will eventually have to respin them and I want to fix the layout somewhat also and change the transistor footprints to wide not narrow. I do not like those small pads to solder to. LOL
I am going to redesign this board. The layout is sub-optimal. But for now, I am going to assemble it and see if I can make 2 filters worthy of use.
The one thing that has always frustrated me with many SSB/CW homebrew rigs is that you get one filter and one filter only. I have seen some use varactor diodes to solve this problem, by varying a voltage on the diodes you can vary the capacitance and this the width of the filter. I thought about going down that route myself for a while, but in the end, settled on having 2 filters as I have lots of relays already.
The plan is for an SSB filter that is about 2200hz wide and a CW filter about 500hz wide. I am also using the good crystals for these also. 4.91520Mhz xtals sorted to within 10hz of each other.
Each crystal was loaded up into my you beaute xtal tester and the frequency measured to within an inch of its life.
My function gen also has a frequency counter that counts down to 1hz resolution. So I measured my crystals and sorted them into piles within 10hz of each other. If i cannot make acceptable filters with such tightly grouped crystals, then there is no hope in hell i can ever make a good filter ;
Out of this pile of crystals I have 6 groups of 8 crystals all within 10hz of each other and a few more groups all within 20hz of each other. Buying these crystals was certainly well worth it. The pile in the bottom right corner are the outliers. It seems even good quality crystals have some in each batch that should be rejected.
Got the parts all sorted, just gotta build it now. Will update this post once i have built and tested each filter.
Part 2: Well i kind of expected that to not take forever, but anyway here is the update.
The build is pretty straightforward and all went to plan
Other than the width being a little wide in both cases and a bit to much ripple, I will say that I am happy with the shape of both filters. I can narrow both up easy enough, just need to change the cap values and the ripple, well it might be a bit of work, I will need to play with transformer values. I used 4:1 impedance transform, but 3:1 might actually be closer to the mark. I will have to play there and see what i can come up with, but a better match will remove some of that ripple.
For now though, I can keep on building the rest of the project as these will be acceptable to get things working. I can make them perfect later. Next job is IF Amps and mixers.
Being invasion day here in Australia with not much doing, between watching the cricket, sleeping and doing not a lot more I managed to build and test the bandpass filter board.
Overall, well its MEH. The filters are nothing to rave home about, they will work for now, but I do think i am going to redesign the board using a different topology and get much better attenuation in the stop band.
I ran all the filters though the bode plotter and remembered to screen grab 3 of them which i will now present here for laughter and ridicule.
40m bandpass filter is a 2 humped camel. The width between the peaks is fine, but i will have to play with the coupling cap value to see if i can take some of that hollow out.
20m bandpass filter looks ok, its just to narrow.
The 15m band filter is also to narrow and the high side attenuation is kind of poor at best.
The 80m filter was also MEH but i forgot to screen grab it, It was double humped, but only -3db down in the middle and the high side attenuation was also poor. Anyway, these will allow me to get building and testing more parts of the radio and think about changing the topology and laying out a new board and having it made. I have 2 other board sitting there waiting to be sent to JLCpcb, the 3rd iteration of the audio board and the first of a final PA board. More on those to come.
Got to do some building on this the other day and knock up the LPF board. Ran it though a battery of tests and other than Elsie screwing up the values of the filters, everything worked at designed. I gotta suck the inductors out and add some turns. Not a fun job but its gotta be done. Gerbers for the board can be downloaded, from HERE: http://robs-blog.net/2018/12/16/4-band-lowpass-filter-board/
This picture will give a bit of an idea on the stackup and how these things well all fit together. Using the 10w ebay PA for now for testing until I find a transmitter design i want to use and lay a board out for it. But as an idea, it will be using a pair of RD16HHF1 conservatively biased for 10w or so.
So today I populated the 2nd IF mixer, the 2nd mixer and some of the switching circuitry and gave all those elements a little bit of a test.
To follow on from the test of the first IF amp, I put a 10mv signal into the first IF amp and tested the whole IF strip through the Xtal Filter and out the 2nd IF amp buffer and it all seems to be working quite well.
