ELEC 332

In the Lab II: Testing

Now that we've assembled our amplifier we need to test and characterize it. This will be similar to what we did in 241, but at much higher frequencies. In particular, the frequencies we are interested in are well beyond the capability of the 241 lab function generator. We have several different generators with sufficient range. We'll use the most powerful one, which is located on the Northeast bench. (Unlike the 241 lab, each bench has a different selection of equipment. The arrangement is described in detail in the Layout section.)

Part 1: Functional Test

The first thing to do is a functional test: i.e. does it work? Except for the modifications for single supply operation, these circuits are straight out of 241, so fire up the scope and function generator and verify that it does what it is supposed to do. (Functional testing is best done at a moderate frequency, so the function generator is fine for this purpose.)

Part 2: The Signal Generator

The HP 8648A RF signal generator only produces one type of waveform (sine wave) but has a much wider range of output frequencies and amplitudes than a function generator. It is also capable of modulating the output (either AM or FM) using either an external or internal signal.

In this part of the exercise we will use the function generator as the modulating signal, so connect the function generator MAIN output to the HP 8648A MOD IN input. Set the function generator to produce a 10 kHz, 2.5 V sine wave. Connect the RF Out of the signal generator to CH 1 of the Tek 2247A oscilloscope, using a BNC-T connector with a 50 Ω terminator.


Producing a signal:

 When the signal generator is turned on, it will return to its previously set of frequency, level, and modulation, with the RF output turned off. Turn on the signal generator and (after the 5 second self test has completed) set the following parameters:
Frequency
10.0 MHz
Amplitude
50.0 mV (Be sure to use the button marked kHz mV rather than the one marked mV emf.
Modulation
AM, 80%, EXT AC, Modulation OFF.


Observe the signal:

 Press the RF ON/OFF button to turn on the RF output. Adjust the scope to give a reasonable display of the output. Using the DMM and counter/timer functions of the scope, convince yourself that the output claimed by the signal generator is verified by the scope. (Be sure to remember to convert from p-p to RMS.)

Effects of Termination:

 Repeat the measurements of the previous step with the 50 Ω terminator removed. Replace the terminator before proceeding.

Modulation:

 Press the MOD ON/OFF button to turn on the modulation. On the scope, observe that an amplitude modulated waveform is produced. Turn the modulation off again for the time being.

Output in dBm:

 Set the output level to -10 dBm (using the MHz dB(m) key). Is the output level shown on the scope correct for -10 dBm in a 50 Ω load?

Other signals:

 Try generating different frequencies, amplitudes, and modulations until you are comfortable with the various controls.

Part 3: Measure Gain and Frequency Response



Midband Gain and Dynamic Range:

 Set the signal generator to produce an unmodulated 1 MHz output. Connect the output of the signal generator to the input of your amplifier and the output of your amplifier to the oscilloscope (with the 50 Ω terminator). Set the signal generator output level to -10 dBm and verify that the amplifier is still working correctly in its new environment.

Increase the signal generator level until the amplifier output begins to show visible signs of distortion. Note the input and output levels. Reduce the level until the sine wave is no longer visible in the amplifier output noise. Note the input level. For several input levels between these two extremes, measure the gain.

Frequency Response:

 Using a -10 dBm input level, measure the frequency response of your amplifier over a range of 100 Hz to 100 MHz. Because the lower limit of the signal generator is 100 kHz, you will have to use both the signal generator and function generator to make this measurement. Choose your measurement frequencies so as to capture all significant detail of the response. I.e. if it's not changing, measurements can be widely spaces; if it's changing rapidly, they need to be close together.

Compare Inverting and Non-Inverting Configuration:

To practice your unsoldering skills, reconfigure your circuit to use the other amplifier. Repeat the measurements, and compare the two halves.