help with impedance matching and transmission

[patrickian01]

hi,

i made some oscillators out of crystals and inverters and it turns out that the wave generated is close to a square wave. I used potentiometers for the feedback and adjusted it a little to get some sort of a traingle wave, although the wave looks sine to me and the oscilloscope gives me a "sine wave detected" text when i connected the output to it.

and as it is, i have tried leaving a connecting wire hanging and when i did an RLC filter on another board, I was surprised that i was receiving a small portion of the oscillating signal on another board, although only for a few inches. here comes the problem, i want to do some impedance matching.

I would try connecting a potentiometer in parallel, forming a voltage divider, and look into an oscilloscope as to where the voltage will be divided by 2, and measure the potentiometer value and it would give me a resistance value nearly equal the output impedance of the oscillator, but how accurate would it be? (a question of the blue though, why is there a need to ground the 3rd pin of the potentiometer?)

and assuming that i would be connecting it to a quarter wave monopole with impedance of approx. 50 ohms, would a simple balun suffice? and the output of the oscillator gives a value of around 600-800mV pk-pk. Assuming that i have a matched output impedance to load impedance (antenna), would the transmitted power be enough to reach a receiving antenna a few meters away from the transmitting antenna?

 

[chuckey]

it is unlikely that a simple oscillator would continue to oscillate if you connected a 50 ohm load directly to it. You must use a piece of co-axial cable from the oscillator and say this is my RF output and the RF earth. measure the voltage from core to screen with your scope. Connect your pot up CENTRE to live, one end to earth (one end is left unused), twiddle the pot until the RF is 1/2 of that seen at first. measure the resistance of the used part of the pot. Now it is likely that it will be , say 5k ohms, so you have to match from 5,000 -> 50, i.e. 100 :1 in impedance or 10 : 1 in value. The easiest way to do it is to use a PI network, a small C across the oscillator end, series L, and 10 X C across the 50 ohm end. The magic bit is to get the L to tune with .9 C at the operating frequency, when it does you will have got your power match and filtering all sewn up !
Frank

 

[patrickian01]

thank you. i will try that one tomorrow and hope it helps.

the thing is, is it, at all, possible? transmitting a triangle-ish wave (though it looks sine to me) which has around 3-4 harmonics at approximately 5 meters? is the 600mV enough to reach the receiver assuming all the losses in the impedance matching circuit, antenna and all?

 

[chuckey]

it all depends on the sensitivity of the receiver and your method of modulation and the efficiency of both the receiving and transmitting aerial. Radio amateurs have communicated across the Atlantic ocean by using morse code with a transmitter power of less then 1 watt.
Frank

 

[patrickian01]

well, i wouldn't be using any modulation actually. I would just like to send the produces waves across the room and at the receiver side it would just be a quarter wave monopole connected to probabaly a high pass filter then to an oscilloscope, just to see that i actually received it.

 

[chuckey]

Again you will need to tap your 50 ohm aerial into a resonant tuned ciircuit and connect the scope across the coil. Remember that the scope and lead has capacitance that will affect the tuning of the coil.
Frank

 

[patrickian01]

I tried tapping the scope directly to an antenna and it only got a bunch of frequencies according the FFT of the scope, I also tried adding a filter and tapping the oscilloscope across the filter and it did better, although some noise were still present, i was able to see on the FFT the frequency that I was transmitting. This was done using simple connecting wires as the aerial. Fine-tuning the filter and using matched transmission lines could probably improve the result.

 

[patrickian01]

i just figured out that i have no coax which has 50 ohms. all i have is a coax with 75 ohms. can i use this coax and connect it to the aerial? how much difference would it make compared to when i use a coax that has 50 ohms?