Friday 22 August 2014

Emperor Mong Defeated!

Well, It seems my problems with the Wispy's audio stage have been to an unannounced visit to my workshop by the Emperor Mong. That and a crappy earth or earth loop between the test set and the variable PSU!

Using Roger G3XBM's AF strip design as detailed in my last post, and listening with a crystal earpiece, I set the test set to about -75dBm on a 28.1261MHz carrier. At first there was nowt but buzzing! Realising that this was probably an earth issue, I switched the supply to the AF strip and my mock-up for 'TennaLady' to a 12v SLAB. I now had a clean output but no tone! I again had to back the input right, right down, this time to nearly -20dBm!!!

Alas, It was but the Emperor, having one last play with me! I decided to swap the port on the test set, to see if that made a difference, and discovered I was feeding the project with the BNC to Croc clip cable, but the test set was connected to a BNC-BNC lead! I quickly swapped to the correct cable, and nearly had my ear blow out!

Reducing the signal level, after adjusting the frequency to something that gave a nice to listen to tone, I was able to go right down to -115dBm before I had to start on/off comparisons!

By doing on/off (switching on and off the RF output of the test set) I discovered that I could easily discern a -120dBm signal, and could just make it out down to -125dBm.

So, the audio strip works as expected. However, as far as the receiver goes, this is using my SBL-1 DBM based 'TennaLady' mock up, not the Polyakov mixer based Wispy. I will try the Wispy receiver chain later, after trying the AF stage hooked up to an LM386 based audio PA, which hopefully will drive a small loudspeaker.

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