Finningley optical transceiver working fine

Although I posted this on the Nanowaves Yahoo group , I forgot to update folks reading this blog on the progress with this optical transceiver kit designed by Bernie G4HJW.

Well, at the weekend I finished building the unit and started testing it. The TX part worked first time, but the RX did not. Time to find out why!

Using logic and common sense, I carefully went through the various stages (8V regulator, later audio stages etc) and tracked the fault down to an intermittent preset SMA pot that sets the FET bias. The FET stage is the very high impedance stage that follows the PIN photodiode.

The error was entirely my fault and easily fixed by removing the part and redoing the surface mount joints. Now the full transceiver is working well (but yet to be put into 100mm optics) and the RX sensitivity is close to that with my K3PGP design RX. In the coming weeks I hope to get a transition piece to connect the transceiver "tube" to a 110mm drainpipe that houses the 100mm lens. I'll then assemble this onto a stable tripod with sighting scope and I'll be ready to look for QSOs.

http://www.earf.co.uk/optoposition.JPG

One thing that puzzled me was how having the detector diode and TX LED slightly off-centre would work. In my mind I thought that the light would not be properly focused onto the devices, so losing sensitivity. Then someone pointed out that by slightly aiming "off beam" by around 1 degree the light would fall exactly onto the position on the transceiver where the LED or PIN diode are mounted.

My first 481THz 2-way QSO

The 481THz kit at the G3XBM/P end of the path

This evening, thanks to Bernie G4HJW, I managed my first 2-way optical QSO over a path of around 10.5km.  For the AM baseband QSO, Bernie used his Finningley transceiver with 100mm optics and I used my simple AM TX  with my modified KA7OEI receiver with unbiased BPW34 connected directly to the FET gate.  For some reason, the RX in my dual tube transceiver (the one on the larger tripod) was about 3-4dB less sensitive than the separate RX, so in the end I used this to copy Bernie at a solid 58-9 with scintillation.

481THz transceiver ready for QSOs

The electronics of the 481THz AM transceiver

My first complete optical speech transceiver has been finished and is ready for a line-of-sight test over a few kms. The pictures show the assembly of both the electronics and the optics. All the TX and RX electronics are built "dead bug" style on a piece of copper laminate and housed in a small metal box. There are holes for the BPW34 detector diode and the high brightness LED that line up with the focal point of each 100mm optics tube.

The optics showing the 2-tube construction with sighting scope

A quick "in strong daylight" test down the road over 0.3km was successful one way: my wife could see my TX beam strongly and I could detect my CW beacon OK, so I am hopeful that a proper 2-way amateur QSO with this kit should now be possible. Sensitivity at night is very considerably better.

G4HJW is set up to do a 2-way test, so this is now likely to be attempted in the next few days. if successful I will then try to make a better, more permanent, version.

481THz optical transceiver progress

My AM baseband optical transceiver build is nearly finished (mechanics and electronics both almost complete), although I have to optimise the sensitivity as it is several dB less sensitive than my stand-alone optical receiver.  I am not quite sure why. All being well I should be ready for a real QSO with fellow local optical enthusiast G4HJW next week.

Finningley Optical Transceiver (G4HJW)

Bernie G4HJW is well known for his innovative microwave designs as well as many other good projects in recent years. This year he has produced another winner for the Finningley Round Table in mid July:  a baseband optical transceiver kit designed with SMA components. For details see http://www.earf.co.uk/nanotrx.htm . I am not sure if Bernie is planning to make kits available more generally. Bernie has tested this design over a 65km line-of-sight path recently.

How I hate drilling boxes

This afternoon I have been "engineering" my dual lens optical transceiver mechanics. This consists of 2 34cm long 110mm drain pipes (with 100mm lenses at one end) fixed together on a tripod mount with a sighting scope attached to one tube and an end piece at the focal points which will support the aluminum box housing the detector, the TX LED and the electronics. I had to work out how to support the two tubes in close alignment and how to fix these and the electronics to the tripod mount. This has involved a fair bit of mechanical engineering, which is the part of the hobby I absolutely detest. By this evening I think everything is ready and fixed together OK. The electronics box has all the holes drilled for switches and sockets. Now all that remains is to build tidy versions of my sensitive detector and AM transmitter.

When everything is finished - the end is in sight now - I will ask Bernie G4HJW for a sked over a fairly short (2-3km) line-of-sight path to check alignment and that everything is OK. Assuming this goes well then it will be time to try a 481THz QSO over a reasonably DX path.

Light beam RX improvements

This afternoon I played around with my light beam RX trying to squeeze a few more dB of sensitivity from it. Well I think I have managed it by further optimisation of the electronics and by optimising the focal length. Now, in the dark, a barely lit LED in the ceiling of my lab is detectable on the bench without optics. With the 100mm lens I can detect a 1kHz subcarrier on the 481THz optical signal when there is no sign at all of the LED being illuminated. Some tests using an LED as a detector were less successful, so my plan is to make a dual optics AM transceiver with 2 drainpipes and 2 100mm lenses. With this I should be able to work the local nanowave stations over line-of-sight paths.

May Projects

All being well I have a window of a few weeks before visiting grandchildren and the like, so I have a couple of new aims:

(1) Convert the 10m halo to a dual band 10m/6m halo
This should be a fairly easy task as I already have dimensions for the 6m halo and I've air tested it in the loft. All that should be necessary is to take down the 10m halo and change the feed and support arrangements so both the 10m and 6m halos are fed from the same coax. The pair of halos well clear in the air should prove an effective QRP antenna system for my 2 favourite bands.

(2) Complete my first optical speech transceiver
Strange though this is, I have made several quite sensitive optical receivers and a couple of reasonable power QRSS3 and CW optical beacons, but I've still not got around to making a complete optical speech transceiver. Locals hereabouts use baseband audio for TX, so this is what I should do if I want to work people like Bernie G4HJW. It would be good to get a few line-of-sight short range (up to around 10-15km) 481THz QSOs in the log-book soon. Based on my optical work so far this is not a difficult task, just one I need to crack on with.

Target for both projects is within 2 weeks i.e. by May 17th. We'll see.