BITXing Some More

I've just about finished the other bits and bobs for the BITX I was aligning yesterday. I've made a digital dial for the frequency readout and an s-meter; the readout is a kit I have had here for ages, it came from QRP Kits:

http://www.qrpkits.com

The build using surface mount components is really tricky, but it works OK second try!


1st time through I'd got some pins on the main microprocessor chip shorted out which was screwing up the multiplexing of the counter display. You really need to use a magnifying glass to see this stuff properly. Soldering it is another game again! I can't get the camera to take a picture any closer, so you'll just have to believe me when I say the components are small!

I've got all the bits of the BITX V3 wired together on the bench here (the mouse is not an essential part of the BITX):



Now, in a nutshell, what you should be able to see below is exactly what I love about ham radio, you can see the mess above which is all my own work, I turned it on, made a slight circuit modification (connected the speaker) and this is what I soon came across. There's only a piece of wire rammed in the antenna connector right now:

Not bad, egh????

BITXing

A while ago I ordered a BITX20 V3 (Bi Directional Transceiver) from Sunil in India:

http://www.cqbitx.blogspot.com/

This is a 20M version of the ever popular QRP design of the BITX. There is a lot of information on the BITX design available, originally designed and built by Ashhar Farhan, 2004. The project details are here:

http://www.phonestack.com/farhan/bitx.html.

There is now a wealth of knowledge on the various forms of this original design available at a Yahoo! group here:

http://groups.yahoo.com/group/BITX20/

I was looking for a good drive for my linear amp project and figured I might as well build this project and use this to test the linear!

Some pictures of my build follow, generally this was very straightforward, although I have built a different version of this before:





Looking at the output of the PA with a constant drive from my audio generator, I've got about 70V pk-pk (the scope is connected using a x10 probe so it's 20V per division), that's about 8 watts. I would have expected a little better from a single IRF510 MOSFET. I used pk-pk voltage / 2 * .707 to give me RMS. Then squared the RMS and divided by 50 ohms.

So in this case I have 70 / 2 = 35V peak. Multiplied by 0.707 = 24.745V RMS

P = E^2 / R, so thats 20.745^2 = 430.355 / 50 Ohms = 8.607 watts.



The MOSFET is being powered from an unregulated supply of about 26.26V DC which is also being regulated down to 13.8V for the main board and the relays on the PSU/Linear board.

I'm now planning to build a s-meter, a frequency readout and then case this project. The output is looking really clean, so once these extra bits are complete I need to stick it on the air and get some reports!

This kit cost me $40 US plus some very modest shipping costs from India. The kit contains the boards, relays, a hard to find transistor, cables, connectors, torroids, enameled wire plus much more. If you are looking for a cheap and easy way onto 20M, it doesn't get much better than this! All other components needed for the build I had here in the shack already.

Balanced Modulator Musings

I've been fiddling further with the balanced modulator from the SSB exciter in my previous post to try and get some better carrier suppression. The first thing I have done is to match some IN5711's - these were the diodes used in the RadCom article I am copying. These diodes didn't make much difference, so I then decided to remove the balancing components from the circuit and re-wind the toriods. I originally used some FT50-43's as I had loads of these, but I have re-wound the toroids with FT37-43's and much tighter wound trifilar wire.

After all this I still didn't like the output much, so I then tried matching some good old IN4148's and still with no balancing components, and probably a more symmetrical layout:

This is now the output of the crystal filter on LSB:


And here on USB:


You can see that the carrier suppression is now very significant, so I think I am quite happy with this aspect of the exciter now.

The next part of this project will be to make the triple balanced modulator (TBM) as described in January 2011 RadCom. This will need five bifilar wound transformers on FT37-43's which I will need to have a good rummage to find! There is also some discussion on VHF transistors and the selection of the MPSA10 - which it just so happens I have about 100 of!

So onwards with this project over this coming holiday given suitable breaks from family commitments.

More soon.

How to generate drive to generate drive

Or perhaps I should call this post "recursion, see recursion".

I finished testing the phasing of the transformers on the linear amp board, you can see from this pic that the input gets split nicely into two signals that are out of phase with each other. These are effectively the inputs to the gates of the FETs. The output transformer is then doing a good gob of mixing these two signals back together again:


So, this suggests that the transformers are wound and installed correctly. So I've installed the FETs on a heatsync that is a little below the board, that bit's worked out OK I think:


I can bias the FETs as expected, I measured the idle current and then increased the FET bias voltage until each FET drew an additional 10mA, the board with the relay engaged and the FETs both biased on is now drawing about 50mA.

