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| Receiving AM Broadcast station - note the IBOC "sidebands". |
Touching a bit on the mechanical aspects: I didn't want to fall in to my usual trap of trying to squeeze 10 pounds of stuff into a 5 pound bag, so this rig has a large form factor when compared to my others. Dimensionally, it's 16" wide, 12" deep and 6" high, so with any luck, I won't be building each module 3 times in effort to make it fit into the smallest available space. We'll see how that goes.
Because of my limited metal working skills and facilities, I've built the chassis from 26 gauge galvanized steel. Getting this rather flimsy material formed into a rigid housing took a little thought and some trial and error, but it's working out reasonably well. Basically, it's a lot like modern cars in that it's a box that gets its strength from being made from smaller boxes. So, while it's very light in weight, it's structurally rigid, and very easy to work with - except for the bleeding part; some of the edges are pretty sharp.
Electronically, I haven't deviated from the topology that I laid-out in my initial block diagram: The touchscreen equipped Raspberry Pi, running Quisk in "kiosk mode" talks to the Arduino that handles all the hardware switching and programming of the synthesizer, while a modified "Softrock" connected to a USB soundcard handles the modulation/demodulation. Ahead of the Softrock is a diode double-balanced mixer that converts the received signals up or down to 9 MHz. This is all working fairly well, though there's still a lot of refinement work to be done.
An example of this is in getting the receive audio sounding decent. My original intention was to take the line audio out of the pi and feed it, through a 10K pot, to the LA4425A power amplifier. This works, but picks up a ton of stray computer noise unless I significantly load the input to the power amp, which reduces gain more than I'd like.
The LA4425A has a rather high input impedance, somewhere in the 10s of K Ohms, while the output of the on-board soundcard is fairly low Z, and I think this mismatch is causing the problem. So, I'm going to try a simple common-base transistor amplifier between the soundcard and volume control.
The common base amplifier's characteristics are: Low input impedance, less than unity current gain, moderate output impedance and relatively high voltage gain; exactly what I think is needed.
I've built enough transistor amplifiers that it's almost become second nature, but I still like to go through "the design process" first, then model the circuit in LTSpice before melting any solder or frapping any silicon.
When it comes to designing a common-base amplifier, I tend to approach it in much the same way as I would a common emitter circuit. R1 and R2 form a voltage divider that's "stiff" enough, current-wise, to keep the base voltage at about 2.1 volts. This biases the circuit so that the drop across R4 will be about 1.5 volts. Dividing 1.5V by 100 Ohms gives us the quiescent current flowing through the device, about 15 mA. Next, I want the collector to be able to "swing" about a volt, so I chose a resistor of 560 Ohms, which has the collector resting at about 3.25 volts. Cool.
Plugging these values into LTSpice, I was able to see that the model confirmed the numbers that I'd come up with. Yes, they're not exactly the same, due largely to my assumption that the B-E Voltage drop was 0.6V, but still well within in the ballpark.
Initially, I ran the model without C3 in place, and the predicted gain was just under 18 dB from a few hundred Hz to well beyond the audio spectrum. Since this isn't a hi-fi, I added C3 so that the gain rolls-off above about 3-4 KHz. Since I had the circuit already "running" in LTSpice, I simply plugged-in different capacitance values until I got the response curve I was looking for, but this could be determined algebraically with just a little more effort.
This response shaping cost around 4 dB in overall gain, which is insignificant in this case because I'll still have more than enough signal to push the LA4425A to it's maximum. Actually, I think I'm probably going to have to reduce the gain a bit... Maybe not, we'll see.
The next step will be building the circuit, plugging it into the rig and seeing if it works as intended. Stand by!
73 de N8NM
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