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.
Failure is always an option as the mythbusters were wont to say, but for me it seems more like failure is always the option. Built every module and tested each individually and then bringing it all together as a whole some of it refuses to play nice together. So, the idea of a multiband receiver might just get canned and i go back to making a single band receiver.
So what is wrong with it? Well, the rf preamp did not really work right. The bandpass filters were always crap, but that is an easy fix, the LO needs buffer amps to drive the mixers properly and the AF amps are kind of ok but i think need a little more gain in there and the attenuator, im not sure its even working. It makes noise but its nothing to write home about.
This audio amp is going to be the death of me. I am on like the 4th iteration of this board and still its not working right. Each time, something that worked now does not and this time it was the preamp. Everything else is working great, the SSB AF filter works perfectly, the AF amp works perfectly, i still have to build the CW filter and test it, but that I think should be ok. So i bypassed the preamp and things are great. Not sure i actually need the extra gain.
So now i have all the individual bits of the receiver working to the point where I can join them all together and see if I actually have a working receiver. If i get time tonight, that is exactly what I will be doing. Its getting close now to actually making noise.
I have built and tested the IF amp board for my receiver project. Its all system goes. The amps themselves are termination insensitive amps “TIA” by Wes Hayward. No reinventing the wheel when these things work so well. Though the 2nd IF amp might eventually become a Hycas so there is some AGC happening there.
I have built projects before with SMD components on them, but this is the first time that I have included SMD components on my own projects. Yeah i know, its nothing extraordinary, its a 5V regulator, but we all gotta start somewhere. Which got me to thinking, perhaps I should do more with larger SMD components and be able to fit more on a board. We will see LOL 🙂
Oh and for anyone who is interested, this is a respin of my band pass filter board using these variable inductors i got. Anyway, all i have done at the moment is build one filter for testing. Testing which still needs to be done, but I will get to that soon enough and post up some results.
So I am sitting here this morning drinking a massive cup of espresso coffee and thought i really should post up where I am at with the receiver project. Its getting closer now to final testing and making noise. In fact, the final board will arrive today from china and as long as it does what its meant to, i might only be days away from hearing beeps
So anyway, onto the IF amps and mixers. I find the whole amp design, input and output impedance matching and how one can effect the other to be slightly confusing. So I went with an IF amp design that eliminated that confusion for me, the TIA amps. Termination Insensitive Amplifier by Wes Hayward
The mixers will be ADE-1 Mini Circuits jobs as I have a bunch of them here doing nothing and figured why not. I do have a board here I am designing with a number of different mixer topologies on it that I will eventually have made and test out homebrew mixer designs more fully, but for now I will use the commercial product.
I built the 2nd IF amp first, so I can have a direct conversion receiver happening in short order once I have a working audio board.
Well it works as designed and does the business obviously. There is a missing trace on the board, so I will eventually have to respin them and I want to fix the layout somewhat also and change the transistor footprints to wide not narrow. I do not like those small pads to solder to. LOL
I am going to redesign this board. The layout is sub-optimal. But for now, I am going to assemble it and see if I can make 2 filters worthy of use.
The one thing that has always frustrated me with many SSB/CW homebrew rigs is that you get one filter and one filter only. I have seen some use varactor diodes to solve this problem, by varying a voltage on the diodes you can vary the capacitance and this the width of the filter. I thought about going down that route myself for a while, but in the end, settled on having 2 filters as I have lots of relays already.
The plan is for an SSB filter that is about 2200hz wide and a CW filter about 500hz wide. I am also using the good crystals for these also. 4.91520Mhz xtals sorted to within 10hz of each other.
Each crystal was loaded up into my you beaute xtal tester and the frequency measured to within an inch of its life.
My function gen also has a frequency counter that counts down to 1hz resolution. So I measured my crystals and sorted them into piles within 10hz of each other. If i cannot make acceptable filters with such tightly grouped crystals, then there is no hope in hell i can ever make a good filter ;
Out of this pile of crystals I have 6 groups of 8 crystals all within 10hz of each other and a few more groups all within 20hz of each other. Buying these crystals was certainly well worth it. The pile in the bottom right corner are the outliers. It seems even good quality crystals have some in each batch that should be rejected.
