Optical RX improvements

G8CYW Optical head (April 2011 RadCom)

I've now bought some NE5534 low noise op-amps so can implement the full G8CYW optical head circuit which many users have successfully adopted. Stuart describes this as "exquisitely sensitive". I have BPW34 PIN diodes but think the sensitivity should be similar to that with the SFH2030 diodes used in Stuart's design.

In the first instance I need to remeasure the noise floor with the optical head in total darkness to see if the changes indeed make it better than the current discrete component circuit.  My first quantitative noise measurements suggest far higher noise floor than I expected.

Assuming this gives improved performance over my 0.4km local "up the road" test range, my next step is to attempt a longer path, so I am looking for a path of around 2-3km to test with my baseband beacon. If this works with decent S/N then I'll retry looking for the GB3CAM optical beacon. At 32km this is a good test of system performance. At the test site at Nine Mile Hill the traffic noise was high (optically and audibly) so I also need to increase the signal level in the earpiece used to align the RX.

There is also soon to be a second optical beacon near Cambridge and this should be a little closer and an easier signal to find. However, I need to do tests before this is installed at Dry Drayton.

Incidentally, I managed to overcome the parallax issue in the optics alignment  so now a distant street light is spot on in the cross-hairs of the spotting scope.

FT-450D Hum (again)

From what I see and hear, Yaesu is still selling this otherwise excellent HF/6m transceiver with a fundamental design flaw: hum created by poor grounding of the display module when dimmed from maximum brightness. This problem was there on the FT450 but whatever improvement, if any, was implemented in the upgrade to the D version,  it has not worked consistently.  This really annoys me: surely this sort of issue should have been picked up in beta testing? The fix being implemented by many owners seems simple, so why the heck doesn't Yaesu get on a fix it?

OPERA (weak signal mode) - a word of warning

Since installing Opera V1.0.4 a few days ago my Inspiron 630m Win XP PC has started to behave very oddly. For years it has worked in a very stable fashion with very few issues in 6 years of use.  Now it is slow (very slow) even when not using OPERA, CPU load is high and running Ccleaner, which usually takes a minute, took nearly AN HOUR to get rid of 156M of files to be cleaned up.  Now, I cannot PROVE the link, but the coincidence is remarkable.

To make matters worse, the software once installed does not appear to be listed in the list to uninstall. Not being a PC expert I am loath to just remove all the obvious files in case this damn program has left something behind that "leaves its mark".

The lesson is DO NOT INSTALL DODGY, UNSTABLE AND POORLY WRITTEN SOFTWARE unless you know what you are doing and you know how to get rid of it safely and completely.

OPERA, as a weak signal mode is interesting, but I am VERY unhappy that it has messed up my good old faithful PC.

SAQ VLF 17.2kHz test 10.1.12

The historic Grimeton VLF transmission over Christmas on 17.2kHz had to be abandoned because of an equipment failure but Lars (SM6NM) says they are planning another test tomorrow (Tuesday Jan 10th) at 1330UTC.  The sequence will be "VVV VVV VVV DE SAQ SAQ SAQ" running for about 30 minutes and no message. No QSL cards will be issued this time. More details see www.alexander.n.se.  One possible receiver to use is SM6LKM's simple VLF SDR that can be downloaded from http://sites.google.com/site/sm6lkm/saqrx .

This was what they said about the Christmas failure:
 
"The reason for the trouble with SAQ on Christmas Eve was most probably a damaged fuse in the excitation circuit for the 500 V DC generator. The fuse was not completely blown but measured a very high resistance (150 ohm). When we lost the excitation current to the DC generator, we also lost the control current to the choke coils in the conductors to the main motor, and thereby the speed control of the alternator was lost.  It was confusing that the problem occurred exactly when we connected the running alternator to the antenna. We were actually on air a few seconds with decreasing frequency until the antenna was disconnected by the protection circuit in the speed control."

Optical setback (of sorts)

RX set-up at Nine Mile Hill on 481THz tonight

Flushed with my local successes (0.4km) last night, I decided to have a go at looking for the GB3CAM optical beacon 32km away from a local high spot where I know others have copied it sending its 1/15kHz FSK sub-carrier data at 481THz (red light). I set up in the daylight so I knew where to aim approximately and had the PC ready with SAQ receiver (0-22kHz SDR) and Spectran all ready. After dark I started to seriously hunt for the elusive signal    .....but with no success.

Failure can be put down to several possible causes:

1. Difficulty in aiming. Although I knew where to aim, I am uncertain that my spotting scope is correctly aligned on the 100mm optical head and that there is no parallax error. Looking for a very weak signal is therefore hard to start with.
2. Inadequate sensitivity in the optical head. Although I could hear the 50Hz buzz from every street light and house light locally, maybe there is another 6-12dB to be had in basic sensitivity in the optical head.
3. Inadequate volume on the recovered baseband audio. I was monitoring the "live" feed with just a crystal earpiece attached to the head and, although this is fine at home and down a quiet street, it was too low a level to hear weak signals over the traffic noise from the main road which adjoined the field gateway at the RX location.
4. Path loss. According to G4HJW, the beacon keeper, the signal varies from 5-30dB over noise at this location, assuming one is aligned correctly. Although the weather was cloudy and good visibility there was drizzle that came in as I was testing.
5. QRM from car headlights. As cars came along the main road near the test site, I got quite bad buzz from their headlights.

