AFSK - effect of overload of tx audio input

The following graphs are spectral analysis of short segments of 1200 AFSK (Bell 202 modulation) which is carrying alternate mark and spaces at the maximum rate.

They demonstrate the effect of overdriving the transmitter audio on spectral distribution of the transmitted modulated signal.

The resultant nullification of the transmitter pre-emphasis contributes to roll-off of the high frequency end of the pass-band which degrades PLL demodulator performance.

The analysis technique can be used to identify such signals off air by performing spectral analysis of the short sync period at the start of each transmit burst.

TNC output

This graph below is output from the TNC (sending a test pattern of alternate ones and zeros). This TNC uses an AMD7910 modem chip. Note mark and space frequencies and the sideband about half way between them. I expected that the level of the mark and space frequency should have been similar, but in this case there is 3dB roll off at 2200 compared to 1200.

Figure 1: TNC tx output spectrum.

Receiver output - 3kHz peak deviation

The graph below is output from a receiver. The roll-off evidenced above is worsened, with about 5db at 2200 compared to 1200. I suspect that the additional roll-off can be attributed to equipment manufacturers tendency to over de-emphasize receivers to improve the SINAD specification.

Figure 2: Receiver output spectrum at 3kHz peak deviation.

Receiver output - overloaded transmitter input

The graph below is output from a receiver when the transmitter input is increased 10dB. A 10dB increase in input should cause about 10kHz deviation in a linear system, but in this case measured peak deviation is 4.5kHz, demonstrating limiting in the transmitter audio stage. The roll-off evidenced above is worsened further, with about 12db at 2200 compared to 1200. The further degradation is caused by the effect of the audio limiter.

Figure 3: Receiver output spectrum with overloaded tx input.

Diagnostic exercise

The following graph was captured off air from a signal that I cannot decode successfully. It is a spectral analysis of repeated frame codes. Note the similarity in the spectral distribution to the controlled overload case above.

Figure 4: Diagnostic example.

Figure 5 shows defective AFSK signals from different stations and captured locally on the APRS channel. Defects readily identifiable are:

• excessive initial sync period (parameter TxDelay too long);
• bad amplitude / frequency equalisation of transmissions two and three (low frequencies higher in level than high frequencies - observe during the data transmission between framing);
• significant energy below 1000Hz in transmissions two and three, probably the result of overload of the tx audio input.

Figure 5: More defects in "waterfall" display

Figure 5 demonstrates that there is a natural concentration of energy during sync because of the code value (0x7E), and whether it is high or low depends on the state of the encoder when it commences to send the first frame code. It is the data part of a frame, where the bit sequence is more likely to be random, that best indicates amplitude / frequency equalisation.

After monitoring the channel for twenty minutes, I did not observe a model signal. However, the first transmission in Figure 5, apart from the excessive TxDelay and slightly low deviation is otherwise good. The blip at the end of the transmission is noise between end of transmission and reciever squelch closure. Note that the squelch will have clipped the front of each transmission.

Figure 5: Another "waterfall" display from Spectran

Figure 6 shows three different AFSK transmissions in the waterfall log in the lower part of the display. The spectrum graph in the top part of the display corresponds to the signal shown at the very top of the waterfall. The red tick marks at the left of the waterfall are at one second intervals.

A description of the highlighted areas follows:

1. A generally good quality transmission, the energy dispersal between 1300Hz and 2200Hz is fairly even, and there is much less energy below 1000Hz.
2. A poor quality transmission, note the taper in power density from 1200Hz to 2400Hz, a sure sign of incorrect pre-emphasis, degrading S/N ratio at the receiver.
3. A very poor quality transmission, high relative power density below 1000Hz, a sure sign of overdriving the modulator and consequent intermodulation distortion distorting the transmitted signal and degrading received signal because of excessive deviation and distortion.
4. A very poor quality transmission, severe taper from in power density from 1200Hz to 2400Hz, a sure sign of incorrect pre-emphasis, degrading S/N ratio at the receiver.

Not highlighted on this figure was that the "sync" period at the start of some station's transmissions were excessive, wasting channel bandwidth. The sync period can be measured against the tick marks. The lowest transmission's sync period is not all shown on the display, and was excessive at more than double the sync period of the top transmission. The parameter that controls the sync period is often termed TxDelay.

How were the measurements done

The spectral analyses above were done with Spectrogram  and Spectran using the PC sound card line input driven from the receiver speaker output.

A TNC Audio Break-Out-Box was a convenient way to monitor and make measurements on received and transmitted signals.

Changes

© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.