Got the parts all sorted, just gotta build it now. Will update this post once i have built and tested each filter.
Part 2: Well i kind of expected that to not take forever, but anyway here is the update.
The build is pretty straightforward and all went to plan
Other than the width being a little wide in both cases and a bit to much ripple, I will say that I am happy with the shape of both filters. I can narrow both up easy enough, just need to change the cap values and the ripple, well it might be a bit of work, I will need to play with transformer values. I used 4:1 impedance transform, but 3:1 might actually be closer to the mark. I will have to play there and see what i can come up with, but a better match will remove some of that ripple.
For now though, I can keep on building the rest of the project as these will be acceptable to get things working. I can make them perfect later. Next job is IF Amps and mixers.
I thought before i added in the IF Amps i would build the xtal filter first and see how it looked. The values of 33pf seemed very small and would end up with quite a wide filter. Typical values for a 2K wide filter are usually around the 100pf mark, so i started there.
Obviously i was wrong, but i am thinking 50pf might be closer to the mark after looking at the bode plot. Its not going to break any performance records and the skirts are kind of garbage for a 5 xtal filter. But, I will see how it goes on receive once i have the rest of the radio built.
As far as xtal filters go, this is one of the better looking ones I have built HEHE, so I guess the others have been really crap. It is kind of symmetrical looking and only 3db-ish of ripple. Might be ok. We will see.
Ok, so after playing with cap values i got the filter about the right width, but there is a lot of variance in the passband. 8db to be exact. Way to much. But as there is no way to impedance match this, there is not a lot i can do. It is what it is as they say and as I am only building this as a receiver, I can live with it.
Ok, I am posting this mostly because I can, but I wont publish the gerbers yet because I actually want to layout the board better, fix some footprints and have the boards remade. Not because they are bad or do not work, but because I know i can make them so much better
So this it my receiver front end. RF comes in from the antenna, through the TX-RX relay into a 0-31dB attenuator module, into a 5 to 15dB RF Preamp.
The RF amp is a pair of J310 as a dual gate mosfet as designed by Pete Juliano here: SimpleCeiver
I might add some more info here later, or will just post anew when i have redesigned the board. The main thing i want to do is to remove those wire links joining the attenuator to the rest of the circuit and replace that with some right angled SMA connectors as the attenuator board comes with SMA connectors on it already. I will probably change the relay also as I now have some better RF relays, not just chinesium grade floor sweepings.
The relays are double pole double throw in a standard 18x9mm package size. The transistors are anything you have in CBE footprint, I used 2n3904 and the resistor is there for current limiting, just pick a typical value, i think i used 4.8K. The trimmer caps I use are 3 leg 100pf max value to tune and align each filter.
SMA edge connectors bring RF in on the left side of the board. By default all relays are in the OFF position and no RF can pass through. The 6 pin header brings control signals and power to the board. To turn a relay on, a 5v control control signal is placed on one of the first 4 pins to turn 1 pair of relays on. The last 2 pins are ground and VCC of the relays you used.
Aligning each filter can be a bit tricky, it is best to use a spectrum analyzer and tracking generator if you have one, or a noise source and spectrum analyzer or even in a pinch if you slow down the seconds/division on the oscilloscope so you can see the filter shape. The reason being, if your filters are not quite tuned right, you will end up with 2 peaks and some attenuation in the bandpass you just do not want.
The filters themselves are Double Tuned circuits. To design the values needed for each band I used some software called DTC.exe which can be found on a CD in the book Experimental Methods In RF Design. I have linked it here for download, but I am not sure if I am allowed to and i will pull it down if asked by the authors: DTC DOWNLOAD
The gerber files for the board can be downloaded from here: Bandpass Filters
Here is a schematic of the filters with values for 80, 40, 20 and 15m.