Re: Volume Limiting and Distortion Reduction for uBITX #ubitx
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Jack Purdum
There are pros and cons to each approach, which is why JackAl has both RF and audio AGC. Indeed, the AGC section takes up a good portion of the JackAl's nanoacres. BTW, we just received the assembled Jack boards which has the SMD parts in place and looks like this: This is what QRP Guys will be selling after we spend a little more time testing everything. After adding the off-board parts (less than $13), the board looks like this: All of the yellow pins are test points to encourage experimentation. You also have about a dozen free I/O pins to experiment with. Jack, W8TEE
On Wednesday, December 26, 2018, 11:26:29 AM EST, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:
There have been dozens of threads in this forum about adding AGC to the Bitx40 and uBitx. Most get hacked in (both detection and attenuation) after the rx audio preamp, to prevent ear damage on a sudden strong signal. Then perhaps add a manual RF gain pot at the antenna port to avoid audio preamp overload. Some used PIN diodes to attenuate the signal at RF (or IF) frequencies, and thus get around the dynamic range limitations of that 2n3904 audio pre-amp stage. Here's the earliest thread to identify the dynamic range issue of that Bitx/uBitx audio pre-amp stage: :? ??/g/BITX20/topic/5945215#32066 Kees (K5BCQ) sold nearly 1000 kits of an AGC design from Don (ND6T). This was audio detection with RF attenuation at the antenna port using 2n7002 FETs. Nik (VK4PLN) was planning to sell a kit with audio detection and attenuation after the audio preamp, I don't know what came of that. ? ? ? Sensing signal levels is probably best done after conversion to audio. Must sense after the 12mhz crystal filter, or strong signals elsewhere in the band will bring on the AGC action.? But if we sense between that 12mhz crystal filter and the demodulator, then we will likely be seeing mostly the unshielded 12mhz bfo. Attenuation could be anywhere between the antenna port and the input to the audio preamp. Putting attenuation at the antenna port avoids overloading the mixers and IF amps, but they generally have enough dynamic range that it doesn't really matter where we pot it. Putting attenuation after the audio preamp works, but does not fix the dynamic range issue of that preamp. Various ways to implement attenuation at RF, here's a good one that? tries to maintain a 50 ohm impedance match, and is quite cheap: ? ?? Allison recommends using a 1n4007 as a pin diode to set the gain in one (or both) of the bidi IF amps, many of us have a 1n4007 in the junk box: ??/g/BITX20/message/45984 Jerry, KE7ER |