Shielded Filter Boards

I have been working on a few different things of late. One of the major ones has been to design some plugin filter boards that I can add shielding cans to. There are 2 different xtal filter IF boards, one using binocular inductors for impedance matching and the other using LC low pass filter matching. The latter is more an experiment to see if it is feasible to get a good passband ripple and impedance match using off the shelf SMD inductors and caps. Its easier to solder in 4 parts than wind inductors.

The other filter is a bandpass filter for a receiver. Again using SMD components rather than winding toriods. In simulation using a Q of 60 for the inductors the losses were acceptable. How they will go in practice is another thing. Worst case scenario is i add a MMIC amp onto the board and add some gain before the filter losses to make up the difference. My hand wound filters of the same topology had 1db loss in them. 2 or 3db loss using off the shelf components would be probably quite acceptable.


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.


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.