Working with RF - Page 1
Forum section: electronics / beginners

author: gamozo
author_rank: Regular Contributor
timestamp: September 26, 2011, 05:22:46 pm
content: 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

author: Psi
author_rank: Super Contributor
timestamp: September 26, 2011, 07:17:33 pm
content: Try x10 on the probe, or even x100 if you have it.

author: gamozo
author_rank: Regular Contributor
timestamp: September 26, 2011, 07:26:37 pm
content: 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?

author: Time
author_rank: Frequent Contributor
timestamp: September 26, 2011, 07:40:25 pm
content: 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.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 26, 2011, 08:37:44 pm
content: Quote from: Time on September 26, 2011, 07:40:25 pmi was about to say this. but i dont think i'm the right guy to say it first.Quote from: gamozo on September 26, 2011, 05:22:46 pmwhere 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.

author: gamozo
author_rank: Regular Contributor
timestamp: September 26, 2011, 08:41:56 pm
content: When I get home I'll draw up my schematic.

author: Neilm
author_rank: Super Contributor
timestamp: September 26, 2011, 09:15:29 pm
content: Quote from: Time on September 26, 2011, 07:40:25 pmRF 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

author: DavidDLC
author_rank: Frequent Contributor
timestamp: September 26, 2011, 09:29:25 pm
content: If you increase your frequency to 3KHz, then you will be on the R.F range based on Wikipediahttp://en.wikipedia.org/wiki/Radio_frequency

author: gamozo
author_rank: Regular Contributor
timestamp: September 26, 2011, 09:35:49 pm
content: I'll just knock it up to 540kHz (the start of the AM radio broadcast spectrum in the US)

author: Zero999
author_rank: Super Contributor
timestamp: September 26, 2011, 10:00:47 pm
content: Quote from: DavidDLC on September 26, 2011, 09:29:25 pmThe 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.

author: gamozo
author_rank: Regular Contributor
timestamp: September 27, 2011, 05:24:11 am
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 27, 2011, 01:14:53 pm
content: 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.VK6ZGOP.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.

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2011, 04:28:42 pm
content: 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.

author: gamozo
author_rank: Regular Contributor
timestamp: September 27, 2011, 05:04:57 pm
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 28, 2011, 03:47:25 am
content: Quote from: gamozo on September 27, 2011, 05:04:57 pmI'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.htmand:-http://en.wikipedia.org/wiki/Crystal_radioOr this one:-http://www2.eng.cam.ac.uk/~dmh/ptialcd/trf/trf.htm#rfampVK6ZGO

author: gamozo
author_rank: Regular Contributor
timestamp: September 28, 2011, 04:13:05 pm
content: 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!

author: vk6zgo
author_rank: Super Contributor
timestamp: September 30, 2011, 05:21:48 am
content: 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.htmVK6ZGO