EMRFD Message Archive 14584

Message Date From Subject
14584 2018-03-15 15:53:20 iq_rx Audio preamp signal handling

The signal handling capability of an audio LNA early in the signal path of a modern architecture receiver is one of several important specs.  Other critically important specs are noise figure, stage gain and input impedance.  In a 50 ohm environment, common base and common gate configurations are attractive, since the input impedance may be set to 50 ohms using device transconductance rather than resistors.  In IQ receiver front-ends, I and Q stage gain and input impedance are held to tight tolerances so that downstream addition and subtraction cancel one sideband.  This is discussed ad nauseam in EMRFD chapter 9.  The downstream signal processing may be analog or digital, the math is the same.

These small-signal audio LNAs may be successfully studied in the simulator and at the bench, a necessary first step in developing a good receiver.  Required noise figure, input impedance, gain and signal handling specs are all intertwined with the rest of the receiver, and there is no one circuit that meets all requirements.  In an HF CW receiver with little gain before the first audio stage, minimum input signal levels are around 10nV, so if the first stage of audio LNA can handle up to 1mV, that's 100 dB, without AGC.

An instrumentation receiver with 60dB gain before the gain control will typically distort in the downstream op-amps before the audio input LNA.  60dB more than kT/q is 26 volts!

When specifying and designing an audio input LNA, I start with required noise figure, input impedance, and signal handling requirements, then set gain accordingly.

Enjoy the experiments.

Rick KK7B

14585 2018-03-15 19:16:00 Ashhar Farhan Re: Audio preamp signal handling
Thanks rick, this is what I am doing. I start with LTSpice and then proceed to make it. I use audio from a 24 bit sound card as the input to the LNA. The step attenuators work just as well. The output is seen on a scope first. However, the best judge of audio distortion is the ear. I can pick up distortion that doesnt even show up on a PC based audio spectrum analyzer.
I realized the importance of your recommendation to put the manual gain control before the detector. A passive attenuator made with  2N7000 in series mode provides a low distortion gain control. However, it is tricky to get it to work with AGC. The AGC loop seems to be unstable. I am not sure why. This needs more work with mapping the audio path phase and gain characteristics. Is there an experiment method to fix feedback loops like that of the AGC?
- f

14587 2018-03-15 21:04:05 Dana Myers Re: Audio preamp signal handling
14588 2018-03-16 11:34:37 iq_rx Re: Audio preamp signal handling
Sorry to confuse you Dana.  I should have used the term "baseband" instead of "audio."  It's the thing between the IQ frequency sampler and the baseband signal processor in a quadrature sampling detector architecture.  Some of us still refer to that frequency range as "audio."  I was referring to the low noise baseband small-signal stuff in the vicinity of roofing filters in a rig such as a KX3, immediately before the ADCs.

Of course, if you limit the use of "modern" to receive architectures that directly sample RF at the antenna, then I concur.  I don't think of those as "modern" since I've been doing them since 1982 for instrumentation applications, and my mentors before that.

Best Regards,
14589 2018-03-16 12:33:51 Dana Myers Re: Audio preamp signal handling

14590 2018-03-16 12:37:59 Dana Myers Re: Audio preamp signal handling
14591 2018-03-16 14:27:08 iq_rx Re: Audio preamp signal handling
Hi Dana,
Yes, I hoped you could detect my tongue-in-cheek smile.  An amusing one for me is "Modern Filter Theory," from about 1959.  In that context, a National NC-155 in Cosmic Blue and Chrome is the quintessential Modern Receiver.  Keep up the good work.
Best Regards,
Rick KK7B
14592 2018-03-16 14:36:57 Dana Myers Re: Audio preamp signal handling
14593 2018-03-17 02:22:56 peter_dl8ov Re: Audio preamp signal handling
"An amusing one for me is "Modern Filter Theory," from about 1959."

I don't have that one on the bookshelf as I was born in 1959. However, I just ordered a replacement copy of the book that got me started in electronics 'The Pegasus Book of Radio Experiments' by FG Rayer G3OGR, published in the early 1960's. It should arrive in time for my birthday after which I will spend some time reviving happy memories.

Peter DL8OV
14615 2018-04-13 16:04:53 sewtfc3hymnor4la5... Re: Audio preamp signal handling
Hi Ashhar.
Sorry about being off-topic but trying to get a handle on these groups .we have 4 out of 7 printed circuit boards designed and assembled the rest are waiting assembly for the Specan Spectrum analyzer. My building partner, who is the computer guy can't get the software that you provided to work either with Windows 10 or Macintosh. he is currently designing something to work from the computer. Is there any advice you can give forgetting the software to work. I Did send you Gerber files a couple of weeks ago for the initial design-they changed slightly since then
Thanks for any advice you might give.
14625 2018-04-14 09:02:27 Ashhar Farhan Re: Audio preamp signal handling
I will take if offline from this topic.
- f