7.1 RF Receiver Testing
The receiver in an RF system is designed to collect an input signal at a specific frequency, filter out unwanted signals, and demodulate the input such that the base information can be analyzed. A typical example is the FM Radio. When you set the channel on the radio dial, you are configuring the receiver to be more sensitive to the channel that has that particular base frequency. It will then demodulate the audio information from the carrier, and play the audio through a speaker.
There are analog (AM/FM radio) as well as digital (WiFi, Bluetooth, Zigbee) receivers, but they all operate on similar base principles.
Figure 1 shows a generic block diagram of a typical receiver
Tech Note: Other receiver types are similar in their block diagrams. The major differences are the demodulation (analog vs. digital) and output (speaker vs data).
In this section, we will provide a brief overview of each element in a typical audio receiver and present some common test procedures for each. We will step backwards through each section, starting at the speaker and ending with the antenna. In reality, you can jump to test any section on its own, but this technique allows us to use the speaker to provide instant audible feedback as we step through each design area.
Receivers typically contain filters and amplifiers that can also be tested individually using techniques previously discussed. Please refer back if needed for more specific tests on each component.
The Speaker (Optional)
The speaker converts electrical signals to sound waves. The easiest method of testing a speaker is to simply connect a function generator to the speaker inputs. Function generators are instruments that can output voltages in specific waveform shapes like Sinusoidal, Square, and Ramp. They are typically low power (<1W) , but should have enough power to test the functionality of most simple speakers.
Function or Arbitrary Waveform Generator like the Berkeley Nucleonics A2085
Cabling to connect ARB to Speaker (typically BNC-to-Alligator)
1. Make sure to study the schematics for the design that you have if available. Clearly identify HIGH VOLTAGES and make sure that you are shielded from any HIGH VOLTAGE areas.
2. Disconnect the speaker wires from the receiver.
3. Check the speaker connections are clear of any contamination or dirt. Clean connectors with a cotton or lint free swab and an electronics cleaner. Allow solvents to evaporate before turning on any electrical devices nearby.
4. Connect the function generator output to the speaker input wires.
5. On the generator, set the waveform to Sine, frequency to 1kHz, and the amplitude to 1V (peak-to-peak) and listen for sound out of the speaker
6. OPTIONAL: You can adjust the frequency and voltage of the generator to test the frequency response and volume of the speaker. Humans can typically hear frequencies from 20Hz up to 20kHz.
A properly working speaker should have a noticeable change in output sound when you adjust the frequency and amplitude of the input signal. If the speaker you are working with does not have sound output, it may need to be replaced.