Self Designed RF Circuit Tx Problems

ok so I designed my very first Oscillator to operate as a Transmitter. For the testing part, the MultiSim simulation went well but in practice I am not able to achieve the same results.

Check the attachment for the circuit I simulated

Or

Here is the direct link

**broken link removed**


The problem is, with the same circuit on a breadboard (without the use of wires for component interconnection; I know high impedance on high frequency), I get no AC signal with my DMM (Digital Multimeter) on the output capacitor. I cant help understand where the problem lies.

On another note I am using Ceramic inductors (green coloured inductors that look like resistors), I dont know if they are the problem. Furthermore all components which I am using are in good state (no faulty component).

My Second theme is, I want to achieve ~150/ 350 MHz, which ever is successful first as the output frequency using a single stage. Also considering the current output Voltage, say if I achieve 150 MHz as the desired frequency and a 2 ft long vertical rod soldered to the output would I be able to transmit the static signal atleast 1~2 metres away? My theory of this Tx is to simply tranmit when the battery is plugged in; no audio waves, no nothing. Similarly if I am able to 350 MHz as the output frequency with the same output voltage with a 10 inch long vertical rod, will I be able to transmit 1~2 metres away.

I need to get this done by today, please guide me through the fault I am undergoing, help would be much appreciated to construct a transmitter and the receiver. What ever information you require feel free and immediate to ask.


Note: I am going with the basics of RF Circuit construction. This means no Op Amps, ICs or other junk. Just plain old RLC & Transistors; for XTALs I have 62 MHz, if I you have some experience with XTALs just tell me where to plug in what.

The overall design shall remain a VDB, Common Base Oscillator (Clapp or Colpitts).

The first problem is that multimeters do not have AC frequency response beyond the video range. (few MHz). Try using some form of receiver and see if you can find the signal. Also, a few turn loop of wire connected to an oscilloscope probe can pick up the signal if it is below the maximum frequency of the oscilloscope.

Well considering my current situation, I am not within easy access to an oscilloscope.

The only other possibility is that I construct a receiver with the LED as its output.
However that is one experimental risk which will cost me time and failure is not an option. But even to do that, I need to know what kind of power I'm dealing with in order to create a suitable Receiver, and what would be its range.

Also I forgot to mention, I have so far observed the circuit practically on a breadboard. If I solder it on a veroboard, will I notice any difference? Except for the circuit being final and all.

I am going with the basics of RF Circuit construction.

Click to expand...

You should also refer to known working circuits from literature instead of choosing arbitrary component values, that only work
by chance and in a simulation. If you want to generate a frequency in the said range, you should better use a RF transistor.
2N2222 has a low gain at high frequencies. "Tons" of transmitter circuits have been published at edaboard, some are really
good, others are mostly working.

Circuits above some 10 MHz can be expected to behave completely different (or not work at all) in a breadboard setup.
Veroboard designs can work, but above 100 MHz also considerably different from theory respectively
simulation, as long as you won't add all trace inductances to your simulation circuit.

To mention some non-optimal properties of your circuit, you can expect a higher gain-bandwidth product at higher Ic values,
e.g. 20-50 mA and also Vce > Vcb.

Actually, I don't think that will work at all.

L1 is DC shorting the base and collector,
The combined inductance of L2 and L3 depends on their separation and orientation,
The total possible DC current is restricted to 0.5mA by R1,
Even if R1 was moved to a more sensible position between L1 and R2, the emitter current is still limited to 60uA by R3.

I'm not even sure it's either a true Colpitts or Clapp configuration either!

Brian.

Actually, I don't think that will work at all.

L1 is DC shorting the base and collector,
The combined inductance of L2 and L3 depends on their separation and orientation,
The total possible DC current is restricted to 0.5mA by R1,
Even if R1 was moved to a more sensible position between L1 and R2, the emitter current is still limited to 60uA by R3.

I'm not even sure it's either a true Colpitts or Clapp configuration either!

Click to expand...

I studied the circuit off Electronic Principles (7th Ed) by Albert Malvino from chapter 23 which is about oscillators. If you want a scanned proof, be my guest.


However, I don't mean to be rude. It's just that I am aware of how incredible, amazing and yet so profoundly advanced the field of Radio Frequencies is; the starting point and the intended direction is overtly hard to navigate.

On my behalf here are the following books I studied:
> Kybett All New Electronics Self Teaching Guide (3rd Edition)
> Giblisco Electronics Demystified
> Practical Electronics for Inventors
> Floyd- Principle of Electric Circuits CC (8th Edition)
> Bertrand Ron' Online Electronics School notes
> Basic Radio- The Essentials of Electron Tubes & their Circuits
> HeathKit Educational Series- Basic Radio (Part 1 & Part 2)
> Electronic Principles (7th Edition)- Malvino

Although I am aware of the amount of books, research work, articles, journals and papers have been written on the themes of RF circuits & design after Heinrich Hertz & Nikola Tesla. But the general feeling I have found is confusion. The more books I read the more oblivious I feel, and thus the feeling of neverending-ness prevails.

