# How to buffer AM signal?

Thread Starter

#### MrsssSu

Joined Sep 28, 2021
263

Dear all, I have actually built an AM circuit in real life to combine high frequency carrier sine wave (200kHz) and signal wave of triangle wave at (3kHz), and the output is on my oscillioscope which I am happy about. However, when I try to resonant the wave on my oscillioscope using LC tank, the output get very nasty and unreadable and even if I actually used a high frequency op-amp (TL072), it does not really have much gain and current amplification plus the waveform becomes distorted. So, I really believe that my Wave shown in the oscillioscope is of very high impendence and I needed to buffer it to feed into LC tank to give beautiful AM signal. May I know if there is a way to increase the output current (lower impendence) of the signal I shown above. It is made up of 200kHz sine wave to form the overall jagged triangle I would really appreciate any help because I have spent quite a lot of time on this haha

Thank you for reading and have a nice day !!

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#### Ian0

Joined Aug 7, 2020
6,331
That waveform isn’t amplitude modulated. It is just two sinewaves mixed together.

Thread Starter

#### MrsssSu

Joined Sep 28, 2021
263
That waveform isn’t amplitude modulated. It is just two sinewaves mixed together.
Yes, that signal will then be sent to LC tank to be resonanted to produce AM signal as shown in mu updated diagram above

#### MisterBill2

Joined Jan 23, 2018
13,206
That circuit shown will not produce anything close to accurate amplitude modulation. It may produce some distorted mixing products because of the diode.
So first, what is the purpose of generating a triangle wave amplitude sine wave?
To accomplish amplitude modulation you need a multiplication method, not an addition method, unless large amounts of distortion (more than 50%) is acceptable.
In the crudest form, with those two signals, change the 200KHz signal to a square wave and use that to operate an analog switch, so that short pulses of the triangle wave are passed through at the 200 KHz rate. Then send that signal to the 200 KHz resonant circuit. That will result in a slightly distorted sine wave at about 200KH.

#### Papabravo

Joined Feb 24, 2006
19,346
...
So, I really believe that my Wave shown in the oscillioscope is of very high impendence and I needed to buffer it to feed into LC tank to give beautiful AM signal.
...
You need to let go of the idea that circuits you create with real components will routinely produce "beautiful" results. That is a subjective term, lacking a precise definition, rather than a technical term. In most cases they won't produce those results. When they approach some level of perfection, as in laboratory instruments, the cost and complexity can be extremely high. There are ways to precisely measure and describe the approach to perfection and you would do well to familiarize yourself with them.

One way to know if you have succeeded is to look at the output of an FFT analysis so you can see the frequency content of the output. In a proper mixer you will have a peak at each of the two input frequencies: and in addition, you will have a peak at the sum of the two frequencies and the difference of the two frequencies. When you look at the output on an oscilloscope it will look quite messy. Oscilloscopes traces do best with periodic signals at a single frequency. They do poorly with signals having a rich, but not infinite harmonic content.

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#### MisterBill2

Joined Jan 23, 2018
13,206
If you examine the signal displayed on the scope screen you will see that both signals are present in their original for, amplitude, and frequency. NO MODULATION HAS OCCURRED! Combining has happened but not modulation.
So what you must do is consult an electronic textbook and learn the definition of modulation, since you do not seem to believe any of us.
I have designed and built amplitude modulated transmitters, as have others, and there is nothing in the circuits that you post that performs modulation.

#### bertus

Joined Apr 5, 2008
22,097

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#### Papabravo

Joined Feb 24, 2006
19,346
Here is what an actual mixer looks like. Notice that the carrier and the Local oscillator have been suppressed and only the two sidebands have appreciable energy in them.

#### MisterBill2

Joined Jan 23, 2018
13,206
The way that the double balanced mixer functions is thet the voltage from the Local Oscillator alternately switches on diagonal pairs of diodes, thus inverting the polarity of the arriving signal. That is effectively multiplying the incoming frequency at the frequency of the LO signal.
Multiplication, not addition, is what makes it modulation.

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