The Filter That Never Was

One upon a time in a land far far away, LOL. Well, this is one of those stories and it reads like a horror story. I have spent all day on this, pulling my hair out and trouble shooting and wondering what the fuck is going on here. Is it me, was it my design, was elsie screwing me with stupid values. Well, hold onto your hat Batman, this is going to be a bumpy ride 🙂

It all starts here. Designing a filter in elsie. Bandpass filter, 7.150mhz center frequency, 400khz bandwidth, chebychev response with 0.001 ripple. All good, simple even. I do not like the inductor values it spits out, i rescale the network to use 500nH. And begin to wind up the coil forms.

Everything goes to plan, i wind up 9 500nH coil forms, LCR meter says I am a genius and I agree with it, not only am i super smart and know everything, I am also good looking and a legend. 😉 That was this morning. Now let me rewind things back 4 weeks ago when this plan started to evolve. This should not be the problem.

After having elsie tell me what to do, I laid out this schematic in Diptrace and proceeded to produce a pcb to use. Schematic looks fine to me, looks the same as what elsie says i should do. This should not be the problem.

This is a small screen shot of the PCB layout, if you close your eyes and squint you can see the series capacitance and the shut capacitance and the inductance are all in the right places electrically. This should not be the problem.

Elsie gave this plot as an idea of what the filter should look like. Looks fine.

Never to take the word of a machine seriously, I checked the design in LTspice, just for sanity sake. Looks fine.

Then i built the filter, just one mind you, there are 4 on the board, but i have learned the lesson many time not to jump ahead of myself, because as the Mythbusters would say, failure is always an option. Becides, things going to shit are the norm around here. Today was no exception, bloody abortion of a filter was on 14mhz. Its like 1/2 my inductance and capacitance just vanished into thin air and left me with a pile of shit. And i had no idea why.

Being the clever idiot that I am, i did say i am a legend right, I began by melting the plastic cases on the relays, I mean pulling all the caps off the board and measuring them. Frequency went up, so that means less capacitance and less inductance. I am the boss of making strays of everything on my boards, so i usually have more than i need. Caps turned out to be correct.

So i rechecked the value of the inductors.  Actually i wound the slugs all the way in and as expected the frequency shifted lower. LCR meter says that i had 600nH. More than i needed but I was still on 14mhz, not 7.

Then after much wailing and gnashing of teeth, I grabbed out an inductor of a known value and measured it. It was wrong. Not just a little bit wrong, but a lot bloody wrong. I calibrated the LCR meter and checked the known value again and BOOM right on value. Checked the other coils i wound and they were 160nH not 500. Well there is your problem. I screwed up royally. I did not calibrate my crap and thus made a huge pile of it and wasted half a day on what should have been a 3 hour tour, a 3 hour tour.

So there you have it. That has been my day, fun, entertaining and well, i learned a valuable lesson. Never trust your meter not to lie to you. And with that, i am off to have a shower and cook dinner. The filter board can wait now for another day. I am done 🙂

Oh one last picture, i put some 500nH crappy inductors in the circuit and yeah its in band. Now i need to rewind my nice coil forms and make some filters that do not suck as bad as this nano vna plot looks.

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I Now WSPR On 3 Bands

This has been a fun little journey. What started out as look at me i can WSPR on one band, is not looking like, look at me I can WSPR on 5 bands. So what we have here is the 3 band WSPR TX that does 20, 15 and 10m and with an extra PA board and a little bit of code i can add 80 and 40m into the mix also, all this from just 2 antenna.

Here is the schematic for the PA board, less the power supplies. It requires 7V for the fets and 3.3V for the bias. The 10K pots and 3.3v bias supply allow for really fine changes in bias voltage. If memory serves me correct, 600mV was enough to run the fets at about 400mA draw each. The fets themselves are for VHF to GHZ range, so running them at HF is not ideal but its all about what you can find. These are 50 cent parts and what is used in many dual band HT’s as their 5w PA. The fact that I am getting 5w out at HF is rather nice.

Here is the PA board sitting in the box I made for the never ending story transceiver LOL. Note the 5A buck converter being used for the 7V power supply.

Here is a more broad view of how things are looking. 2w out using the old LPF board that was more loss than pass. Built new filters and things are now much better.

Fresh built LPF board without the losses of the old board means I am not wasting 1/2 the power in the filters. I was always going to be happy with a couple of watts, but the fact its making 5w, is nice. Note the heatsink stuck to the back of the PA board. A tone of vias are uses to wick the heat away from the fets and into the heatsink. Its been running for 24 hours straight and temps have remained steady around 50deg C.

Close enough to call that 5w. Its a little less on 20m, more like 4. But that is ok.

And finally, no spots on 15 or 10m, that is to be expected, but we have a number of spots on 20m. So i am calling this a success. Learned a bunch thought this project and the outcome was better than I expected.

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Corona Virus Update

Corona virus, whats it mean to me and my home brewing? That is an interesting question and one i am asking myself daily. Sure it looks like I might have more time on my hands with the chance of being locked down growing daily. So that is the positive. The negative is I am also watching the economy tanking and along with it my income. I am a contractor, our clients are all pulling work so that means I will not be earning much at all. This is ok, I have some savings and will survive, but it means that I will lock down my budget to essentials and so I cannot see me starting any new projects for quite sometime.

However, I do have 4 or 5 projects already going on at various levels of progress, so i have plenty to do for now and I have a lot of parts so that is not going to be an issue. I think what will happen is that once i have worked through the projects in the cue, I will end up going back to doing manhattan builds with through hole parts. I have a lot of board and parts, so I am good for a couple years at least. About all it will really mean is that I wont be buying any fancy PCBs at all.

Oh and I have a lot of kits here also, so I can bring them out when things get desperate and have something to build. All in all, i will be ok, but if things get really tight, I might have to shut down my blog, or move back to cheaper VPS hosting rather than having dedicated server hosting. One stupid flu and the world is turning to shit.

Stay classy everyone.

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NANO VNA Calibration

So long story. I have had some trouble getting the Nano VNA to hold a calibration and display the correct information. I always had a -10db offset when using certain SMA leads i had there. It would display fine with semi rigid SMA leads, but, others would show -10db. Like the leads had loss in them.

Well anyway, I think i have resolved the issue and got the calibration right. 0.5db loss in my leads would be about right and that is what is showing now, they are after all cheapest crap leads from China, not high end leads you would use in a lab.

So anyway, popper calibration procedure.

Open: Open Load on S11 port.
Short: Short load on S11 port.
Load: 50 ohm load on S11 port.
Isolation: 50 ohm load on both S11 and S21 ports.
Through: Shortest high quality 50 ohm cable connecting S11 and S21 together.

Then save that. That is it, that should then give you fairly accurate, well as accurate as the NANO VNA is results. As you can see by the plot of a 40m bandpass filter above, it looks about what you would expect from a known design that has low insertion loss. A couple of dB, made up of the lead loss and the filter loss. This means my filter has about 1dB insertion loss. That is something I can live with.

Anyway, through no fault of my own, I broke the mini USB socket off the original NANO VNA i bought and tore the tracks off the board. Yeah not really happy with myself but it is what it is. So i bought another one, this time it seems to be one of the better clones, it even came with shielding and a battery. So anyway, i plan this time to ruggedise my NANO VNA and mount the whole think into an aluminium box, and have an panel mount USB port on the side that will take the abuse of me pulling and stretching and inserting a lead in and out on a regular basis. So i ordered one of these as well, yeah the new VNA came with USB type C not that stupid micro USB rubbish. Anyway, by the time i mount this in a box, i will never have to worry about breaking the damn socket off the board again.

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