6
4.3
Power supply options
Originally, the Si5351A Synth module kit has an LM317LZ voltage regulator on-board, that produces the
3.3V required by the Si5351A chip from a 5V supply. There are also a pair of BS170 transistors which act
as level converters between the 3.3V Si5351A I2C bus and the microcontrolle
r’s 5V data lines, where the
module is used with 5V systems.
The ProgRock is designed to run the microcontroller at 5V. The recommended assembly is to leave out
the LM317LZ and associated components from the Si5351A Synth PCB, and instead use them on the
ProgRock PCB as a 5V regulator. The 3.3V required by the Si5351A is then produced by the noise filter
circuit (actually it produces about 3.5V, which is within the Si5351A’s specification). The advantage of this
approach is that the noise filter produces an exceptionally clean supply for the Si5351A. Furthermore, it
means ProgRock can be used with higher supply voltages, in the range 7 to 37V. Practically speaking, the
LM317LZ will need to supply around 30mA current, mostly to the Si5351A; if you use the LM317LZ to drop
the voltage from 37V to 5V the power dissipation will be almost 1W which will be too high for this tiny
TO92-packaged voltage regulator. So limit the supply voltage to 12V, in practice.
You may wish to power ProgRock using a 5V supply. In this case you would short out the LM317LZ with a
wire jumper.
ProgRock is also compatible with the QRP Labs OCXO/Si5351A Synth module kit. This is a kit-built
27MHz Oven Controlled Crystal Oscillator reference version, of the Si5351A Synth kit. The heater circuit
for the oven consumes quite a lot of power (too much for an LM317LZ), so in this case you should wire
ProgRock for 5V operation, and build the OCXO kit exactly as described in its assembly instructions!
The following lists the various power supply options you may want to consider.
4.3.1 5V LM317LZ regulator on ProgRock, noise filter provides 3.3V to Si5351A
(RECOMMENDED)
This is the recommended method for powering ProgRock and the Si5351A Synth module kit!
Here you should leave off the LM317LZ voltage regulator, two resistors and 0.1uF capacitor that produce
the 3.3V on the Si5351A PCB. Instead, 3.3V will be provided by the noise filter circuit on the ProgRock
PCB. In the Si5351A kit, the components to leave off, are labelled IC2, R5, R6 and C1. Some of these
components are then used in the assembly of the ProgRock PCB.
The rest of this assembly guide assumes that this is the option you choose.
