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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