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Posted: 10/16/2008 10:13:45 AM EDT
I know antennas and radio but not circuits

First, the device in question:





This is a wireless flash trigger for photography.  This unit attaches to the camera, and when triggered by either the camera contacts or the manual pushbutton on the unit it sends a radio signal out to receivers which set off the flash units.

I'm having trouble with them and need to improve the signal performance from the transmitters.  These are obviously very low powered devices and with a really poor antenna (that squigly circuit board trace as best I can tell) so in true amateur radio fashion my idea was to wecsog on a 1/4λ antenna and see how it did.  Two problems though... I know very little about circuit design so I'm not sure where to attempt to modify things here on this obviously cheaply made device... and second I'm trying to figure out the actual operating frequency without benefit of a spectrum analyzer.

I do have a frequency counter but just trying to pick it up via an antenna on the freq counter was giving me erratic results.  From the results I got with the freq counter and throwing in some educated guessing I think they might be in the 350MHz range similar to garage door openers.  I also found an advertising description from a similar set that described them as "420MHz".

The transistor (?) attached to the squiggly line is what I'm guessing to be the output amplifier, but I don't know which terminal would be used for antenna and which for ground, or if that's even what I'm looking at for sure.  I may have a photo gig this weekend and need these to work if possible.  I'm looking at replacing the system but funds are very short and don't have time to get anything else before the weekend anyway.

ETA:
Flash of genius today led me to try my antenna analyzer on wideband as a ghetto spectrum analyzer... pretty sure I located the TX frequency which was 315MHz.

Decided to go ahead with a non-destructive modification/test.  There's a solder pad on the board that is attached to what I think is the antenna trace on the board.  So I cut a section of scrap duckie antenna for 315MHz and soldered to that solder pad.  Going to compare range of this modification to an unmodified unit tonight.  Still open to more informed opinions.


Link Posted: 10/16/2008 7:20:54 PM EDT
[#1]
At first glance, it looks like a modulated oscillator circuit. The 4060 IC is a counter with an internal oscillator section limited to 10 mhz. The strip line inductance indicates a much higher freq for the actual circuit. I dare say that the counter is being used to send a pulse of 1's and 0's to the oscillator. You are correct in saying it appears to be on garage door freqs, since the board looks like some of the ones I have checked out. Operates under FCC Part 15 that limits the RF field to some set value. Presumably the RF transistor is that or a 3 terminal RF oscillator (unlikely), so more voltage across the inductive strip =  more RF power. As to changing out chip resistors, good luck.

RS
Link Posted: 10/16/2008 8:03:26 PM EDT
[#2]

Quoted:

Flash of genius today led me to try my antenna analyzer on wideband as a ghetto spectrum analyzer... pretty sure I located the TX frequency which was 315MHz.


The metal can with "R315" on it is a SAW ("Surface Acoustic Wave") Resonator - Basically, the UHF counterpart to a quartz crystal - and is what controls the transmit frequency. So, the "315" marking should be a good clue as the frequency...

It looks like there is no RF amplifier transistor - They're directly coupling the SAW oscillator transistor (located at 4:00 with respect to the SAW) into the antenna.

And yep, the thick, squiggley trace is the antenna. Directly connecting another 1/4 wave antenna to it is probably going to detune it, or possibly kill the oscillator. You might be able to couple the new antenna very, very lightly into it, through a resistor or very small value capacitor.
Link Posted: 10/16/2008 9:07:02 PM EDT
[#3]

Quoted:

Quoted:

Flash of genius today led me to try my antenna analyzer on wideband as a ghetto spectrum analyzer... pretty sure I located the TX frequency which was 315MHz.


The metal can with "R315" on it is a SAW ("Surface Acoustic Wave") Resonator - Basically, the UHF counterpart to a quartz crystal - and is what controls the transmit frequency. So, the "315" marking should be a good clue as the frequency...

It looks like there is no RF amplifier transistor - They're directly coupling the SAW oscillator transistor (located at 4:00 with respect to the SAW) into the antenna.

And yep, the thick, squiggley trace is the antenna. Directly connecting another 1/4 wave antenna to it is probably going to detune it, or possibly kill the oscillator. You might be able to couple the new antenna very, very lightly into it, through a resistor or very small value capacitor.

Glue some small maybe 1/4 wave lengths of fine wire to the inside of the plastic housing. They may couple to the antenna through the air and improve their performance. Like those cell phone booster antennas that are made of plastic.film/copper strips. Soldering a similar strip to the ground of the device and gluing it in the case may help also.
Link Posted: 10/16/2008 10:39:03 PM EDT
[#4]

Quoted:
Soldering a similar strip to the ground of the device and gluing it in the case may help also.


That's a valid point...1/4 wave antennas work against a ground - Might as well provide a good one.
Link Posted: 10/17/2008 7:28:52 AM EDT
[#5]

Quoted:
The metal can with "R315" on it is a SAW ("Surface Acoustic Wave") Resonator - Basically, the UHF counterpart to a quartz crystal - and is what controls the transmit frequency. So, the "315" marking should be a good clue as the frequency...

It looks like there is no RF amplifier transistor - They're directly coupling the SAW oscillator transistor (located at 4:00 with respect to the SAW) into the antenna.

And yep, the thick, squiggley trace is the antenna. Directly connecting another 1/4 wave antenna to it is probably going to detune it, or possibly kill the oscillator. You might be able to couple the new antenna very, very lightly into it, through a resistor or very small value capacitor.


i am ar-jedi and i approve the above post.  

ps to the OP:
see, for example, www.hoperf.com/pdf/HB315 F-11.pdf, specifically Section 4, Figure 1.  that's about what you got there.

you can also buy 315MHz TX modules, which when coupled into the modulation circuit above, *may* provide you with more range.  e.g. link

finally, i think that board you posted pics of was assembled in the dark by a 12 year old human pick-n-place machine.  i guess i am way too used to looking at high quality PCB's and PCBA's. i especially like the fact that there are no copper pads on most of the IC leads.  that's sweet.

ar-jedi
Link Posted: 10/17/2008 9:06:55 AM EDT
[#6]
That is, in fact some of the worst soldering joints I have ever seen.  My professor in college when I was an electronics major would have flunked me if I had turned in a project that looked like that.  

I personally think a blind 12 yr old would do better than that.  I can solder better than that by touch.
Link Posted: 10/17/2008 10:15:21 AM EDT
[#7]
Thanks for the posts

I'd already made the mod when I saw the replies.  I cut a section of rubber duckie antenna with a little piece of wire on the end and tuned it for 315MHz, and soldered to the solder pad that I indicated.  Compared to the unmodified transmitter (I have two) I got noticeably better reliability in an intentionally-difficult situation.  Still not perfect but better.

This morning I've dissected the receivers and it was a straightforward operation to identify and disconnect the 1.5" long strip of circuit board trace that constituted the receive antenna and connect a 1/4λ piece of wire.  There was a convenient (unused) hole in the case for antenna mounting.

Part of the genesis for this project (other than the basic need to get them to work) is that I've seen almost identical looking transmitters that have a small antenna sticking out the top.  From the information discussed here and a little more critical look at the circuit (it's easier in the large size pics actually) it seems to me that those solder pads for signal and ground right there together would probably be intended for an antenna connection.  If that's a capacitor across that "loop" with the squiggly line that would be an LC circuit correct?  Perhaps that solder pad is just at the 50Ω point in that circuit.

The receivers are obviously designed to use an external antenna but it was simply deleted and a machine screw used to fill the hole instead.
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