New toys today. Portable tomorrow to give it a go.
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.
I am currently in the process of building a dummy load capable of 500 watts. With the amp project capable of potentially a couple hundred watts, being able to dissipate a lot of power for extended periods of time is kind of needed. Yeah i want to run full key down for like an hour into a dummy load to really stress test the whole amp design.
The dummy load will use 25 100 watt 50ohm resistors in a 5 in series 5 in parallel configuration, mounted on a huge heat sink with fans. Hopefully this will dissipate the amount of heat that will be generated. Anyway, this got me thinking about the dummy load that i built many years ago using 20 5w ebay resistors in parallel and how well it actually performed. Back then i did not have the test gear to actually characterise it. But i have had a VNA for a while now, well, not really a VNA more a glorified S11 meter that measures via vectors. 🙂
So i plugged it in and gave it a look and well, to my surprise it was not all that bad. 1.05 to 1 from 1Mhz to 60Mhz, which is the limit of measurement for my VNA. I am only interested in HF so more than good enough for the kinds of builds I make. I think it would be usable into the 2m band, but from there on parasitic’s would really take over and make it more or less unusable.
After doing this measurement, i kind of decided to order and better VNA and be able to do 2 port measurements and have extended frequency range. I have some other things i want to build like rf taps etc that would really benefit from having a full characterisation done. Oh and filters.
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.