[Laser] led txr

[stuart.wisher at talktalk.net]

Hello all again,


 Tim wanted to know about the frequency response of the receive heads I have been building. I find Clints circuit to be best and the K3PGP circuit nearly as good. I have modified both to capacitor coupled reverse bias (12V) circuits with the SFH2030 pd, using 10meg from anode to 0V, 1nF from anode to fet gate and another 10meg to 0V from the gate. Not surprisingly, the frequency response looks just like an RC low pass filter with 100kHz or so breakpoint. I did not look much higher in frequency since I purposely keep the sub-carrier to a minimum (around 25kHz) bearing in mind warnings that it will not work as well as baseband.


In addition to the 5mm water-clear red led that I made the 6.5km comtact with, at such high signal strength and with a potential 24dB more gain to be had, (when I discovered 12dB more signal at 33V reverse bias and had narrowed the 4inch lens dia to 1 inch), I have now managed to get a power led to perform as a photo diode.


The LED in question is the golden dragon 1W led. You need to do a bit of "micro-surgery" and cut through the lead to the anti-parallel protection diode in the led that prevents its use in this application (you can see the gold lead to be cut set in the silicon gel), then I have found an enhancement at 43V reverse bias.


I have built another transceiver using this power led and find it to be just as sensitive as the previous led, and interestingly, just as sensitive as the SFH2030 purpose designed receiver, having done a very careful field comparison test the other night. The advantage with the power led is no penalty in light output as was the case before.


As an aside, what about this secondary lens that some advocate? I started with this but then using something as simple as the magnification formula that says the smaller the object (real or virtual), the smaller the image, which equals more light  on that tiny little dot where the distant receiver is. Yes with a secondary lens you get a good light show over the countryside, but how much light is going where you want it? At first I was unimpressed with the little dot I was getting using just one lens, but the light meter bore good news, the actual intensity was greater in this tiny dot. A good demonstration was when the beam from a two lens system was visible projected on to a distant wall. I then brought my one lens system to project on the same wall at the same time, there was a much brighter dot in the middle of the larger patch, qed. Of course the same leds at the same power were being used.


Sam Jewell (RSGB) has expressed interest for his GHz column, I think I sent him some sound files from our recent 25km test)
I have contacted Giles Reed (tech ed RSGB) and I am writing all this up for publication in RadCom.


Having used the led transceiver a few times now, I cannot think why, for the type of contact and range  we acheive, anyone would want to line up two sets of optics when one set will do both. Over the next few days, I have tests scheduled with two other stations over ranges of 40 - 60km, not a lot for those with access to mountain tops and dry air, but try it in England in autumn! 


Using the transverter and led transceiver feels much like microwave radio now, you point the (one) "antenna" and tune in on the agreed frequency and mode.
We have found FM to sound very clear over 34km so far (using the transverter but not yet the transceiver head, it started to rain and cut the test short).


There are now seven transverters in use or under construction in NE England, Barry Chambers G8AGN, and Sheffield group are joined in now. One of them is getting boards made. 


I hope this has answered all recent points, I will tell you how we get on over larger distances when the weather improves.


Stuart G8CYW