Constructing a circuit that would trigger my cordless house phone intercom circuit?

[Buk]

Note the "L", so it's an inductor. probably part of the matching network.

pretty sure the L301 (and L303 on t'other side) refer to the color coded axial inductors; not the switchback traces.

 

[Buk]

Higher, here means 3A vs 300 mA.

Here, in my house, "higher ranges" means 20mA and 200mA:

Again, I thought this was implied by my mention of 2000uA

 

[Buk]

Tantalum has a small size, good ESR and high capacitance density. They are unforgiving with overvoltage and reverse polarity.

Thanks. I was just trying to work out why they would use an expensive tantalum capacitor in preference to a standard electrolitic in what is obviously a built-to-a-price commodity item.

I haven't managed to work out what capacitance it is:

The marking read 476J 20P53.

 

[rjenkinsgb]

476, like resistor code but picofarads:, so 47 + six zeos: 47 000000 pF or 47uF
I'd guess it's 20V rated.

 

[Buk]

476, like resistor code but picofarads:, so 47 + six zeos: 47 000000 pF or 47uF
I'd guess it's 20V rated.

Thanks.

That means its not even particularly high capacity for is physical size compared to alu/polymer at < half the price. again I'm wondering why tantalum?

 

[Nigel Goodwin]

Thanks. I was just trying to work out why they would use an expensive tantalum capacitor in preference to a standard electrolitic in what is obviously a built-to-a-price commodity item.

Small size, easier machine manufacturing, and in china the cost is probably no different - and may even be cheaper?.

 

[Buk]

Small size, easier machine manufacturing, and in china the cost is probably no different - and may even be cheaper?.

There are non-tanalum 47uF electrolics in the same sized smt mount.

And other electrolitics on the board both SMT and thru-hole mount:

 

[Buk]

I'd guess the track length is tuned to 1/4 wavelength at the operating frequency - they could then act as "metal insulators" to prevent RF being passed through the external power cable.

Is this a reasonable explanation of what you were describing?

When a transmission line is 1/4 wavelength long, it is essentially 90 degrees long to the signal - meaning that the signal goes through 90 degrees of its 360 degree cycle. When the line is shorted, this causes a -1 reflection (the signal is inverted and reflects back towards the input). A -1 reflection is the same as a 180 degree phase shift. Therefore, if you launch a signal into the line, it travels 90 degrees to the short, the short induces another 180 degree shift, then the reflection travels back another 90 degrees to the input. Thus, the total round trip is 360 degrees. This means that the signal coming back as a reflection is exactly the same as the injected signal, so no current flows (in theory), which is the definition of high impedance.

And could I confirm this hypothysis by measuring the length of the tracks, to see if the frequency makes sense in this context?

 

[rjenkinsgb]

And could I confirm this hypothysis by measuring the length of the tracks, to see if the frequency makes sense in this context?

Yes, that's it - just remember you have to allow for the velocity factor in that material. Somewhere around 0.66 at a guess, making the physical length probably around 2/3 of the electrical length.

 

[Buk]

Measuring the length (in pixels) it comes out at 571. Measuring several known lengths it comes out to ~17pixels/mm *4 = ~135mm => 2.2 GHz. (Without any velocity factor adjustment)

The phone operates 1.9GHz.

Of course, I perhaps could include: the length of the lead to the inductor? The length of short side (parallel to the edge of the board) of the input terminal? The long edge of same?

I'm also intrigued by the thick white graphic lines. The description I quoted mentions that the antenna needs to be shorted. The board is 3 layer, so the graphics could indicate a middle layer direct connection between the terminal and the inductors, which would provide the short?

 

[rjenkinsgb]

An impedance discontinuity can have a similar effect, to set the resonant length; just the change from the fine track to large pads could do it.

End loading capacitance from the other connections will also reduce the frequency, so it does look like a possibility as a resonant filter.

Edit - more info relevant to a series connected 1/4 wave line:

Quarter-wave impedance transformer - Wikipedia

en.wikipedia.org