But there is one issue and I am not sure what the cause is. The 470R resistor from voltage divider on Q6 does not go straight to ground and trying to find its ultimate path to ground has been elusive.
To make matters worse there are q Q10 in the schematic and Q10 and 21 on the board, there are 2 diode on the board not on the schematic and I cannot for the life of me work out what the actual cause is here. Either one of the Q10’s is not being biased on or there is some part missing or either VR7 or VR2 are not providing the emitter ground path on the emitters of those Q10 transistors.
Anyway, I think what i will do is pull both the Q10’s out and related circuitry, part of which is for a signal meter and just hard wire the 470R voltage divider to ground. That will at least leave me to be able to complete the receiver and actually test it.
At one point today I had fed a 9mhz signal into the IF strip and was getting a 600Hz tone out the audio amp, so that in itself is promising as I was worried about the dodgy design 2nd mixer. Which looks like a ring mixer but does not have trifilar windings on the inputs and outputs, but some crazy bifilar winding on one side and a variable resistor and cap divider on the other. I am not sure about this mixer design and I have not done any google foo to see if i can find it, but it does work
Oh and for the diodes in the mixer i did not use the typical 1n4148 diodes that everyone uses, but rather I used a Schottky Diode 1N5711, it probably means i have way to much drive from the BFO as the switch on of these are much lower, but its working and for now, I call that a win, particularly so when i really do not understand the mixer topology being used.
Oh and as far as audio goes, of the 2 AF stages, I am only using the LM386 first stage because the TDR2003 is motor boating or oscillating or just acting like a tard and I am not sure why here either
I do hope the next post will be a video of it receiving signals. Failure is always an option here, but so far this build has been very challenging, but its been rewarding and even if it ends up being a pile of junk, i will have learned a lot.
Ok, this has been a fun section to build. I spent the better part of an hour scratching my head over this and wondering why it was not working. Well, this is how I approached this section of the build.
First thing i did was rummage around in my parts bins for canned inductors. I do not have a lot of these things, but i do have a few and those are of a few different types. From what I understood from the few bits in information i have been able to glean from the internet, they used 10.7Mhz IF cans. This then narrows it down to 3 or 4 actual IF Transformers.
Of the ones i have in my parts trays, only one looked like it had suitable values for a 7mhz filter, with windings that measured 120nH and 2.2uH. That was going to get me ballpark and allow me to simulate the required capacitor values.
The fun begins when you go to buzz out the actual pcb, because the input series cap is not on the board, but its on the schematic, first issue that did my head in. The output series cap is on the board and on the schematic. Simulating without the input cap seemed to make no difference at all. So i just left it out rather than cutting traces and soldering it to the bottom side of the board. This is not the first time in this build that i have found such discrepancies either. In fact, it would be a miracle indeed if someone could stuff the board with parts and it just worked.
Not feeling very confident at this point, I thought it would be best to simulate the lowpass filter just to make sure it was ballpark and surprise it was actually quite ok. Perfect in fact.
Ok now this is where the fun began. First thing i do when building something is buzz it out to make sure there are no shorts. Well the low value windings on the canned inductors are only 100nh. They look like a short to ground at DC. So sticking my continuity meter on both the antenna pins caused it to go beep and I spent the next 30mins trying to track down the mystery short to ground, before i worked out what was happening
So finally I wired up some power and injected a signal and started to trace it though the board all the way to the input port of the mixer footprint. Initially there was nothing, so i probed around and found RF at a mystery pad showing a 6.8uh inductor, which is in series with the 7pf coupling cap. After some head scratching, I think that this meant to be a capacitor so that you can series 2 caps together to get a lower value. I dropped a wire link in there and things finally worked.
0.02V signal injected at the antenna port, through the lowpass filter, through the bandpass filter into the RF Preamp and we have 50mV. I do not think the losses in the filters are all that high.
So that end this nightmare 🙂 Its all worked so far, just some IF amps, IF filter and 2nd mixer to go to have the receiver working. Still plenty that can go wrong from here, but its getting there.