10 Mar 2013

Optimised 481THz receiver

As the weather was miserable here today, I decided to work on the K3PGP derived 481THz optical receiver this morning. I wanted to ensure it was working at maximum sensitivity with subcarrier tones around 100-1500Hz. With some adjustment of the coupling capacitors and the addition of an HF roll-off capacitor on the collector of T2, the sensitivity now appears to be excellent. My test is an AF modulated red LED on the ceiling of my building shack, in almost total darkness, with the test receiver (less any lenses) on the bench about 1.5m away. If the receiver is working credibly then the tone can be heard in the headphones when the LED is barely lit. In the case of this latest receiver I cannot even see the LED lit at all in a darkened room, yet the tone is audible in the receiver. When comparing this with my previous best optical receiver there is around (guess) 6dB more sensitivity. This design uses "blog standard" components: nothing selected for low noise, and not an IC in sight. I am now waiting for some decent weather - and no snow please - to test this on my NLOS test path.


LY2SS said...

Which photodiode you have used before Roger? If it was SFH203
could this improvement due to narrower beam of SFH213? Comparing electrical parameters 213 ant 203 are almost the same (except beam with).

73! Zilvinas LY2SS

Roger G3XBM said...

Hi Zlivinas,

The original diode was the BPW34 but I think both the change to the SFH213 and the detection circuit noise reduction have both led to improvements.

Roger G3XBM

LY2SS said...

I see. Thanks for clarification Roger.