Saturday, July 30, 2016

SMD BITX Audio amplifier

I quit playing with the new toy (3D Printer) long enough to populate and test the AF amplifier section of the SMD BITX.

As you can see even though it is built with mostly SMD components, the layout is much larger than it really needs to be. But since I plan on putting it in an old CD drive case there was no real reason to shrink it down any more.  I do have another project in the very early stages that shrinks a transceiver down a lot more.

After getting the AF stages built, it sounded good with a low level test signal injected, no hiss or background noise.  But, I decided to test it to see what the output response looks like.  I connected up my sound card based audio spectrum analyzer-signal generator, and started the Visual Analyzer program. I didn't bother calibrating the input level, so values are just relative for measuring stage gain. 

With everything connected but the signal generator turned off and the Volume pot all the way up I had a fairly flat baseline out to over 16Khz. that had a value of about -115 dB at around 1500 Hz. Not sure where the 1 Khz. spikes are coming from, probably from something in the computer. But after applying the pink noise signal I could not see them in the output at any volume setting.

After turning on the pink noise generator and averaging the signal, the amplitude at the same frequency increased to -83 dB, a 32 dB gain at  1.5Khz.  The maximum gain was around 500 Hz. with a gain of 39 dB.  The 6dB response is from about 200Hz. to 1.2 Khz.  The higher frequency response is more than I would like, but I will live with it for now.  If I start hearing a lot of hiss after I get more stages built, will see about changing the response.
Next to build and test the microphone amplifier.  Then I will have to get one of the early si5351 VFO test circuits I built a couple of years ago modified to work as a test VFO and BFO.

Now I NEED to get back and play with the new toy some more.

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