Short note: Filter and oscillator requirement in FDM

In an FDM system the channels are separated only by frequency, so the quality of the filters and carrier oscillators decides how much crosstalk and distortion occur.

voice->[Mod]<-osc fc1  ->[BPF]-+
voice->[Mod]<-osc fc2  ->[BPF]-+-> composite
           (from one master)   |

Filter requirements

• Sharp cut-off: SSB generation needs filters that pass one sideband and reject the other, which is only a few hundred hertz away (e.g. 300 Hz lower edge of voice). Very steep skirts are needed, so crystal, ceramic or mechanical filters with high Q are used.
• Flat passband and low ripple across 300–3400 Hz so all speech frequencies have equal gain.
• Linear phase / low group-delay distortion, especially for data and video channels.
• High stop-band attenuation (about 60 dB) to keep adjacent-channel crosstalk low.
• Guard bands (900 Hz per voice channel) relax the filter slope.
• Stability with temperature and ageing.
• Multi-stage modulation (group → supergroup → mastergroup) is used partly so that filters work at low frequencies where sharp filters are practical.

Oscillator requirements

• High frequency stability and accuracy: in SSB-SC any carrier offset between transmitter and receiver shifts every voice frequency by the same amount; only a few hertz of error is tolerable for speech and less for data.
• All carriers are therefore derived from a single crystal master oscillator by frequency multiplication/division (frequency synthesis), so they are locked to each other.
• Low phase noise and harmonics, since spurious outputs cause interchannel interference.
• Synchronization: a pilot carrier is sent so the receiver can lock its oscillators to the transmitter's.
• Carriers must have stable amplitude for constant modulator gain.