Friday, 22 August 2014

Wispy AF strip rebuilt

The two transistor (common base - common emitter) audio amplifier strip for the Wispy transceiver beacon has been rebuilt as a self contained little module on a small precut piece of Veroboard. This makes a nice looking little module, especially since I took the time and trouble to add proper PCB pins for the external connections!

Common Base - Common Emitter AF amp

I have to confess however that the layout is not as compact as I had planned, due to an error in designing it. But it fits neatly on the little precut piece of board.

Following on from discussion last night with Roger about how he checked the audio level, I connected a high impedance crystal earpiece to this amp, and then fed it with an AF signal at 800Hz from my Marconi 2955. I could quite clearly hear the test sets 1mV signal, along with a lot of noise and other switching artifacts from the test set! (They're not the cleanest of machines!). Knowing i'm looking for about -120dBm sensitivity on the receiver, which equates to about 0.2uV, I decided to see if I could go lower. So, another previous project was put in line -

Homebrew 50 ohm Attenuator Bank

Now, this is of course designed for 50 ohm use, so some inaccuracy has to be expected, plus I'm relying on my own ears! But I found I could still make out the tone even with a further 28dB of attenuation switched in (possibly more, but thats the most I can be confident at), which is a signal level into the amp of around -95dBm, or ~4uV. I suspect with a cleaner input it could be lower.

Other bits of electronics on the go today included a 10m long control cable for my LDG RT-11 Auto remote ATU, this required soldering a pair of 9-way D-types, which is always a fiddly job. Ive also added a brass rod and plate (made from a 2p piece) to the gate input of the FET electroscope, which has tamed the beast somewhat!

Next task is to add more gain to the AF strip, to allow the direct drive of a small loudspeaker. I already have a small general purpose audio power amp for this, based on an LM386 chip. That should complete the audio stages of the Wispy project, and allow me then to move to the mixer again.

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