antenna accident

What is the time scale? It's not amplifying the input signal the way you want it to- the output voltage peak might be larger than the input voltage peak, but the signals don't look anything like each other. The output looks like ramping triangle waves like audioguru said earlier.

dknguyen said:

What is the time scale? It's not amplifying the input signal the way you want it to- the output voltage peakmight be larger than the input voltage peak, but the signals don't look anything like each other. The output looks like ramping to triangle waves like audioguru said earlier.

Click to expand...

is 5ns per division for the antenna input and 500us for the amplifier output

So the circuit isn't amplifying the signal properly at all. The output voltage is higher than the input voltage (amplifiers do that, of course). But the input and output waveform should also be the same. In this case, they aren't even the same frequency. The output frequency is much lower and is ramping the output signal because it can't react fast enough to the input signal.

The circuit is doing something...just not what you want it to.

dknguyen said:

So the circuit isn't amplifying the signal properly at all. The output voltage is higher than the input voltage (amplifiers do that, of course). But the input and output waveform should also be the same. In this case, they aren't even the same frequency. The output frequency is much lower and is ramping the output signal because it can't react fast enough to the input signal.

The circuit is doing something...just not what you want it to.

Click to expand...

does the 100nF at the input from the antenna has something to do with the ramping?

micael said:

does the 100nF at the input from the antenna has something to do with the ramping?

Click to expand...

It's your entire circuit that is causing the ramping right down to the wiring. At high frequencies like 433MHz, you really need to know what you are doing because all the parasitic inductances, resistances, and capacitances start having huge effects. You can't prototype circuits like that on a breadboard- you need to get a custom PCB made because breadboards have way too many parasitics for high frequency circuits. You need capacitors, resistors, inductors that work the can do their job at high frequencies. Your op-amp is also too slow to handle these frequencies (and I think someone else said you can't get op-amps that are fast enough) There's nothing you can do with this circuit to make it work- you need better components.

If you have math equations for your circuits, the frequency variable is a very large number. This makes it so all the tiny numbers for parasitic capacitances, inductance, and resistance because they get multiplied by the very large number for frequency. You usually ignore these numbers because they are so small (most of the time you don't even draw them on your schematic) so they don't affect your circuit, but with high frequencies these numbers are large enough to be noticable and start affecting your circuit.

thanks all for your help,
using 1M resitor for negative feedback ur suppose to get high gain of the amplifier (1000000) that runs the output into saturation even when the difference at the input of the amplifier is quite small (mV)....may at least mak this assumption to explain why the output from the comparator is higher than the input?

You have simply made an old fashioned crystal AM radio. Like I said earlier, "Your circuit might be using the opamp's input transistor as an AM detector diode."
Then the opamp is amplifying the small amount of 670Hz modulation that is AM modulating the high frequency.

simulation problems

Can someone please help with these two simulations?
I ve tryied to do the simulations using PSPICE and TINA for windows with 433MHz and 23mV amplitude but they both crashed...

The 1st circuit doesn't have an input coupling capacitor so the signal generator at the antenna shorts the input of the opamp to below the minimum DC voltage where it works.

The 2nd circuit has the generator at the antenna shorted to the power supply.

The 3rd circuit has the opamp incorrectly powered.

All the circuits use a low frequency opamp that has no gain above 1MHz.

All the circuits use a low frequency opamp that has no gain above 1MHz.[/QUOTE]

any suggestions on an amplifier that might be used at 433MHz to allow the simulation?

micael said:

All the circuits use a low frequency opamp that has no gain above 1MHz.

Click to expand...

any suggestions on an amplifier that might be used at 433MHz to allow the simulation?[/QUOTE]

Basically opamps don't work at UHF - haven't you been listening to ANY replies in this thread?.

You need an RF amplifier, which should be tuned to the required frequency - but if you expect 741's to work at UHF I don't see as you have a prayer of building UHF circuits!.

Micael

When you have realised that a 741 does not work at 433Mhz (I think someone may have said that in an earlier post), have a look at this circuit which DOES WORK at 433Mhz. I also posted it here some months ago in response to someones request for a 395Mhz amplifier.

It is intended for 432Mhz, and has a gain of about 20dB.
It is built out of what was in the junk box, using "ugly" construction. It is not pretty, but it is stable.

The coils are short lengths (about 25mm) of 1mm diameter coper wire. The variable capacitors are about 10 pF.

JimB