4.91520 Crystal Filter For CW

So i was talking with a mate about crystals yesterday, he has an old 2m rig chock full of pairs of repeater crystals and the discussion was about overtone and fundamentals and all that. So for shits and giggles I knocked up a test fixture to measure some crystals on the VNA. And with the best crystals I have was getting the typical plot for a crystal.

And one thing lead to another and I built another crystal filter, this time for CW. I kind of did not make my target, of 500hz and was closer to 1200hz, but that is nothing changing the values of the caps cannot fix. Ripple is nice, the best  i have made yet, the shape is acceptable and the stop band is down near the noise floor of the VNA, so all in all its not a bad looking filter and should be usable once i make it narrower.

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Op Amp Gain Bandwidth NanoVNA

I have been thinking for a while about using high speed op amps as IF amps in a receiver. As mental as that might sound to some, it actually makes practical sense in someways. Gain is easy to set, impedance is easy to set, being that the IF is at a fixed frequency you are not worried about being broad banded and can tailor the circuit to suit by using a suitable op amp with sufficient bandwidth to do the job.

And that is where the problem lies, op amp gain bandwidth is given at unity gain, IE a gain of 1 and as soon as you start adding gain, you start losing bandwidth. This means that you need a unity bandwidth of Gain x MHz to be somewhere close and then you also need a op amp with a fast enough slew rate to deliver the waveform amplitude you desire.

Now there are what are called current controlled op amps that give much better gain bandwidths above unity, but they are kind of expensive and so that leaves using voltage controlled op amps and working around all its limitations, but as you start to get up there in unity gains above 300 MHz even they start becoming non cheap items also.

So a few weeks ago I was on one of the Chinese parts sellers just looking at all the different crap they have and for some reason I ended up in the op amp section and found an op amp with a few hundred MHz unity gain bandwidth for pretty cheap. And by cheap, i am talking in the 40c each kind of space. So i bought a few to try out.

So i built up the non inverting circuit as shown above. Which is quite simple to set the gain and the impedance’s just by changing the value of a few resistors.

This is the circuit built on the test board. While the op amp is an SMD part, its SOIC 8 so its big enough that even a dummy with coke bottle glasses could hand solder, but I am kind of slack in that regard so I used paste and hot air, i mean why not. LOL

You can see from the S21 gain plot that there is usable gain from 40m to 10m. I am not sure what that notch is, but i suspect that its an artifact from the nanovna, because a manual sweep of that section of spectrum using a function generator and oscilloscope did not show that dip.

Oh I should say that I have the gain set to 6x for this test. And slew rate was not an issue for 7MHz to 30MHz, with the op amp able to deliver 1.3v peak to peak quite happily. Below that, particularly around 80m, the op amp could not deliver much more than 500mV peak to peak. So for a 9MHz or 12MHz IF amplifier, the op amp might be a credible option.

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Measuring Gain Bandwidth With NanoVNA

The NanoVNA is more than just a fancy SWR meter for checking your antenna. Its much more and a very useful tool for the home brewer building amps and filters and the like. Now i have been buying things like crazy for various projects that I would like to build in the future. Often these parts are spec’ed for bands not of interest to me. So what do you do? Well you measure them at the frequency of interest and see how they work yourself.

Below is a MMIC amplifier part that I found for really cheap. By cheap I am talking in the 40 cents per range, so i bought 100 of them. I mean why not, they are spec’ed for 100mhz to 3gig with +30db of gain. Worst case senario they are kind of useless at HF and I will have parts for when I actually want to build things at VHF and up.

So anyway I had a test board built with 4 different circuits on it for testing out various parts I have here and it includes Op Amp, Mosfet and BJT amp circuits. So i decided to start with the MMIC and see what it can do.

The good thing about MMIC gain blocks is the fact that they have such a low parts count. 2 blocking caps an inductor, bypass cap and gain setting resistor. Initially i set the bias resistor a little to high and was getting a lot of distortion, so I halved the value and boom it was providing 24dB of gain at 7MHz, which is my go to frequency for all these sorts of tests. Next though, I wanted to see how the gain bandwidth was. My bandwidth of interest is HF so 3 to 30MHz, so just for shits and giggles I measures it out to the 6m band.

The test setup was Port 1 of the NanoVNA to the input of the test board, the output of the test board to the RF Sampler i made the other day, which was also connected to a dummyload and then back to Port 2 of the VNA. Then an sweep of was performed and the S21 Gain was measured.

Things actually looked quite nice and rather flat. A few dB down at 80m and 1 dB down at 6m. That is pretty good for a part with a minimum frequency of 100MHz. So all in all, this part is a winner and something I can use in a project sometime soon.

