The AF Unit

The AF Unit contains TX circuits - microphone preamplifier, TX audio source switch and gain control, and RX circuits - LPFs, volume control and two power amplifiers - one for the phones and the other for the speakers.

The audio RX path linearity is very important to effectively use our ears/brain when chasing for the weak CW signals in the pile up. Another important parameter is the noise level - the noise (either in band or out of band) should be as low as possible. The low noise and hum level in TX audio allows to apply more processing to the signal and low noise RX audio prevents operator from fatigue during the long contests. So the goals were the same as with the Hi-End audio - high linearity and low noise.

So the choice to use parts usually found in the Hi-Fi/Hi-End audio equipment is not a surprise. Most part of the signal path is differential to keep low noise and hum, and lower even order distortions. The low noise differential OP amplifiers (OPA1632) are used in LPFs. The high quality volume control ICs (PGA2311/PGA3411) and the Hi-End headphone audio amplifier (TPA6120) provide excellent performance.

I used E-MU0202 sound card with the PC FFT spectrum analyser software to measure AF Unit parameters. First of all I tested my sound card (E-MU0202) to determine the limits of my test set.


The signals of two audio generators were mixed and fed to the E-MU0202 sound card. As you can see the worst case second order IMD level is -125dBPEP and the third order IMD is better than -122dBPEP.

The Neon's AF Unit were temporary connected to the T03DSP DSP Unit (it uses the same DAC as the Neon - CS4392). A special firmware were loaded to the DSP Unit. It generates two tone signal (for IMD measurements) or single tone one (for harmonics measurements).

The AF Unit was tested with the three volume settings - 4Vp-p output voltage, 0.4Vp-p voltage and 0.04Vp-p voltage.



The audio spectrum during two tone test at the headphone amplifier output (4Vpk-pk output voltage). The IMD2 is at -110dBPEP and the IMD3 is at -123dBPEP (probably limited by the IMD of the soundcard).


The audio spectrum during single tone test at the headphone amplifier output (4Vpk-pk output voltage). The THD = 0.00083%, the level of the largest harmonic (the second one) is 105dB below the test signal. It is still questionable if it originates in the AF Unit or inside the sound card.


The audio spectrum during two tone test at the headphone amplifier output (0.4Vpk-pk output voltage). The IMD2 is at -104dBPEP and the IMD3 measurements were noise limited at -107dBPEP


The audio spectrum during single tone test at the headphone amplifier output (0.4Vpk-pk output voltage). The THD = 0.0025%, the level of the largest harmonic (the sixth one) is 100dB below the test signal.


The audio spectrum during two tone test at the headphone amplifier output (0.04Vpk-pk output voltage). The IMD2 is better than -84dBPEP and the IMD3 measurements were noise limited at -88dBPEP


The audio spectrum during single tone test at the headphone amplifier output (0.04Vpk-pk output voltage). The THD = 0.024%, the level of the largest harmonic (the second one) is 82dB below the test signal.

The signal to noise ratio is measured on the following spectrum. The sound card was recalibrated so, that 4Vp-p signal corresponds to -10dB amplitude. The noise voltage at the headphone amplifier output is below 25uV in 10..20000Hz bandwidth. The 50Hz mains spur can also be seen at very respectable -117dB level.


The audio spectrum during single tone test at the headphone amplifier output (4Vpk-pk output voltage). The SNR in the 20000Hz bandwidth is 100dB.

Note: the headphone power amplifier is able to produce 12Vp-p output voltage at maximal power.


The audio noise spectrum at the headphone amplifier output.50Hz, 150Hz and 250Hz mains spurs can be seen. The sound card calibration is the same as on the previous spectrum.

I rarely use the speaker, so a simple LM4950 is used to drive the speakers. No attempts were made to measure AF Unit parameters at the speaker output.

The TX audio chain is fully differential from the microphone to ADC. The use of differential signal connections with the DSP board solves possible grounding troubles and associated noise.

I see no reason to measure the TX audio path. It uses the same parts and principles as the RX one and the linearity/noise requirements are less strict, so it should match them by design.

The schematics can be downloaded here


The AF unit board top


The AF unit board bottom

All materials provided here are copyright by Oleg Skydan UR3IQO (unless otherwise explicitely specified). In all cases, materials are provided for the purpose of self education
and training in Amateur Radio. No use may be made for commercial purposes without permission of the author.
Please send any comments and questions to Oleg Skydan UR3IQO.

This page was last updated on 22.03.2016 00:40