Rob’s Receiver Part 2: Band Pass Filter

Bandpass filter as designed in Elsie. In practice was a little low in frequency when built. I changed the end caps to to make them a little lower to bring the frequency up. The bandpass is rather ripply and if i remember latter I will Bode Plot it on the red pitaya.

Here is the schematic. It works but does not have the smoothest bandpass. I will see how it goes for now and if its not good enough, redesign it.

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Robs Receiver Build: RF Amp

So i have started to build the actual project now rather than build and test different blocks. Its about time that I actually built something from beginning to end. So lets start with the schematic.

A blank canvas all ready to go.

The first stage built, but not working in this photo. The J310’s i have, have a different pinout to the data sheet and i put a cap where a resistor should be 🙂

Got things fixed and working in this photo.

Maximum gain from the dual gate rf amp.

Minimum gain from the RF Amp.

So far things are going swimmingly and next job is to wind an output transformer for this stage and then start on the bandpass filter.

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IF Filter

So after building the IF Filter haphazard looking like a spastic, I connected up the test gear to run a Bode Plot to see the shape of the filter. I then proceeded to sit here wondering what i did wrong for the next hour as my 500hz CW Filter is almost 4K wide and probably ok as an AM Filter. LOL. Well, slow as slow can be it dawned on me, I grabbed 470 capacitors out of the box, thinking they were 470pf not 471 capacitors which are 470 pf. So in my haste and not thinking i have 47pf capacitors in my filter not what i originally planned.

 

On a positive note, the shape is not all that bad and not to much ripple even though the input to the filter is not correctly terminated. That is another headache, trying to get accurate measurements of in and output impedances and matching right when you have not built the preceeding stages. So I am going to switch to Termination Insensitive Amps for the IF and make my life easier.

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MPSH10 IF AMP

So i had another swing at the MPSH10 in LT Spice and what it says should happen and what actually is happening are 2 very different things. I got it sorted and it is working fine. But I am not sure if i run with this or revert back to the 2n3904. For now I will run with this and see how it goes, I just have to measure the input and output impedances. Which as of next week or so, will be easier when i have a vna. The circuit is a little different to what was in the schematic in the previous post. I will update the schematic if I end up running with these.

I build this a little different, I had these squares sitting there i made so I used them.

Oscilloscope output, green trace is the signal in, yellow signal out.

 

Output from the spectrum analyzer.

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Oh The Humanity

So i sat here this morning with a bag of 50 crystals playing a game of what is the resonant frequency. Thoroughly boring I can assure you and I am not sure how the ladies at uBitx Labs in India can sort through thousands to get sets of 8. But alas I have done enough for now to get 2 sets that are matched at a guess to within 50hz, being that my frequency counter is only good to 100hz.

One thing that I would not have predicted is the distribution in the resonant frequencies. Its like buck shot, all over the place and the other thing I would not have predicted is the number of dead crystals or ones with so low Q they might as well be dead being 8, that seems like a very high death rate to me. I still have 2 more bags to sort, but thats not happening today, i have 16 matched crystals I need for 2 filters and am moving on to the next thing.

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Crystal Frequency Tester

Need a tool, make a tool as they say. Well i need to sort these 100’s of xtals into groups of similar frequency for use in xtal filters for an upcoming homebrew receiver project I have started. I have like 200 12mhz to sort through, so i should be good for a few filters at least. Its just a matter now i sitting here for a few hours and sort them all. Not looking forward to that. 🙂

No rocket surgery here, just your basic oscillator and buffer circuit that can be found all over the internet. Its simple, it works and it is reliable.

Built it up Manhattan style on a scrap of copper clad pcb. Yeah i love MeSquares to all you MeSquare fan boys out there. LOL

The Joy Of Oscillation, it works. 3 xtals checked, 100’s left to go.

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More Metal Work

So i bought a new metal shear and thought I should take it for a few laps around the garage straight away to see how she goes. Well, it cuts a nice straight line obviously LOL. Anyway, I made a little case to house a AM broadcast band highpass filter in. While the fit and finish are not perfect, it is certainly serviceable. I did learn a bit and should be able to make nice slide in and out clam shell type boxes with much better tolerances.  Something i also did was add in some locating pins on the bending break, this made repeatability of the bends much better.

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