Sunday, October 24, 2010

Made the first QSO...

...With the homebrew VHF FM transceiver! A big thank you to CT1BBP who answered my call and was patient enough to let me test some changes in the radio controls while QSoing!
Audio report was: "looks you are in the bottom of a well but 100% copy!"
That's nice, I am happy, will improve later on the audio modulation part and also the PA with ideas from YO5PBG (he answered my question in a previous post for a PA "ninja" :).


Meanwhile I cleaned up the soldering table and placed some unfinished project's on top for the next few days.

A 20A power supply (linear), a speaker as a mic and the VHF/UHF receiver with an VCR tuner module.

Must "clean" this old experiment boards to give space for new projects....

Saturday, October 23, 2010

VHF trx on the shelf

Here it is....
On top of it, the 88-108Mhz FM receiver and the APT weather satelite receiver. On the right the Si570 VFO and the a TS-50.

Now, there's a little issue with the transmitter... I already knew it but kind of forgot... repeaters have a country wide 12.5Khz spacing but the PLL has 10Khz so I am going to miss some access to repeaters, for now I offseted a litle bit the reference cristal do get the 145.012,5Mhz needed for local access. Since this is not an elegant solution I am now looking for another PLL design probably leaving this one for a separate receiver (it's just a matter of programing the PIC).

Regarding the PA...I allready measured 2W on a 50 Ohm load (specs for the 2n3553 say 2.5W at 175Mhz and 28V, I have 13V and 145Mhz), with my uncalibrated test probe...anyhow it hit's the repeater nicely. I sorted the lack of TO-39 heat dissipator for the PA transistors making my hown model, here's the look of it:












Just roll a peace of thin piece of metal sheet around a 8mm drill and tighten a little to fit the transistor (around 7-8m according to 2n3553 specs).
I first tryed with aluminum kitchen foil but it get's to flimsy:

Monday, October 18, 2010

VHF FM transceiver - live tests

IT WORKS!!!

I have now around 700mW output, not the wanted 1W but for now is enough, (I already managed to get 1W but for too much heat dissipation), and I can hit the local repeater with the antenna inside the shack.

video

If you look at the signal meter, the first level, (1/4 scale), is local signal interference and the second level (around half scale) is the repeater signal before it stops transmiting.
I made some audio loopback tests (recording the audo from the repeater transmission) and it looks good, I mean, it's inteligible :)

Now, the world is not perfect and there's still issues/problems to solve:

1 - Place heat dissipators in the output transistors
2 - Sort the problem in the PLL subsystem, sometimes there's no display indication and or garbage and PLL will not lock. Look's like it was a bad solder joint.
3 - There's some delay (half second or less) between hitting the ptt and getting audio via the repeater. Will have to see if it's a delay in the output stage power on and/or PLL unlocking or some delay at the repeater level.
4 - Check modulation level deviation and harmonic generation... will have to study a little bit more on how to verify this!
5 - Make a live QSO.
6 - Finish the project box. Assemble the project box
7 - Put in the shelf....

And now, so that you don't to the same mistakes I did:

1 - Check if your transmit monitor radio is in FM and not SSB mode while doing transmiter tests....
2 - Check if you realy are in the correct repeater tone. I was on the wrong tone and took me a lot of time to undesrtand why the repeater wasn't activated.
3 - Just because when you hit the PTT you listen to some sound on your monitor radio that doesn't mean it's your signal coming from the repeatar. If the monitor radio is close enough you probably are geting a lot of signal localy!
4 - Watch out for loop current's.... in doubt remove the ground connection on any point to point cable.

Stay tuned, for more fun!

Addendum: I can hit the local repeater with the dummy load connected...it's leaking somewhere....

Friday, October 15, 2010

Box for the PIC programer

I had the pic programmer (for 16F84 and similar type) for some time on top of the computer box, built it in a rapid prototype board, so decided to give him new shiny box so I can place it in the "not so frequently used equipment shelf".

Now, why to spent around 10 Eur in an aluminum box when you have some other for around 1 Eur, that's ten time's less...and you can eat some tuna or sardine fish! Yes, I am writing about the mighty tuna tin!

Here's the outcome:

Inside:



PCB come from the VHF PLL board left over, and, by the way, the "tuna" was very nice!
Schematic at: http://speakyssb.blogspot.com/search/label/PIC%20programmer%20for%20APRS%20tracker

Meanwhile, made some more tests in the VHF 1W amplifier and now I only need more 1.2V to get the 1W mark! Maybe next few days...

Wednesday, October 13, 2010

VHF amp...first schematic


After 3 burned 2n3553 I am starting to get the "touch" on building VHF amps... or not...