The problem I have now is that the board includes a RF sniffer to close the relay, my signal generator (homebrew of course) only generates about 150mV pk-pk output which isn't anywhere near enough drive to close the relay, if I use a cable to drive the relay (i.e. short out the relevant bits of the circuit), I'm not seeing much gain in the FETs as I dont think the drive is enough for them either.

So I need to build an RF amplifier for my signal generator before I can test my RF amplifier - recursion see recursion.

Hence another small project is born.

HF Linear

Well, yet another project has started today; I've been collecting bits for the WA2EBY amplifier. This design claims 40W from a 1W drive accross all HF Bands. I got the PCB's from Far Circuits in the USA http://www.farcircuits.net, the toroids came as a pack from Kits and Parts in the USA also http://kitsandparts.com/. I've bought from both of these suppliers before and their dispatch to the UK is very reasonable and fast.

I've had to order a load of Silva Mica caps from Rapid electronics in the UK for the Low Pass Filters, as I didnt have anything suitable in stock; they are supposed to arrive today, but aren't here yet.

The main amplifier board is going to look a little scruffy, but here are a few picks from the build so far:






The last image shows the board under test, I havent inserted the finals yet and I've just stuck a 1nF capacitor between where the Gate and the Drain will go on each FET. This should allow me to check the phasing of the transformers and the other bits before I smoke any expensive parts! I've just watched the realay explode as I thought it was a 24V coil, but it seems it was a 5V coil and it didn't take too kindly to having 28V accross it!!!

I'll post more when I've got a replacement relay, I don't have one right now.

Would you Adam and Eve it?

Well,

As big a believer in QRP as I am, I'm not sure I'm able to actualy get my head round this. 6V p-to-p out of my QRSS beacon, that's about 100mW. This feeds through my ATU (mainly to convert from BNC to PL259!) then half way round the house in RG213; then up the side of the house and into a Windom.

Here is my call very readable at the QTH of ON5EX, thanks Johan! Thats a whopping 420km distance. Not half bad! The beacon frequency looks quite steady too! I'm well impressed:

QRSS Beacon Alignment

I've had a go at aligning this QRSS beacon today. The output from the small transmitter seems very clean and I've set it to the recommended level of just over 6V p-to-p. I did manage to set the output pot completely to max when I first powered it on, lots of heat but no smoke before I realised! I dont seem to have done any damage though...

I've stuch a 50R load on it and a scope probe, here's the output, very clean:

I've adjusted the frequency output to as near as I can to where I want it to be, and I've also watched the output using the Argo software and set the frequency shift of the callsign transmission.

So I guess it's time to connect the antenna and leave it running. There are quite a few grabbers on the internet, so I need to understand that a little more and then find out if and how far my 100mW is getting!

QRSS Beacon

I've been building a QRSS beacon kit that I purchased from Hans Summers:

http://www.hanssummers.com

It's a neat little package, which came as a complete kit. Mine is built for 40M.

Here's the kit with only the IC holder in place:

And here the kit is neary complete; only problem is I have L1 and L2 the wrong way round!


I've added some veroboard as a wider board to fit into the box I had in mind for the project, and included a 6V regulator in there to enable me to power this from the same source as all the other junk in the shack:


And here is the kit boxed and ready for alignment, I've also added a power switch and power on LED, plus a switch to put the keyer into 12wpm CW for testing or 6s dot length for QRSS running. Theres also a second LED to indicate that the kit is running in QRSS mode:


I've had a poke about with the scope and all looks as expected, I need to align the output frequency and spend some sime setting up the frequency shift.

I'll post more as I do that.

Mark.

Huff n Puff

I've been considering having a play with Huff n Puff VFO stablizers for quite some time; there are plenty of resources out there to explain this concept:

http://groups.yahoo.com/group/HuffPuffVFO/

http://www.hanssummers.com/huffpuff/minimalist/2chip.html

I've been reading with interest the experiments done by Hans Summers:

http://www.hanssummers.com/

And have had a go today at building his 2-chip VFO + Huff Puff; which is an exceptionally clever design.

Here's a picture of the schematic as drawn by Hans:



I've built it using veroboard, and paid no real attention to layout:



Now the output from this VFO is, as you would expect, rock solid:



What I'm planning to do is build a QRSS beacon and receiver for the shack:

http://www.sdrham.com/

Next is to build a Huff Puff stablised VFO on 10.140MHz for the QRSS receiver, and then think about how to build the beacon....

More will follow.

