Spring 2007 Issue

CTCSS Modulation
and the LC Phase Modulator



How can you improve the quality of CTCSS modulation while using LC phase modulation? Here WINK explains the drawbacks and offers
a couple of solutions for improving the
quality of the modulation.



By Virgil Leenerts, WINK

Figure 6. Plot of the GE 450-MHz exciter with FM modulation of the CTCSS tone.

CTCSS modulation of a LC phase modulator has been used successfully in FM communications systems and from a practical point of view works well. However, those who would like a higher quality voice modulation may notice that the LC phase modulator has some voice-quality degradation when the CTCSS tone is combined with the voice response. The reason for voice-quality degradation with higher distortion is that the modulation index for CTCSS tones is in the same range as the voice response (sloping 6 dB per octave) and results in distortion products in the modulator itself due to nonlinear (tan x) phase shift curve. Note that modulation input level to a PM modulator is directly proportional to the modulation index.

Figure 1, a plot of modulation index, shows the comparable levels of modulation index for voice response and CTCSS. The CTCSS frequency range shown is from 60 to 200 Hz. For example, the voice frequency of 500 Hz, and the CTCSS frequency of 100 Hz, results in a 2:1 ratio of modulation levels.

Figure 2, a scope trace, is the time-domain view of the 500-Hz and 100-Hz tones as applied to the PM modulator. One may note that the deviation level of CTCSS is nominally a tenth of the voice deviation. So why this 2:1 ratio? It is because the modulation index is related to deviation divided by the modulation frequency.

The scope trace in figure 3 is the time-domain view of the 500-Hz and 100-Hz tones as detected by the FM discriminator. The amplitude of the 100-Hz tone is a tenth of the 500-Hz tone, as expected.

The levels of distortion products are not particularly high and normally are not an issue for the communication system. However, for higher quality applications they may be an issue.

Figures 4 and 5 show 10 dB per division, and the frequency sweep is from 0 to 2 kHz. The resolution bandwidth of the HP3580A spectrum analyzer was 10 Hz. The 500-Hz tone is at about 10 dB, with the 100-Hz tone at 20 dB. The 100-Hz tone is not down 20 dB from the 500-Hz tone as is present at the output of the FM discriminator, because the plots were made at the de-emphasis (200-Hz corner) output. As can be noted, the distortion products look like AM modulation of the 500-Hz tone and its distortion products at 100-Hz intervals and are generally 40 dB below the 500-Hz tone.

To eliminate the distortion, isolation of the voice and CTCSS tones is required. This is not possible with a single-stage LC phase modulator. Two possible solutions are another LC phase modulator stage for the CTCSS tones or FM modulate the oscillator with the CTCSS tones.

Of note is that CTCSS tones are an FM response with a required constant deviation over the CTCSS-tone frequency range. Thus, CTCSS is like a separate sub-tone channel with the voice response being the other channel. Thus, the best case is for the voice channel to be PM and the CTCSS channel to be FM. The isolation with an additional phase modulator stage will meet the isolation requirement; however, it does not have constant frequency deviation over the CTCSS frequency range unless the amplitude is correspondingly changed to maintain constant deviation.

In the case of the GE 450-MHz exciter, the channel element (ICOM) has a temperature compensation circuit that shifts the frequency of the crystal for frequency stability as a function of temperature. The TC circuit is estimated to be able to pull the crystal about 5 PPM. One PPM is about 12 Hz in this case.
 

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