Well, after some problems, at some point in my tests, I manage to get more than 1w out....but just for a glorious 1s before another blown 2n3553 ruin my day, guess they can't withstand the drive I was putting. Why did I not keep this "successful" schematic? Probably with some small changes it would work nice but I started changing things and then never again got that output value.
Never giving up and after some more hard testing, yes because, every time you change some biasing you have to tune all the stages again, I now get 0.5W output.

Total current on the 4 transistors
That's roughly 13W dissipation... to much for 0.5W output. It's good to have a current meter when testing...if you get more than 1.3A, for a few seconds, in a 2n3553 you must expect that he would die... never the less I have been seeing them withstand some 0.5-0.8A for short bursts. Just don't touch them after... I have more finger burns than transistor...

Voltage at the 50 Ohm load (5.01V is 501mW, 7.07V is 1W)
I am less than 2V away from total success!

This amp has 4 stages after the buffer and all of them use 2n3553 transistors (my choice).
There's a lot of dissipation on all stages except the last one that works in class C so there is for sure a lot of improvement to do. My main issue is getting the right level to drive last stage without warming to much the transistors before.

The following schematic is here just as a placeholder and needs a lot of improvement but I need a base to work and record the changes made.
I am open to suggestions for removing one transistor from the schematic and specialy for transistor biasing and inter stage coupling...

If some VHF amplifier "ninja" reads this, please get in touch... I like to learn!

Friday, October 08, 2010

VHF amp...and the transmiter schematic

I am now in the process of building another version of the 1W VHF amplifier for the FM transceiver, well, in fact, it's still the same version since I didn't managed to get 1W... YET!
Today I made some tests with a 2n2553 and a 2n2866.... it melted solder in one of the base resistors (never I had seen such a thing, normaly resistors burn), a 27 Ohm one, made with 3 different resistor.... but the power was: 1.6mW thats mili! So I still have to learn a little more on VHF amplification... 2 more 2n3553 went belly up...it's always the emiter junction that breaks :)

Anyhow, Pedro - CT5JZX, today sent me a schematic he found for me to have a look and get some ideas! Funny, it's basically the same schematic I use on the transmitting part, that's normal, most parts of my schematic came from Miguel - PY2OHH site.

Here's the said schematic linked from PY2OHH site:


My schematic is only different in the following parts:

Tone encoder: I have a diferent design, but the connection place is the same, also I use the same IC.

VHF oscilator buffer (BFR91 with 100k to base): I have two, one for the PLL and the other for the output amp part. Both connect to the same place via DC decoupling caps.

Oscilator: I don't use the trimer for frequency adjust, I placed a fixed cap after some tests.

Mic amplifier: I don't use an electro mic, so don't need the polarization circuit.

PLL: The same

I am trying to make the final output with 3 2n3553 or 2 and one 2n3866 transistors but if no success I migh just copy Miguel design!

Anyhow, if you need an VHF FM transmiter, this is a design to follow, tested by two...at least!

Wednesday, October 06, 2010

VHF FM transmiter... On air tests

YES! It works! In fact this is my first second transceiver ever built (the Pixie was the first)! The others or aren't finished or are separated units!

The crappy photo and the lousy video:


video

There's no image in the video since it's difficult to hold the camera while talking to the mic some 2m away! The sound comes from the TM-D700 with the antenna about 3m away from the vfo buffer and output amplifier disconnected.

Now, there's still some improvement to do on the tone encoder, the final amp and connect the rx/tx circuit. The tx vfo is some 40Khz offside from display frequency but I think that's because the PLL crystal is not trimmed to frequency yet.

Tuesday, October 05, 2010

Updates on the VHF transceiver I

The VHF FM receiver part is done! I made some more tests trying to put the APRS packet decoder working with this receiver but no success. Will leave only the discriminator out and them with more time in the future will see what's the problem. Never the less, with the computer sound card I can decode nicely the APRS packets.
Here's how it looks line now:
The top PCB is the transmitter one that will occupy me for the next times.
I found a 4.5 Mhz cristal in an old car radio that will fit nicely in the PLL transmiter board...what a luck since the part was getting dificult to find and I was making tests with an ceramic filter doubling as oscillator.

Now some other things...

I still think in making a simple, standalone NAVTEX receiver. After some sucessfull experiments my only problem is to get a cristal combination (for filter and LO) that allows reception of 490 and 518Khz transmissions, I want to avoid PLL, DDS or any other more complex method for the LO frequency. In the past I made some experiments dividing frequency but doesn't hurt to try some more like the ones bellow (external site image):





PA1GSJ blog has also some topics on this subject of finding crystal combinations, and also subharmonic mixers (easier than making a frequency divider). You can read more at: http://draaggolf.blogspot.com/

If you have any idea on a crystal combination that migh work on 490 and 518Khz I would like to ear... preferebly those frequencies plus 1.5Khz to avoid inverting mark and space. Thank you.