As I finished up the receiver on my 17-12 SSB transceiver, I started to wonder -- how good is it?
Sure, I could hear stations on both bands, and when I got started as a homebrewer that was enough for me. But now, I find myself wondering about receiver performance. Did I get the gain distribution right? Do I have too much gain ahead of the mixer? Ahead of the crystal filter? Is the receiver generating too much noise? Can I hear the band noise? If not, why not? Do my circuits lose linearity in the presence of strong signals? What is my dynamic range?
This is a big complicated subject that takes time to master.
I am just beginning. I found the video above to be very helpful.
I was jealous of this fellow's audio spectrum analyzer, but then Tony G4WIF told me that the the analyzer that this fellow was using was really just a sound card and some software. I quickly found a similar piece of free software that lets me do the same thing he did: Look at the audio output of my receiver and watch what happens as I put an RF signal of varying levels into the antenna port.
I am using Visual Analyzer, a free program out of Italy:
https://www.sillanumsoft.org/
The author of the software seems very cool"
My name is Alfredo Accattatis; I love electronics and software, and I have been working for years in commercial companies as software/firmware engineer and software designer. I've been writing programs for embedded systems (with DSP and MICROCONTROLLERS), for PC, for Avionic Computers and even for Mainframes, using C, C++, Pascal, Ada, REXX and assembly. I starting write VA during my free time just for fun and using (also) my DSP experience. The program was and is completely FREE.
More info about Alfredo here:
https://www.sillanumsoft.org/about_the_author.htm
Do you folks think I need to buy the kind of True RMS Audio Voltmeter that is being used at the end of the above video?
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Hindsight is always 20-20. That's what's nice about homebrewing is that you can go back and fix anything. You got past the "time to shoot the engineer and get on with the project" phase. Now it's just a bit of tweaking...
ReplyDeleteThis is an informative video. With the meter used and many other similar meters, the dB scale on the meter could be used for measurements and the math becomes addition and subtraction. The noise was -6dB on the -20dB scale for a reading of -26 db. The signal plus noise was +1.2dB on a -10dB scale for a reading of -8.8 dB. The difference of the readings is 17.2 dB.
ReplyDeleteThe Aglient U1272A meter shown near the end of the video will display measurements in dB which may be helpful. The frequency response limitation which was mentioned in the video should be considered. While the Aglient U1272A has a bandwidth of 100kHz, the accuracy degrades as the frequency increases. Other digital meters may have the same effect.
A VTVM is usually peak to peak reading and is not suited for this measurement. Older analog bench instruments like an HP400 or HP403 for this type of measurement can be found at reasonable prices. This may be an option as the as the Aglient shown in the video has a new list price of about $440.