Broadcasting FM and AM Radio with the UHFLI

May 6, 2013 by Sadik Hafizovic

Following my previous blog post, people advised me to try to broadcast FM radio too. For this, I had to retreat into Zurich Instruments' shielded radiation lab, so that my experiment would remain concealed. It then took about 5 minutes to broadcast tunes from my mobile phone's 3.5 mm jack using the UHFLI Lock-in Amplifier. Here's what the setup looks like.

ZI_UHFLI_Lock-in_Amplifier_FM_broadcast_setup

Figure 1: Broadcasting FM radio at Zurich Instruments.

These are the steps to set everything up:

1. Connect the mobile phone to the UHFLI's Signal Input 1 using a 3.5mm-jack-to-BNC cable.

Figure 2: 3.5 mm jack to BNC.

2. Connect the antenna cable to Signal Output 1.

Figure 3: The radio antenna, for which the cable should be on the order of 1 m in length.

3. Configure Demodulator 1 to capture the audio signal (yes, this is a hack - but it does work).

The settings are:

Frequency 0 Hz
Bandwidth 30 kHz

4. Configure the Signal Output.

This works as follows:

Setup Demodulator 2 as the signal generator
Select Osc 2
Set output amplitude to 1V, disable all other outputs
Enable Signal Output 2

5. Configure PID to modulate Osc 2.

Follow these steps:

Set Input to Demodulator X, 1
Set Output to Osc 2
Set Setpoint to 0 V
Set P=300 kHz/V, I = 0, D = 0
Set Center to 106 MHz
Set Limits to +-500 kHz
Enable PID

What's nice is that you can actually tune your broadcast station to your radio!

In order to do the same with AM, you need to set Osc 2 to 106 MHz and to adjust the PID configuration:

Set Input to Demodulator X, 1
Set output to Output 1 Amplitude, Amplitude 2
Set Center to 0.5 V
Set Limits to ±0.5 V