Considerations for 27 MHz RF Amplifier

[dark_soul]

Hi, I'm experimenting in the RF field, for that, I would like to implement a radio control system at 27 MHz, but my system communication is to short around 2 meters (6.56 ft), and I want at least 30 m (98.40 ft), so, I need one RF amplifier, but when I try to make that with 2N2222A or BC547B, the gain of the amplifier is very low, in some case less than the unit, my questions are:

What Kind of transistor I need to use for this frequency, in the case that the mentioned transistor was ok for this frequency what are the considerations for the design(could you give me an example).

Researching about RF amplifier I found for the desing of the RF amplifier I need the Y-parameters, but when I looking for diferent RF transistors, in some case the datasheet contains the Y-Parameter for determinated Q-point at certain frequency, in all case this parameters is above 27 MHz, how can I found the Y-Parameters for the intereseted Q point @ 27 MHZ?

I attach my design for the amplifier, in simulation the gain at 1 KHz and 27 MHz are almost the same, but physically the gain at 27 MHz is to poor around 2*Vi.

I hope you can give me some tips for the design.

For all thank you.

Hector.

 

[JimB]

First a couple of questions:

What are you using for an antenna at the transmitter and the receiver?

What are you using to drive that (audio) amplifier which you showed in your post?
ie, what is its output power, what form of modulation is used?

What are you using for a receiver?

JimB

 

[dark_soul]

Thanks for your reply

Hi, JimB thanks for your answer, I'm attaching some pictures about the transmitter waveform taken from the oscilloscope, and a schematic of my receiver.

I'm using the PC output to generate the 1 KHz signal, and now my carrier signal is 23 MHz, still I need adjustment to have 27 Mhz, but now I'm working with this frequency.

I hope with this pictures you can try to help me.

Thanks.

Hector

 

[JimB]

Well, what can I say?
I can start by telling you a few of the things which are obviously wrong with your circuits.

Transmitter.
The free-running LC oscillator will have poor frequency stability.
The "modulator" stage has no supply to the collector of the transistor.
The "amplifier" stage may give some more gain at 27Mhz if C7 was not a 100uF (electrolytic?) which is probably more inductive than capacitive at 27Mhz.
C6, 1uF is not appropriate for this circuit.
30cm of wire is a very short inefficient antenna at 27Mhz.
There are no tuned circuits to remove harmonics from the transmitted signal.

Carrier scope pic.
The 27Mhz (23Mhz) signal is distorted showing that there are many harmonics present.
(Harmonic = signals on multiples of 27MHz, ie 54, 81, 108Mhz etc.

AM signal scope pic.
This is NOT an AM signal.
It is an audio signal (1khz) and an RF signal added together.
This is NOT the same as an RF signal modulated by an audio signal.

RF Amplifier scope pic.
The amplifier ahs more gain at audio frequencies than at RF.

AM Receiver
30cm of wire is too short ar 27Mhz.
There is no"earth line" for RF currents to flow in the LP1, LP2 C1 tuned circuit.
Theoretically it will not work at all.
A single tune circuit will have poor selectivity at 27Mhz, the receiver will hear (very badly) all signals from 25 to 30Mhz.
Q1 and Q2 are unlikely to have much gain at 27Mhz.
All in all this receiver will have very poor performance.

I am sorry that this is all negative, but there is nothing good that I can say about any of this.

JimB

 

[dark_soul]

Again thanks for your experience comments, so now my question is, do you want to help me with the design step by step.

unfortunately here Mexico there are not some kind of courses to pay about this topics, thats the reason that I have to looking for forums help.

Thanks.

 

[davenn]

Again thanks for your experience comments, so now my question is, do you want to help me with the design step by step.

unfortunately here Mexico there are not some kind of courses to pay about this topics, thats the reason that I have to looking for forums help.

Thanks.

The first thing you should try doing would be to google 27MHz transmitter, there's dozens and dozens of links to practical circuits

But seriously this is a really difficult way to do what you want. You would be much better off using 433MHz or 2.4 GHz prebuilt modules
google them too there's many to choose from

Cheers
Dave

 

[JimB]

Again thanks for your experience comments, so now my question is, do you want to help me with the design step by step.

The thing is, are you doing this to be able to communicate for some real purpose, or, are you doing this as a design exercise in RF transmitters and receivers?

If you just want to communicate some data from A to B, buy some pre-built off the shelf modules.
It will be easier, cheaper and more reliable.

JimB

 

[dark_soul]

My Reasons to make my own design: first this is a hobby and I really want to learn this part of electronic that my teachers when I was student only give me pure theory and nothing of practical.

second:

If you just want to communicate some data from A to B, buy some pre-built off the shelf modules.
It will be easier, cheaper and more reliable.

If I decide this way here in Mexico this kind of modules the most cheapest is around $ 1,700 M.N. (aprox. 131 USD) so really is for nothing cheap, in comparation with single pieces 2n2222A (less than 0.5 USD), capacitor and resistences (0.01 USD ).

Now, Why 27 MHz? because the antenna is more short and the digital oscillosope that I have has maximun 40 MHZ of BW, watching the RC cars design it has no more than 2 transistor + IC (guess it is to generate de 4 diferente signals up, down, left,right) think is not complicated.

The google circuits still don't work for me yet,and in many cases omit the values for the inductors, but help me to find the way to follow, for example the receiver attached in the last post, if changing the BJT by JFEt MPF102 works well, not like a superheterodyne but it's acceptable in a AM comercial Band.

Anyway appreciate your time to answer me.

Thanks.

 

[JimB]

OK, in that case I suggest that you have a look at and adapt some of the circuits here:

http://talkingelectronics.com/projects/27MHz Transmitters/27MHzLinks-1.html

The receivers are super-regenerative types, not exactly the cutting edge of RF technology! but capable of good results for what you are trying to do.

JimB

 

[en2oh]

27mhz amp

You are a very patient teacher. (Jim)
As an aside, have a quick look on eBay. As has been said before, there are many options that will help you both learn and accomplish your goals.

It might be worth mentioning that the number or components usually has little to do with how many channels (Up/Down Left/Right etc) there are.

Good Luck,
Doug

 

[dark_soul]

1 MHz RF Amplifier

Hi again, after the comments about the transmitter and receiver, I'm worked for try to better some things, first I want to have a functional transmitter in the AM Comercial Band, for this I made an oscillator stable and now I could say that I have a really modulated signal, but still remains poor.

With the attached circuit I have not more than 30 cm (aprox .98 ft), so I'm looking for you help again for you can give me some design tips to have 100 mW or 200 mW with one amplifier.

I don't know if I need to use a common emitter (bjt), common source (Fet) or a tuned amplifier to have the mentioned power.

I hope you can help me with a example design.

Thanks,

Hector.