Let it Snow

Really bad weather has hampered progress a little; I've had no post for days as I guess they cant get very close to the house. Here's some pics of my chief technical assistant, plus the damn crazy dog. This is geddy cat, he tries to help, but doesn't succeed much...

This is Saki the dog, You can see just how deep the snow is; half of the dog is missing!


I'll post more when I receive some of the parts I'm waiting for.

RadCom Dec 2010 SSB Exciter

I saw the SSB exciter in the Hombrew column of RadCom Dec 2010 and thought I'd have a go at building it. I had a rummage in my components and found a 9MHz Crystal Filter and two suitable oscilator crystals; 8.9985 and 9.0015, so this seemed like a good starting point.

The first item I built was the Mic amplifier:

I built this using vero board; the output seems quite clean and tidy, and I make the gain about 18dB:


I've built the main part of the exciter and I'm seeing OK carrier suppression through the balanced modulator, however I think this can be better. I've used some BAT85 diodes in the modulator, but I didn't match them first, so I think I need to match some more and see if I can suppress the carrier some more. This image is taken from the spectrum analyser with a 1.5KHz tone injected into the mic amplifier. We should expect to see the carrier plus a signal at +1.5KHz and -1.5KHz (they are the upper and lower sidebands). The balanced modulator should be adjusted for minimum carrier at the output:



The Upper and Lower sidebands aren't quite the same amplitude for some reason, but the carrier is some 30dB down in the noise, so that will do for now. The output of the Crystal filter looks like this with USB selected:



And like this with LSB selected:



So, I need to try some different diodes in the balanced modulator, then I need to try a range of audio frequencies to try and figure out where the skirt of the crystal filter is to ensure that the carrier and the unwanted sideband are set in the correct place for maximum rejection.

Here's the board as-is:



So once I'm happy with the SSB exciter I need to mix this RF output with a VFO and try and get a signal on the Air!

I'll post more when I've figured out the best band to aim at.

Mark.

DigiModes interfacing

I dug out an old radio interface that I built some years ago, the design was actually part of an article on eHam.net:

http://www.eham.net/articles/7007

Anyhow I've had this bit of kit to pieces today and will be doing more fiddling with it tomorrow.

I have always had difficulty getting the TX and RX LEDs to function correctly so I've added some separate circuitry to do that today. Somehow I misread a relay and I've managed to regulate down the +13.8V supply going into the box to +5v - this is driving a 24V relay! Muppetry!

So tomorrow I'll play some more and swap out the relay for a 5V unit I have here.

Mark.

WSPR & The ATU Syndrome

Well, been back with the ATU today, and after some discussion have come away 20 quid worse off and with a Mostly Fine Junk (NFJ) 949E. This includes cross needle metering and an internal dummy load.

I've tried the tuner and unlike the previous one from Comet, it will actually tune my antenna on all expected bands, so I conclude that the other one was highly suspect.

More playing with the FT-950 today, and more menu items 54 & 55. I now have these set so that the radio is in PKT-USB and both 54 & 55 are set to 1000Hz, this seems to be working as expected and is accurately displaying the TX + audio offset on the radio. WSPR is now working find business and will be chuntering away to itself whenever I'm not using the radios inside the shack.

Trying to get to grips with Logger32 on digital modes now, so have printed some parts of the help file out and promptly ran out of ink in the printer. Guess I'll need to go get some tomorrow.

The Shack 11/10/2010

Here is a picture of the shack on 11-10-2010.

You will see that I have a lot of test gear; this is because I do quite a bit of homebrew and am very interested in QRP operation.

Starter Post

Well, as a licensed Ham for some years, I recently decided to get active again and thought my musings may be of interest to others.

Most recently I've got myself an FT-950 which I plan to use primarily for data modes.

Today I have been musing over the data mode on the FT-950 and most specifically menu items 54 & 55. It seems for the default settings of these menu items to work, you need the radio in PKT-USB mode; by default the data mode was in PKT-LSB. I was failing to decode signals I could hear in standard USB mode with that setting.

Seems that the radio will work well in the dedicated data mode, just some understanding of the carrier offsets involved are needed to figure out which mode the radio actually needs to be in.

I left the radio running WSPR for a few hours today, my signals (only 10W out) were heard far and wide including Japan, and I was receiving from all over as well.

I came home with a Comet CAT-300 ATU today, but it seems to be the most useless ATU I've ever come across. It wont tune anything and actually provides a higher best SWR when in-line than without the tuner at all. I conclude that it's faulty and will be taking it back tomorrow.

I'm going to take a picture or two of the shack and post them now.

Mark. G0MGX