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No Transformer Impedance Transform

So i am in the early design phase of a new project. Something all secret squirrel that I am working on with a friend. Anyway, so the problem i faced was how to transform impedance from 50 ohms of the antenna, filter etc to the 1500 ohms of the input to a NE602 mixer. There are eleventeen different ways to do this and the way i ended up going about it was to add a low pass filter element after the double tuned bandpass filter I am using.

This has a bit of a bonus effect of also improving the high side slope of the bandpass filters shape. So I fired up RFsim and designed the match, and was confused about the shape of the low pass filter. I am not sure why there is some sort of resonant effect making the frequencies around the cut off go higher than 0dB.

Kind of confused about the simulation RFsim was giving, i fired up LTspice and simulated there, were the resonant effect became apparent, with the 9dB increase in the pass band. I assume that is some kind of artefact in the simulation. I will know soon enough when i build and measure the match.

So then i added in the rest of the filter into LTspice and well I think that should work well enough and is a simple solution that does not require the winding of any transformers. Oh and the reason why I am avoiding transformers, is that this is going to be build all in SMD components and I want to avoid the winding of any inductors at all costs.

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LPF Board Measurements

Here are some measurements of 3 filters off the low pass filter board I designed and built. First harmonic of each band is at least -50db down. The 4th band which in my case was for 15m, was well out and I need to look at it to work out why, cap values is going to be my initial guess LOL. I built this thing and have never used it LOL. Though I got some new boards coming from JLCpcb this week and i hope to have a transmitter happening that I will use in combination with the HackRF One as a receiver.

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Air Core Inductors

There was a post over on EEVblog that got me thinking. What do you do when you are unable to, for various reasons not be able to procure iron core toriods. Well, obviously air core inductors are a thing and highly practical for the low values of inductance used at VHF and UHF. But, what about at HF?

Well it turns out that as long as you do not mind a little bit of size to the inductors, they are still a vary practical option. So what i did was grab out some 1mm enamelled wire and wind 9 turns on a bit if 22mm OD pipe i had laying about to see what sort if inductance such a coil would create. Turns out about 2uH. That is highly usable value in low pass and band pass filters in the lower HF range. And with some thought on the actual construction and mechanics of the coil, very usable inductors could be made this way if you are unable to buy iron core toriods for whatever reason.

Anyway, that is some food for thought. There is nothing magical about iron core toriods, air cores will work just as well and should have a reasonable Q equal to or better than a toriod. So if you are stuck, there is a solution.

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Filter Capacitors and Voltage Rating

As i explained in a previous post, I am building an amp that is going to be capable of doing more than 100w. No biggy for most people but when everything you have built previously has been qrp there becomes a whole new set of challenges. With qrp you can just she will be right mate and do what ever you like “Almost” and it will work to some degree. So with this in mind, i set forth on working out what voltage rating i would need for the capacitors in the low pass filter.

What i really wanted to know, was the WHY behind the decisions others had made in their design choices and I started to ask others WHY as well. Often the reply was just use Mica Caps. Did not matter what power level, the answer some gave was just use 500v Mica and you will be right. No explanation, just this is what I do.

So that just never sat well with me, the answer cannot always be use $60 worth of 500v caps in your low pass filter. It took a while but i finally came across some information that seems credible and more importantly reliable and my gut feeling was right, Mica while good, might often be over kill, but still there are some caveats that are important to understand.

Firstly the voltage that a capacitor in a LPF sees is the peak to peak voltage of the AC wave form. Might be obvious to some, but its not information that is easy to find on the internet. So knowing that we can then use the power formula to find out what voltage might be present in the filter. Power(watts) = Voltage^2 / Load(Ohms) where in this case, the voltage of the signal is RMS Volts and once you have solved for it, you need to convert RMS to Peak to Peak by multiplying by 1.4.

So in my case 100w = 70.7vrms/50ohms :: 70.7*1.4 = 100v peak to peak. Now obviously there can always be an impedance mismatch between the low pass filter and the antenna and with that a change in the signal voltage. In my case i also looked at 100w at 25ohms and 100ohms load to get a better picture of what might be expected of the capacitors in the low pass filter. Being that I am actually designing the amp deck to be capable of 400w even though i am only running it at 100w, it means the capacitors need to be able to withstand a peak voltage of about 350V.

So onto the next point, I wont be using Mica Caps, for the very simple reason that $50 worth of caps is a lot of money to throw at something that might be a dismal failure. I will be using SMD ceramics and here is something i learned just recently, ceramic caps derate in capacitance the closer you get to the maximum voltage rating. According to a TI white paper i was reading, you should probbaly derate ceramics in filters by at least 1/3. So I need at least 360V rating, that means using 1Kv or 2Kv ceramic caps should be perfectly fine. The good thing is, Kv rated SMD caps are cheap, in the sub 50 cent range in 1 of qualities on mouser and they are NP0 and 5% tollerance. So i have solved this problem now and can move onto other things like output transformer and core sizes i need to use for the power level.

More fun.

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