thanks for the comments Dan. what sort of value would you use on the input->ground resistor?
i have been testing the board with my part 15 rig (the only am transmitter i have currently) and it seems to be working great. this first unit is 10kc brickwall for broadcast band spacing but i am going to build a second one that will be switchable between 3.5kc, 5kc, and 7kc for hf operation. i still dont have my general license so it is kinda low priority for now.
Hi:
I have been away from this thread for a while. Your circuit looks very nice and etching a board is a step ahead of me!
Sometimes op-amps lock up if the input pins do not have a reference to ground or a supply (i.e. they are truly floating and you have high gain). If I build an op-amp circuit with balanced plus and minus rails, then a resistor from the positive or negative inputs to ground usually helps to center the inputs to ground.
On the SCF I built (Thanks to Stu-AB2EZ for the idea), I put it after my audio compression and limiting, and gave a few db of headroom as checked by a scope. This availed the most signal to noise. I skipped the all-pass filter to correct for the phase shift and just used a diode to clip the negative-going spikes that result from the phase shift. As you see, this is not a perfect form of an elliptical LPF, but sure is easier that designing and building multi-stage FDNR's. For transmitting work, I feel the elliptical filter is better than the gentler forms. It provides more energy up to the cutoff, at the expense of very fast phase shifts near the corner frequency.
Thanks for posting your progress!
73
Dan
