C2.6
T856/857 Circuit Operation
M850-00
31/09/98 Copyright TEL
2.3.1 Two Point Modulation
Frequency modulation occurs by modulating both the VCO input and the synthesiser
reference input. This process is called two point modulation and ensures a flat modula-
tion response from 67Hz to 3kHz (2.55kHz for narrow bandwidth).
The PLL has a fast response time, allowing a Tx key-up time of <30ms. Because of this
fast response time the PLL sees lower modulation frequencies superimposed on the
VCO as an error and corrects for it, resulting in no modulation on the carrier. At modu-
lation frequencies greater than 300Hz the loop cannot correct fast enough and modula-
tion is seen on the carrier. The response of the loop to VCO modulation is shown by f
2
in Figure 2.5 below.
To achieve low frequency modulation, the reference oscillator is also modulated so that
the phase detector of IC740 detects no frequency error under modulation. Thus, the
synthesiser loop will not attempt to correct for modulation and the audio frequency
response of the transmitter remains unaffected. The response of the loop to reference
frequency modulation is shown by f
1
in Figure 2.5.
Figure 2.5 T856/857 Two Point Modulation
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