Well.. After studying the schematics for everything, I decided that I really wanted as much gain as I could get out of the RXAMP. If you recall, there are three options for gain. I initially went with the "middle option" which was 18 dB of gain, 22.0 Ohm bias resistor. I decided to instead replace it with the 10 Ohm 24 dB gain option. What I got was a lot more than 6 dB of gain. The radio came alive. I mean from nothing to "Hey there is RTTY Contest going on! WOW!"
I wonder if I didn't fix something else with it when I did that. I'm almost wondering is the 22.0 Ohm resistor was bad or there was a bad solder joint, or whatever. The rig needs more gain in RX, probably still! So I highly recommend that all builders use the 10 Ohm resistor as is in the schematic as the original value and do not substitute gain down/resistance up based on the values in the table. The front end attenuators will fix any overload situation. The higher gain is desirable. I think most computer sound cards have more gain (like an amplifying buffer) in front of their CODEC's at audio than the DSP Board in the SDR-Cube does. Thinking about this even slightly, it make more sense to distribute the gain at RF (until the point of saturation, though) than at audio since it's so bloody easy to instead pick up 60 Hz (or 50 Hz for you Europeans) noise instead with big gain at audio.
This is actually good that the Cube designers did that. What this means for external SDR front ends is that either more RF or AF gain from the rig will be needed though. It's a simple hack to hack in the RXAMP to the Ensemble II RXTX for example.. I'll have to document and post that at a later date.
Subsequently after I it "came alive", I've tested the RF/Antenna path all the way back to and through the PAF-3 (TXPA) and it seem good. I also calibrated the si570 oscillator (after the video was taken) against WWV @ 10 MHz. It was about 7.5 KHz off... which is about the worst si570 I've ever used. But it's all good now. I suppose getting forced to become intimate with the sr63ng wasn't a bad idea in the grand scheme of the project...
The audio on the video is pretty low. I would have still liked more audio gain (and will probably add it in my cube) but it is not nearly as bad as it looks in this video. I don't understand what was up with the camera.. it was loud enough to hear in the next room.
I also took a narrow band CW video.. but the audio is even lower. I'll do that again when I button up the project in a few days.
One thing that takes getting used to but is not nearly as bad as you would think is not having AGC.... You'll find that you think the rig has less gain than it does.. because most rigs will crank the gain way up with no signals. Once you start using this rig.. it's not that big of a problem.
I still have to do I/Q balance on RX and TX and do all of the lab measurements I've been threatening to do so there is still several more weeks of entertainment on the 'blog for this project.
The Cube also does OK listening to SWL broadcasts beating against the carrier in USB/LSB.. I'd still like to figure out a way to do real AM though.. that will be difficult in the current 8 KHz bandwidth.
I'm thinking that PIC32MX touch screen platform with 16 KHz or better 22.1 KHz could fulfill the promise of a complete HF IF. This is close and will be very nice for CW ops and OK for SSB..
That could be a next phase for this "small embedded no-PC" SDR concept. Of course, an Android tablet has crossed my mind for a platform as well :O)
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