Transceiver Audio Test Setup

Receive Audio Test Setup

This test measures the audio response of the entire receiver system, including the deemphasis circuit. In an ideal radio, the frequency response would show a rolloff of 6dB/octave across the audio passband. Between the packet low tone of 1200Hz and the high tone of 2200Hz the rolloff should be 5dB.

In each case, the radio was driven to full quieting a signal generator modulated by an audio generator. The generators provided a constant 3.0kHz peak deviation at all test frequencies, and no preemphasis was applied. The audio frequency was set to 250Hz, 500Hz, and then in 500Hz steps up to 3kHz, and the the receiver's audio output was measured by an AC voltmeter across an 8 ohm speaker. The radio volume control was kept at a constant level around 9 o'clock on the volume control, as is typical for packet use.

The audio voltages at each frequency were plugged into a spreadsheet that calculated the response in dB relative to the level at 250Hz, and plotted the charts. Note that the dB values are not the same on all the charts; some radios have a more complex response curve that requires plotting positive, as well as negative, values from the 250Hz reference level. In nearly all cases, though, the chart covers a 20dB range, so the shape of the curves are directly comparable. In the cases where I was measuring "flat" (i.e., discriminator) audio, the charts typically cover a +1 to -1dB range.

Transmit Audio Test Setup

This test measures the audio response of the entire transmitter system, including the preemphasis circuit. In an ideal radio, the frequency response would show a boost of 6dB/octave across the audio passband. Between the packet low tone of 1200Hz and the high tone of 2200Hz the boost should be 5dB.

In each case, the radio's microphone input was driven by an audio generator, and a service monitor measured the transmitted deviation. The generator output level was set to produce about 2kHz deviation at a 1.5kHz modulating frequency and was kept constant throughout the tests; this level ensured that the entire frequency range could be measured without the transmitter audio circuits starting to clip. The generator was set to 250Hz, 500Hz, and then in 500Hz steps up to 3kHz, and the peak deviation was measured at each point.

The deviation at each frequency was plugged into a spreadsheet that calculated the response in dB relative to the level at 250Hz, and plotted the charts.