Marti Electronics SRPT-30 User Manual page 39

RF SYSTEM & CONTROL
Refer to Block Diagram Drawing 702-117. The fundamental RF generation of the SRPT-30/40A takes place on the
TRANSMITTER SYNTHESIZER board in a circuit known as a Phased-Locked Loop (PLL). The final output
frequency (Fout) is generated by the Voltage Controlled Oscillator (VCO). Fout is determined by the Reference
Frequency (Fr) and N by the relation: Fout = Fr x N.
N is made up by the internal n and a counters of the Frequency Synthesizer IC and by P, the divide-by-128
prescaler. The value N is equal to: N=n x P + a. We can now write Fout in the form: Fout = Fr x (n x P + a).
Therefore, with P as a constant value of 128, the n and a counters can be programmed in such a way that the output
frequency Fout will always be an integer multiple of the Reference Frequency, Fr. The a-counter will always be a
number from 0 to 128, and the n-counter will be a number from 1 to 1023.
The Reference Frequency, Fr, is generated by the 12.8 MHz TCXO (Temperature Controlled Crystal Oscillator)
and the internal R-counter of the Frequency Synthesizer IC. This relation is simply: Fr = Fosc / R, where Fosc = 12.8
MHz and R is the programmable R-counter.
The desired output frequency and the Reference Frequency information is sent from the operator via the front panel
to the microcontroller. The microcontroller will set the n and a-counters to yield the requested output frequency, and will
set the R-counter to yield the requested Reference Frequency. The Reference Frequency is rarely changed, so typically,
Fref is treated as a constant and only the output frequency is changed.
The output frequency is modulated by the injected audio at the input of the VCO. The amount of modulation is
determined by the Modulation setting. The VCO will alter the output frequency in deviation and rate corresponding to
the amplitude and rate (frequency) of the input voltage signal (audio). This is commonly known as frequency modulation
(FM). Since the loop filter has a low frequency response (1 Hz), the PLL will not track the modulated signal and as a
result, only the VCO output will change.
The RF power and power control circuit is shared by the TRANSMITTER SYNTHESIZER and the TWO-STAGE
RF POWER AMPLIFIER. The output of the VCO is sent to a controlled amplifier and is then sent to the TWO-STAGE
RF POWER AMPLIFIER for final amplification. To maintain a steady and constant RF output (over temperature and
voltage changes), a sample of the RF output power (Forward coupling) is sent to the Automatic Power Control (APC)
circuit and compared to a reference output power setting. Any delta changes are instantly compensated for in the APC
circuit and an adjustment is made in the controlled amplifier. A MAX POWER setting can be user adjusted to limit the
final RF output power.
The low-pass filter (LPF) following the final amplifier will filter out all spurious harmonics to a level lower than –
60 dB. The Reverse coupling samples any return power and is sent to the microcontroller. Since the Forward coupling is
also sent to the microcontroller, the VSWR can be determined. The microcontroller will "foldback" the output power if
the VSWR exceeds a value of 4. Finally, all front panel alarms and indicators are sent from the microcontroller.
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