Digital Sampling S Meter for uBITX

25 Mar

I got some time to work on my digital S-meter/AGC concept. I’m running the Arduino ADC in continuous mode at 9600 conversions/sec. The converter works in the background and generates an interrupt when a new sample is available. This ADC samples the receive audio from the top of the volume control and performs a fast peak-detect function with controllable decay. This drives the S-meter display and also forms the basis for a feedforward AGC that will be implemented with a  D-A converter driving the volume control on a TDA7051A power amplifier chip.

In the process, I fixed the bias on the audio preamp stage to give about another 10dB of dynamic range. This will be very important with feedforward AGC, as everything up to the volume control always operates at wide open gain, so the audio preamp is the stage that overloads first.

I’m having some problems with the Arduino locking up since I implemented the interrupt-driven continuous fast A-D.  I might be asking too much of the Nano. I’ll be working on this; an 80 mHz STM32 board is waiting in the wings if that’s what it takes.

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2 Responses to “Digital Sampling S Meter for uBITX”

  1. DuWayne April 11, 2018 at 8:05 pm #

    I have been working on a TFT display for the uBITX and have something that works well for me. I wanted to add more meter functions, but there is only one free analog pin available. I am looking at a stand alone meter using a pro-mini and a I2C OLED display. Have one working that looks very nice. It has four analog inputs and 4 digital inputs that can be used to select display mode. Have some pictures on my BLOG, also some pictures of the TFT display in an earlier post. Would be interested in some of the things you are doing to yours epically the AGC circuit.
    DuWayne KV4QB

  2. w3jdr April 12, 2018 at 5:03 am #

    DuWayne
    I’ll have a look at your work.

    My S-meter uses the spare analog input. I migrated to the Bodmer fast & compact ILI9341 library and that made a difference in overall display performamce. I’m planning to install an I2C analog port expander but I still plan to use a seperate TFT meter, possibly with its own Nano, maybe as an I2C slave, which might also become the port expander…we’ll see. I want to display TX PWR/SWR, voltage, current, etc. To go any further, I have to mount an I2C port interface connector.

    I’m really pleased with the cheap 400 ppr optical encoder. Tuning is smooth as silk and the S-meter dynamics are really smoth too. Now on to the digipot and the feedforward AGC control.

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