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3. Design
Please refer to the circuit diagram on the previous page. We can divide this diagram neatly into 4 pieces.
At top left, are the two 10-pin header sockets for the Si5351A Synth module kit. On the actual PCB, many
of the other components such as the microcontroller IC1, sit underneath the Si5351A Synth module, to
save space. The Si5351A chip uses a 27MHz reference crystal, multiplied up by a phase-locked VCO to
600-900MHz, then divided back down to produce the output. There are two PLLs on-board and three
output division stages. Since the VCO feedback and division stages have fractional divisors, the output
frequencies can be set with high precision. The Si5351A chip is controlled by commands over an I2C bus.
More description of this Si5351A Synth module kit is provided in the associated assembly manual for that
kit.
At top right, is the microcontroller IC1, an ATtiny84, 8-bit Atmel AVR processor. This is the smallest
(physically, and logically i.e. memory space etc) AVR processor that would do the job in this ProgRock
application. The internal 8MHz RC oscillator is used, as timing is non-critical. The microcontroller is pre-
programmed by QRP Labs with firmware to control the Si5351A. The operator sets up the frequencies
using the 4-way DIP switch to select a single binary-coded-decimal digit, then press the button briefly to
enter it into the microcontroller. The LED provides a visual feedback of the button press. Frequencies are
stored in the non-volatile EEPROM memory of the microcontroller, so that they are preserved when the
power is switched off. The operating instructions below discuss the set-up procedure in more detail.
At bottom left, is the LM317LZ voltage regulator, IC2. In the recommended assembly approach, this
component is taken from the Si5351A Synth kit, it is NOT installed on the Si5351A Synth PCB. Instead it is
used on the ProgRock PCB as a 5V regulator. Please see section below for further discussion on power
supply options. It is important to decide which option you will use, BEFORE you start assembly of the kit.
At bottom right, is Alan Gray G8LCO’s wonderful noise filter circuit. This circuit goes beyond the popular 3-
component (resistor-capacitor-transistor) capacitor-multiplier technique of active filtering for power
supplies, and provides higher performance: better filtering and better voltage regulation.
In Alan’s own words:
When ProgRock was started Hans had the wish to minimise the phase noise due to noisy supply lines
modulating the synthesiser. It is fairly well known that some IC regulators can be noisy at LF and that
some batches are better than others.
So ProgRock has an IC regulator to provide voltage regulation and current limit protection and an
additional circuit to further smooth the supply to the synthesis IC. The regulator output goes to a potential
divider which then feeds the base of Q1. Q1 then drives Q2 and Q3, the ring-of-three transistors acts as a
unity gain voltage follower with a high input impedance and a low output impedance. The top resistor in the
potential divider is split into two resistors so that we can use two filtering caps to form a low pass filter to
reduce noise to the input of the voltage follower which then becomes the output. Filtering starts at a few
Hz, -60dB around 80Hz and increasing attenuation at 12 dB per octave.
Note that the capacitor Cx is NOT fitted (or supplied). The place on the board is provided just in case there
is found to be some performance advantage of including this capacitor.
