Clock outputs
CLK 0, CLK 1 and CLK 2 are the three clock outputs from the Si5351A synthesizer. They are
unbuffered, direct connections to the Si5351A chip. As such, they should be connected with care,
so as not to damage the Si5351A chip.
The outputs are 3.3 V peak-to-peak squarewaves, with a declared output impedance in the
Si5351A datasheet of 50-ohms. What this appears to mean in practice is that if you connect a 50-
ohm load, the output will be reduced by 50%, to 1.65 Vpp.
For best phase noise performance, as well as least crosstalk between Si5351A outputs, it is
recommended to use loads of at least 1 K-ohm.
USB connection
ProgRock2 has a micro-USB connector for connecting to a host PC terminal emulator, for
configuration of the ProgRock frequencies.
+5V power may also be supplied by the micro-USB connector. There is a diode feeding the +5V
connection of the USB cable to the +V supply voltage of the ProgRock2. So if you connect only a
USB cable, ProgRock2 will be supplied from that. If you are supplying an external voltage below
4.4V damage can occur, please see section above regarding power supply.
The USB connections are also available as pads on the edge connectors.
1pps
If a positive-going 1pps signal from a GPS module such as the QRP Labs QLG2 (
) is connected here, the ProgRock2 frequencies will be GPS disciplined to a high
accuracy. However CLK 2 is used by this process so becomes unavailable for your independent
configuration. CLK 2 is configured to 9,999,999.5 Hz for the purposes of the GPS discipline
procedures. GPS discipline mode is only cleared after cycling the power (when no GPS 1pps
signal is connected).
This input pin requires a signal of not more than 3.3V.
If you are using the 5V output of a
QLG2, then a 3.9K series resistor is recommended (value not critical, 3.9K was what was used
during testing and development). Alternatively the QLG2 can be configured by jumper wires to
provide 2.8V output logic, this would be suitable for direct connection to the ProgRock2 input.
Unused connections
SWDIO, SWCLK are In-Circuit-Programming connections used only by the factory during initial
download of the bootloader into the ProgRock2 module. They have no further use.
SCL, SDA are the I2C bus (used for processor communication with the Si5351A) and have no
further use currently. They are provided on the edge connector for possible future expansion
possibilities.
TXD, RXD are a USART transmit/receive signal pair but are unused. The pads are provided for
possible future expansion.
ProgRock2 manual 1.00b
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