So, not to be beaten, I will do things to address all these problems and try again quite soon. I think I need a better local test beacon that I can use to (a) check aim, check RX sensitivity and adequacy of the recovered audio.

Quantitative tests with 481THz kit

Today, in daylight again, I did a repeat of my recent tests with the baseband pre-focused red LED TX (20mA), looking for the signal 0.2km along the road. This time I also took my PC and looked for the signal with Spectran and the SAQ SDR receiver. The audio tone was around 40-50dB over noise on both systems.  At this range I would have expected far better S/N. The noise floor looks high. When I blocked off the lens the noise floor went UP about 15dB. Why I wonder?  The picture shows the test set-up. If you click on the image you can see the pre-focused red LED pointing out of the house window and the receiving kit in the bottom foreground. Based on these results speech communication, even with just the same LED and no external lens on TX, should be possible to at least 5km. With 100mm lenses at the TX end, considerably further should be possible.  Below are the test results.

UPDATE: After dark I repeated the range test, just using the crystal earpiece to listen to the signal detected by the PIN diode head. At 0.4km, the greatest distance I could go line-of-sight from my home QTH, the audible signal was STRONG, which is very encouraging.

See my website https://sites.google.com/site/g3xbmqrp/Home/opticalcomms for more details and links.

OPERA - new weak signal beaconing mode

There is a new very weak signal beaconing mode being trialled currently on 136 and 500kHz called OPERA. The current version is available as a zip download at http://www.mediafire.com/?w74a061m48nv806 but this is a beta version and updates are happening quite fast at the moment, so best check the RSGB's LF reflector for the most recent version from Graham G4WGT.

Initial indications are this is even better than WSPR, which as we all know is an excellent QRP beaconing mode. I managed to decode SM6BHZ (just under 1000km) and PA0A tonight when using just my 20m separated earth electrodes (no antenna in the air here at all) on 136kHz.

Over the weekend I may give this a go with a few mW ERP on 500kHz and, when I get my vertical loop re-erected, on 136kHz too.

Early days yet, but a most interesting mode that does NOT need critical PC timing (unlike WSPR) and that can use the PTT line or VOX control to key the TX i.e. it is simple to interface rig to PC.

Optical receiver head completed

481THz optical receiver with 100mm lens

I now have a 100mm lens set-up in some grey drainpipe with 4x sighting scope attached on a tripod together with a "simplified version 3" KA7OEI head (of sorts) with a BPW34 detector. In my version the cascode stage is just followed by a single common emitter stage driving a crystal earpiece, although I intend to add an emitter follower so I can connect to my PC (to use with Spectran/SDR) or a transverter.

A 5mm 20 degree red LEDusing just its own built-in lens was shone out of the stairway window aiming down the street in daylight with 1kHz modulation. It was taking around 20mA. I then walked with the kit to the far end of the street and the signal was solid (S9) in the RX earpiece at a distance of about 0.25km. I now need to find a more distant test range and see how far this can achieve. Depending on results I hope to go looking for GB3CAM next week.

Lightbeam receiver progress

Today I did the part of projects I hate most - drilling holes in aluminum boxes. I can never find the right size drill bits, they are blunt and so on. Despite all this, I managed to get a small metal box complete with BNC socket, phone jack and switch mounted on the end of my optics pipe and carried out some first tentative tests. I am using part of a simplified version 3 KA7OEI optical head based around a BPW34 PIN photo diode. In my case there are no op-amps with discrete transistors used for the amplifiers and buffers following the FET cascode stage.

In the next day or so I want to add the sighting scope and find a way of attaching the whole optical unit onto a stable tripod. At this point and can try some more serious tests for example looking for my own 1kHz beacon at greater distance and, hopefully, GB3CAM at 22km if the system works OK.

The output of the head can feed either a crystal earpiece (to listen to the baseband tones or speech received) or into a PC so that it can be used with an SDR to copy various modes up to around 22kHz.

Winter project list

In this first post of 2012 I thought I'd list the projects that are "on the to-do list" for the coming few months. As always, this is subject to change without notice as my interest and enthusiasm goes through waves:

1. Lightbeam RX. The aim is to detect the GB3CAM 481THz optical beacon at a distance of 22km from my nearest highspot. So far I have breadboarded several good sensitivity optical detector heads and the next stage is to complete the packaging of this into a screened box and attach it to my optics which also need a bit of engineering to complete. Target is this to be completed next week.
2. Light beam transceiver. Following on from (1), this will either be a transceive head with a transverter or an FM transceiver that I can duplicate so I can talk to others who can borrow the second unit.
3. Rebuilt VLF earth mode beacon transmitter. In a few weeks time I will be able to put out a stable signal on 8.97kHz  (the usual VLF test frequency) rather than 8.76kHz. My intention is to rebuild the whole beacon TX so I can run 10wpm CW, QRSS3, 30 and 60 modes as well as WSPR. I may also add a 100W PA option later. With this kit I hope to improve my personal earth mode DX record of 6km.
4. Improved LF loop and E-field probe antennas. I want to erect a more permanent external antenna for VLF and LF grabber work and mount these away from the house. Up to now I have used my 80m square vertical loop on RX but this is currently down.
5. Case up the 137kHz transverter. This has been a rat's nest on a piece of copper laminate for too long!
6. 28MHz WSPR transceiver based on a 14.060MHz crystal doubled in a DSB direct conversion transceiver circuit.

Now, how many of these projects get done is a matter of conjecture, but I hope at least 50% of them will get finished.