Fets and Amplitude Modulation/Demodulation

FETs and Amplitude Modulation/Demodulation

Introduction In this experiment we will build an AM radio transmitter. In the process, we’ll learn about modulation of signals, demodulation of signals, and Field- E↵ect Transistors (JFETs and MOSFETs). Before anyone gets too excited about this, let me point out three important and possibly disappointing facts:

1. The FCC quite understandably gets their undies in a knot any time someone broadcasts interfering unlicensed radio signals in the middle of a commercial radio band. Because of this reasonable concern, we’ll be quietly broadcasting our signal a few feet and calling that good.

2. The method we’re using for amplitude modulation is possibly the worst working method for modulating an RF signal. The goal here is to learn something, not to make a high-ﬁdelity radio.

3. The current ampliﬁcation stage of this transmitter is a very basic im- plementation of a “class D ampliﬁer”. It works by turning a switch on and o↵really fast, basically, and if you want to amplify a signal well this is not how to do it. Again, the goal is to learn something. Sorry.

Procedure We’ll be using the circuit shown in ﬁgure 1. It’s quite possible to build the entire thing and have it work, but I’d recommend building and debugging one section at a time. We’ll be using high-frequency signals on this circuit, and breadboards are lousy for high-frequency signals so expect some weird behavior. You can minimize weirdness by keeping your connecting wires and electrical components short and tidy.

1. Start by building the Amplitude Modulator. The “Carrier” signal should be a 1MHz 1-Vpp sinusoidal wave from one of the nice digital function generators. (We have only 3 of these in the lab at this time, but this signal will be set to a ﬁxed value throughout the lab so it’s possible to share using T connectors.) The “Signal” signal should be something in the audio frequency range, about the same amplitude as the carrier, generated by a second function generator. This signal is coupled to the biased gate of the J111 through a capacitor. The J111

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Figure 1: The circuit we’re using for this lab.

is an n-channel JFET, so its Drain-Source resistance is proportional (or at least monotonically related) to the negative voltage at the Gate. Set up your oscilloscope so that you can view carrier, signal, and TP1 at the same time. The vertical gain on TP1 should be on the order of 100-500mV/div. Adjust the bias trimpot until TP1 shows a nicely modulated signal. (There will be some distortion — see point 2 in the introduction section.) Take your time ﬁguring out what’s going on and how this all works! There are three important ideas here: bias on the Gate, coupling the signal to that bias, and a voltage divider output. Save a screen capture from the ’scope for your lab writeup. 2. Now build the current ampliﬁer section. The signal at TP1 is a rel-

2 atively high-impedance signal: if you connect it directly to anything requiring current (such as a magnetic-dipole loop antenna) the results will be disappointing. For this output stage we will use that signal (with another bias-adjust potentiometer) to switch the IRF510. Note the similarities between this section of circuit and the ﬁrst section. The IRF510 is an enhancement-mode n-channel MOSFET, so positive gate voltage turns it on — note the corresponding di↵erence in the bias-adjust potentiometer connections. There are several options for the antenna. Ideally, the antenna should be 4 high, on a ﬂat ground plane roughly 2 on a side. (An order- of-magnitude calculation should convince you to be content with the horribly inecient magnetic-dipole loop antenna provided.) The 100⌦ resistor in series with this antenna may strike you as unnecessary. It is unnecessary, in one sense: the IRF510 can switch 5A without overheating, the wire loop will get warm at 5A but not melt, so with a high-power supply supplying that 15V you could make a nice big radio signal! Do NOT, however, omit the resistor! See point 1 in the introduction of this lab, and recall that nobody in the FCC has a sense of humor. Add a probe at TP2 to your collection of ’scope signals, and tune the bias trimpot for this section until the signal at TP2 looks about as good as the signal at TP1. (Again, there will be some distortion, and again we don’t care: see point 3 in the introduction.) Once the signal looks good, bring an AM radio to within a couple feet of your antenna and tune the radio to your carrier frequency (1000 kHZ). Adjust your “Signal” frequency to verify that your transmitter works: the AM radio should make annoying noise at the signal frequency.

3. (Optional, but irresistable) Replace the input signal with the signal from the headphone jack on your phone or MP3 player, and play your own music on your own radio station.

4. So far, we’ve amplitude-modulated a carrier signal and used that modulated signal to transmit sound over radio. The other half of radio communication is the demodulation of the modulated carrier wave to recover the original signal. Demodulation is much easier than modulation: at its simplest1 demodulation requires merely a half-wave rectiﬁer followed by a low-pass ﬁlter.

1And when have we avoided simplest in this lab so far?

3 Build the “Demodulator” section. Rather than deal with the very small signals that would either doom our e↵orts or require a much longer lab exercise, we’ll skip the whole part about radio reception and just connect the demodulator directly to the transmitter. For the diode, use a germanium signal diode. Ge diodes have a turn-on voltage of about 0.25V rather than the 0.7V of Si, so they work much better than Si for this purpose. Add TP3 to your ’scope signals — you’ll have to take one of the other signals o↵, probably the carrier signal which is pretty boring anyway — and verify that the output of this demodulator is roughly equivalent to the original signal. Save a screen capture of the ’scope signal for your report.

Write-up Explain, for your write-up, exactly how this whole circuit works. Explain the bias adjustments. Explain how and why we use capacitive coupling of the signal at the FET gates. Explain why the modulator modulates, and how the demodulator recovers the original signal.

Clean up Take your circuit(s) apart and put components away. If either of your FETs no longer works, do not put it back in the drawer — just dispose of it. (If they were working when you ﬁnished, they’re probably good and can be re-used.) Put the antennas away also, and clean your area.