Therefore on my accord I planned out to not only make a Transmitter & Receiver myself but make it simplified. So even if I have to mentor a 5 year old, he would start from a basic pulse Transmitter & Receiver, graduating upto audio Transmissions and thus charter his own path, leading upto maybe radars or whatever.

Hence I would have to practice what I would preach and education is the real idea behind all of the effort.

A general fact I observed even after looking up all the Transmitter & Receiver (mostly audio all over the internet, morse pulses or RC car circuits are a jem) circuits over the internet was; the assembly of the components by parts would take 1 hour. What's more philosophically interesting is, if someone knows the right values with the right parts, the circuit works. Theory is just the conception which produces all the desired values.

Coming back to the orignal theme; I have drawn Transistor Load Lines (AC & DC), calculated currents & voltages, power and impedances of the circuit I am trying to build. Now of course if I am on the verge of inventing something I wouldnt be on the forums now would I. I would appreciate help at least in the form of the important math formulae that effect the desired results of the circuit; what ever I seem to have missed out.

My Confession is, I have not indulged in Bode Plotting, Smith Chart or any of the high end precision calculations.

I would be pleased if someone guides me how to get this transmitter working. For sometimes, Experience superseeds Time.

In no way did I intend to insult your efforts, in fact I admire that you accept the complexity of RF when so many on here grossly underestimate what is involved.

I still can't see that the schematic will work though. The tuned circuit values suggest it should be tuned to a much higher frequency than 51.5MHz and the transistor bias does not make sense. I also doubt the simulation is correct as it seem to indicate a DC voltage at the output when it is capacitively coupled.

I can see how the same components can be rearranged slightly to make it more likely to work but at the moment it just doesn't make sense. Are you sure you copied the schematic exactly?

As I said, I am not trying to be negative about your efforts and I have not run a simulation myself to check the results but almost 40 years of experience give me the 'gut feeling' that something is wrong.

Brian.

As you may have already understaood from my above answer, I basically share betwixt doubts regarding practical operation of the circuit.

Giving a residual DC voltage in the AC output just shows limited accuracy of the iterative transient solution, but it doesn't necessarily
prove it's wrong. Generally, an oscillator can work with Vcb = 0 and a low Ic, but surely not with 2N2222 for a frequency considerably
above 50 MHz and possibly not at this frequency with a real device.

You should consider, that 2N2222 is characterized as a switching transistor, you won't get reliable ft vs. Ic data for the part. But even
if the simulated transistor model is correct and the circuit works at 50 MHz, the output power will be very low. After all the
literature, it should be at least clear, that a linear oscillator is restricted to an efficiency below 1.

So again, select a suitable small signal RF transistor with ft ≈ 1 GHz or above, place it in a standard oscillator circuit and
operate it at a bias point that is specified to achieve a sufficient gain.

Ok so I am re-evaluating my entire circuit from scratch. Just 1 quick question, how much does the antenna length matter in terms of range & signal strength. E.g what if I use a 10" antenna on a 40 MHz transmitter & receiver. What effects will I get? I didnt study much about antennas though.

Btw results of new circuit coming soon

The antenna efficiency is not a function of length alone. Longer length does not mean longer range, in fact the opposite is true in some cases.

Although almost any length will work to some degree, you get best results for transmitting and receiving when the antenna is resonant at the frequency you are using. The length of antenna is the same as the wavelength you are using (λ/C) but you have to be careful how you feed power to it and extract voltage from it. Normally, you either have to tap into the wire a some point along it's length or use a transformer to match the impedance to your circuit. You also need to consider that at resonance, the current and voltage distribution along the antenna is 'stationary', the voltage peak and current peaks remain in the same place but do not coincide, the voltage peaks as the current reaches minimum and vice versa. Multiplying the two will still give the same amount of power (W=VxI). You have to make sure you feed your circuit into a point that matches the V and I points.

I'm guessing that what you really want to use is a quarter wavelength antenna with feed point at the base, such as used on 'walkie talkie' sets. In this case, work out the wavelength of one cycle at the frequency you have chosen then divide it by 4. You feed the signal at the base of the antenna with a ground plane below and insulated from it. If using co-ax, connect the inner to the antenna rod and the braid to the ground plane. You can either adjust the distance from the bottom of the rod to the ground plane or alter the angle at which the ground plane slopes downward from the feed point to adjust the impedance to get best results.

I recommend you read the "Radio Communication Handbook" from RSGB for more information. It covers antenna theory and has lots of practical designs. I also get royalties as co-author of it but don't let that put you off

Brian.

λ/4 for 40 MHz is about 2 m, I guess you're restricted to an electrical short antenna in any case. It can be tuned to the
transmitter frequency by an series inductor, but the circuit has a high Q, so the tuning is critical. Some 27 MHz wireless
devices are also using loop antennas.