May 16, 2008

RX Audio Interface to your PC

Whenever doing AF analysis with help of any audio spectrum analysing software at your pc, you're probably facing several interfacing issues:
  • 50Hz hum
  • RF interference from various sources
  • Bad ground/ ground loops
  • gain/impedance mismatching
The first 3 are referred to as 'pin 1' issues in the world of professional audio. In one word, proper grounding architecture & shielding, or the lack of.
Mind you, the audio input on your sound card is usually already a disaster having bad or failing ground contacts. Next the pc has analogue and digital circuitry, there's a cable to your TRX and there it is susceptible to RF. Not to mention the many ground loops etc..

I have been successfully using Spectran, a freeware audio analyser programm by I2PHD and IK2CZL.
When the audio is directly linked to my sound card input you get the below result (volume settings at zero):

My sound card is an old 16bit Soundblaster compatible type. Any current 192KHz/24bit High-End sound card will lower the noise floor by some additional 10 -15db. For the required measurements this old 16bit card has proven to bemore than adequate.

How to get a 'clean picture'
  • Use properly shielded cable
  • Install an audio isolation transformer
To conquer the 'pin-1' issues I opted for an audio isolation transformer. These usually are 600 Ohm, which might introduce small mismatches. I terminated its output, added some RF filtering on the output and finally put it in a box with both 3.5mm Jack as well as std RCA in-/outputs:

With the RX Audio Interface inserted (as near at the sound card input as possible), you get the following result:

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