Working with RF

[gamozo]

Greetings,

I've yet to really play around with some RF stuff, and I've just started to figure stuff out. I have a simple 555 timer (I know, it's crude) pumping out a 1060Hz signal, and a small little diode receiving the signal and then it is 'amplified' (still have slight issues there) by a single transistor. I've notice loads of issues when I try to probe the circuit. The second I connect up my scope, the signal is changed. Is there any good way to make probing a bit more practical? Also, what's a good way to amplify a signal coming from an antenna?

Thanks in advance

[Psi]

Try x10 on the probe, or even x100 if you have it.

[gamozo]

Yeh, I have 10x, I should get some new probes some day, but I might get a new scope. I'm correct when I think that having it on 10x greatly increases the resistance, thus much less disruption in the circuit?

[Time]

1 kHz is not RF.  I have a hard time believing your probe would introduce any loading to your circuit.  Maybe I am missing something here.  Impedance is the word you want, not necessarily resistance.

[Mechatrommer]

1 kHz is not RF.

i was about to say this. but i dont think i'm the right guy to say it first.

small little diode receiving the signal and then it is 'amplified' (still have slight issues there) by a single transistor. I've notice loads of issues when I try to probe the circuit. The second I connect up my scope, the signal is changed. Is there any good way to make probing a bit more practical? Also, what's a good way to amplify a signal coming from an antenna?

where do you probe? at the diode or at the transistor? since this is very small signal, probing at the diode may change the amplified signal dramatically, i suspect. can you change to an opamp instead of the transistor to amplify the signal and probe at the opamp output? or what about the normal LC tank as receiver? sorry i'm no expert, cannot give you precise answer.

[gamozo]

When I get home I'll draw up my schematic.

[Neilm]

1 kHz is not RF.  I have a hard time believing your probe would introduce any loading to your circuit.  Maybe I am missing something here.  Impedance is the word you want, not necessarily resistance.

RF design techniques don't really need to be considered until you are looking at 10s of MHz.

If you are only using a x1 the chances are that the capacitance of the probe has caused some additional loading on the signal you are probing which is causing the signal change.

Neil

[DavidDLC]

If you increase your frequency to 3KHz, then you will be on the R.F range based on Wikipedia 

http://en.wikipedia.org/wiki/Radio_frequency

[gamozo]

I'll just knock it up to 540kHz (the start of the AM radio broadcast spectrum in the US)

[Zero999]

If you increase your frequency to 3KHz, then you will be on the R.F range based on Wikipedia 

http://en.wikipedia.org/wiki/Radio_frequency

The definition is purely arbitrary. It technically becomes RF once the signal is transmitted further than the near field, which is about one wavelength from memory so 1kHz is RF is you put it into a huge antenna and transmit it over 300km.

[gamozo]

Here are the pictures of the circuit:

Transmitter (with the 555), receiver, raw oscilloscope graph (blue is the 555 output, green is the boosted RF signal), and the same oscilloscope graph but with a digital 750kHz low pass filter.

I switched my scope probes to 10x mode, changed the freq to 540kHz, and then I realized that my diode was for some reason connected in between transistors (no connection made), everything looks much better now.

[vk6zgo]

I would suggest that you Google for "simple radio circuits" on the Internet,to get a better idea of what you are trying to do.

What you have doesn't differentiate between your required signal,the local AM Broadcast stations,your Compact Fluorescent Lamps,the Ham next door,the CBer two houses down,or a taxi going past!

A tuned circuit will increase the level of the signal you want,& decrease the ones you don't want.

Tuned circuits are the basis of most RF circuitry,(although they may appear in many guises,that is still what they are!)

OK,you have a digital LPF ,but that is just shutting the door after the horse has got out.

 Also,Q1,as drawn has no biasing circuit,so you may not have a lot of gain.

VK6ZGO

P.S.:- The Receiver output should be the modulating waveform for AM,or a dc voltage for an unmodulated signal.
Your picture only shows a sine wave at the carrier frequency,so it seems doubtful the thing is working as a Receiver.

[Zero999]

You need a low pass filter on your 555 otherwise the circuit will broadcast on many frequencies due to the harmonics. This isn't the best way to do it, a cleaner waveform can be produced with a single transistor oscillator.

[gamozo]

Yeh, I'll work on filters tonight. I don't know if I mentioned it, but I've yet to do any filtering or tuning. I'm just not done yet. I'll check out some transistor oscillators too.

[vk6zgo]

Yeh, I'll work on filters tonight. I don't know if I mentioned it, but I've yet to do any filtering or tuning. I'm just not done yet. I'll check out some transistor oscillators too.

I'm sorry to sound curmudgeonly,but tuning & filtering are the whole essence of RF design.
As you are looking at the bottom of the AM Broadcast band,forget about making a transmitter ,make the Receiver tunable,& tune in a real radio station.
Again,I suggest you have a look at how it is normally done.(Google for Crystal Sets).
You don't have to re-invent the wheel!

Here are a couple of useful websites:-               http://earlywireless.com/crystal_sets.htm
                                               and:-               http://en.wikipedia.org/wiki/Crystal_radio
                                                    Or this one:-http://www2.eng.cam.ac.uk/~dmh/ptialcd/trf/trf.htm#rfamp

VK6ZGO

[gamozo]

Thanks for the links . I'm going to be getting an LCR meter soon so I can make my own inductors (I like the physics behind EE, so I don't like just turning wire until something works). I'll pick up some magnet wire tonight anyways and twist away!

[vk6zgo]

You don't really need an LCR meter to get started,you can calculate the approximate inductance using one of these formulae:-

http://www.rfcafe.com/references/electrical/inductance.htm

VK6ZGO