Exciter Rf Circuitry; Frequency Control Loop - Tait TB8100 Service Manual

2.5

Exciter RF Circuitry

2.5.1

Frequency Control Loop

Reference
Oscillators
Clock Buffers
Twisted Ring
Counter
Low Pass Filter
TB8100 Service Manual
© Tait Electronics Limited September 2006
Refer to Figure 2.10 on page 49
Audio modulation of the exciter synthesizer is implemented in the
frequency control loop (FCL). It uses a three-point modulation scheme
involving the FCL_VCXO and VCO signals.
The FCL consists of reference oscillators, clock buffers, twisted ring counter
phase detectors, low pass filters (LPFs), ADCs, the FCL processor and
digital-to-analogue converters (DACs).
Modulation to the FCL_VCXO reference oscillator requires the use of the
FCL_VCXO_CTRL and SYN_VCO_MOD signals to apply:
a constant DC offset to the FCL_VCXO signal until it achieves
frequency lock to the internal referenced TCXO;
frequency modulation to the FCL_VCXO and VCO simultaneously
from the transmit audio signal; the transmit audio signal has a range of 0
to 3kHz.
The modulated signal from the VCXO is attenuated by the bandwidth of
the loop filter in the low pass filter (i.e. 150Hz). To obtain flat modulation
across the audio band, the VCO is also modulated simultaneously to obtain
a composite high pass filter response.
between the frequency modulation gain characteristics of the VCXO and
VCO.
The TCXO and VCXO signals are squared up and buffered as digital
signals using hex inverters.
The VCXO and TCXO signals are phase shifted and multiplied by XOR
(exclusive_or) logic. This is achieved using a twisted ring counter, which
also divides both signals by four.
There are two output signals from the twisted ring counter. Both signals
have the sum and difference frequency contents of the TCXO and VCXO
signals, but there is a 90° phase difference between them.
I and Q low pass filters capture the difference frequency contents down to
DC and integrate them to form two triangular waves, which are 90° out of
phase with each other. This frequency is equal to a quarter of the difference
frequency content of the TCXO and VCXO signals.
The in-phase triangle frequency is referred to as "I channel" and the
quadrature-phase triangle frequency is referred to as "Q channel".
and Figure 2.11 on page
Figure 2.9 shows the relationships
Reciter Circuit Description
50.
47