So after much rooting around, I have this finally working and making ham radio noises. Its not brilliant, but it will do the job for now.
More things being tested for the hand held project. These are 1A power inductors. I need something much more low profile than a toroid to use in the low pass filter and also something not through hole. So i grabbed these power inductors to test to see how much loss they are going to induce. Turns out they are not all that bad and at least as good or better than the hand wound toriods i used on another board.
Other than the corner frequency being in the wrong place because i used the wrong cap values, the loss is less than 2db and that will be good enough for the kinds of girls I go out with. Oh and the hand wound type 2 iron cores? 3db loss. Go figure.
Ok, the long and the short of it. It actually works well. And by well, i mean it can hear me talking on the home station, it can make a tone from injected signals yadda yadda yadda.
So what was the problem, well ended up being a dry solder joint, the most basic of all problems. I had pulled it apart, cut traces, did eleventeen other things to it trying to work out why it was ringing its head off and making no sounds, and it turns out the LO input pin was dry and not making contact.
So what next? Well i start to integrate things onto one board now and start actually assembling something I can use to make a QSO with. Wish me luck.
They say a picture is worth 1000 words, well here are 4 pictures so that has to worth a novel or 2. Anyway no opus this post, just the meat of it. I built 2 xtals filters, in each case i grabbed 8 random xtals out of the parts bins, no matching. The xtals are ebay specials 100 for $1 and are probably rubbish that did not meet specification and was taken out of the rubbish bins at the factory. So the highest quality man cannot buy in other words.
The difference between the 2 filters comes in the matching, filter one is matched with the traditional ferrite inductor, though i used binoculars as i prefer winding those for transformers over toriods and the 2nd filter was matched using low pass filters. The filter plots speak for themselves, the low pass filter matched has a better shape but -15db less attenuation in the stop band than the ferrite matched filter. And the losses were about the same -7db. -3db is 2200hz. So with some matching the crystals i should be able to improve the flatness in the passband and probably remove some losses.
But other than that, if you can live with -50db of opposite sideband suppression, the grabbing random xtals out of the box and matching with low pass filters might actually be a rather simple and crude way of IF filtering. For now, i think i will stick to winding the transformers, or buying 4:1 transformers off Mouser.
I have a need, a need for speed. Well, not really, but I have a need for filter components that are small. I am building a hand held after all for 40 and 20m and you just cannot fit large toriods into a tiny box you can carry around very easy. Well, you might get them in there with some careful design, but the mechanical durability of thin wire being bounced about is going to be suspect at best.
And what if i want to sell 100 of these things? Who wants to sit there winding a bazillion toriods? Not me, that is for certain, i do not even want to wind 2 for a prototype. HEHE. Anyway, that got me to thinking, and my thinking i mean i really did not think a lot at all. I got on the confuser and did some jazz hands on my parts suppliers website and poked my peepers at what they stocked to see if there was an off the shelf solution to my problem.
So they have RF inductors, but the problem with these is the low Q. And we all know that Q stands for Quicksand and with a low Q your signal starts to sink into the mud and you might not hear the ducks anymore. Ok, so the question is HOW LOW CAN YOU GO. So i looked and I looked and in the end i found a 1.5uH inductor that had a Q of about 25. TWENTY FIVE you might rightfully exclaim and yes you would be right, that is a bloody low number. But such tiny inductors are always doing to have a low Q much like many hams have a low brow and low I and Q when they down convert their thoughts to base band.
So how bad is it. Well not as bad as you might think. -6db loss is -5.5db loss more than the same filter i built using hand wound T50-2 toriods, which on second thought is actually massively crappy. But hey, i still gotta stick this in a handheld box and have it mechanically secure. Thankfully there is a solution.
Back in the dark old days before time began, the Motorola company or was it Bell Labs, either way, it does not matter all that much as I am just spinning a tale here of those that do, or as I like to call them N Type and those that don’t, the P type and someone added in B J and T into this alphabet soup of inter-related terminology and created the PRE AMPLIFIER.
I need to have some gain in the system anyway so if i put some gain, i don’t know lets say somewhere between 6 and 10db before the filter, i can make up for those filter losses before we have even started. BUT WAIT you might ask, that is going to cost more, and yes sirreee Bob you would be right. But i can take those 2 SMD Trimmers out as they are not going to be needed and replace them with 2 1 cent caps and that just saved me $1 on my BOM cost.
WOO HOO i can do radio design me.
Finally got this right. Well kind of. I need to add some capacitance out of it to bring it onto 40m because my inductors were a lower value than i thought I wound them at. But other than that, the shape looks fine and very low insertion loss.
As designed but using the incorrect inductance values.
This is the baseline, i still have not worked out how to reference things back to 0db.
Gerber files for the PCB Boards: bpf_singles
A quick update. Im happy with that. Shape is fine, low side attenuation is good and will kill any AM broadcast.
I had reason last night to use the Nano VNA for something and while i was at it, i grabbed down an Xtal filter i had built ages ago and put it on for measuring. If i recall right, and I would have to go and look in old blog posts, when I measured this using the Bode Plotter in the Red Pitaya it looked alot like Bart Simpson and had 5db of ripple in the pass band and very lossy. Well, better tool, better measurements and this does not look all that bad to me. The ripple is acceptable and actual loss in the filter is around 3db which is more than fine. Its not quite narrow enough for a CW filter, but its good enough to get started with.
So i am back into receiver design mode for the time being and figure i would try a different band pass filter topology. Elliptic filters have some rather nice attributes. Steep skirts for a low order filter, a relatively fixed level of attenuation in the stop band at the expense of some ripple in the pass band and stop band. The other major con is non linearity of the phase response. How much of an effect that will have on a desired signal is yet to be determined. The filter design below shows a 3rd order elliptic filter, if i can build something that looks something like that with acceptable loss figures, then it might be a good candidate to replace the weakly attenuating double tuned filters with a Butterworth response that I was using.