Using 10 MHz clock reference with the HF2 Instruments

Phase synchronous operation of a variety of frequency sources and analyzers is a common requirement, which is usually solved by distributing a 10 MHz clock signal to which the individual instruments are phase locked. This ensures not only a perfect frequency match between the referenced instruments, but also guarantees a constant phase relation amongst all signals. This blog details how the HF2 Instrument platform (HF2LI, HF2PLL and HF2IS, see can be referenced to an external 10 MHz clock, but also how it can serve as a master clock generator for other devices.

1.  Reference two HF2 Instruments to each other

To reference two or more HF2 instruments to the same 10 MHz reference is straight forward. Simply connect the ZSync Output of one device to the ZSync input of the other device by means of an Ethernet cable. For the receiving instrument(s) the “external clock” setting in the user interface ziControl needs to be selected on the Connectivity tab.

2.  10 MHz clock reference for third-party instruments

The 10 MHz reference clock signal is available at the RJ45 connector of the ZSync output of the HF2. Port 1 is the clock signal, port 2 can be used as ground reference (see manual Ch 6.3). To access the signal, it is necessary to solder a RJ45 connector to a BNC cable as shown in the figure. The unshielded part between connector and cable should be kept short to prevent bad signal integrity. The output signal is a 3.3 Vpp TTL signal. A 20Ω series resistance may be added to match the 50 Ω impedance of the BNC cable. This signal can now serve as a 10 MHz clock reference for all devices using AC coupling on the input, which is the case for most instruments in the market.

With 50 Ω termination, the resulting signal level is  approximately 0.8 Vpp or 2 dBm.

Both options, with and without 50 Ω termination, were successfully tested with an Agilent 33250A arbitrary waveform generator.



Remark: It is worth mentioning that the main signal output can also provide a 10 MHz signal. If one of the signal outputs is not used in an experiment, it is possible to set the frequency of this output to 10 MHz and use it as a clock reference for another device.

3. Referencing an HF2 Instrument to an external 10 MHz reference

As described in the HF2 user manual, a 10 MHz TTL signal can be directly connected to the Clock In input on the rear panel of the device.

Often, a 10 MHz clock signal is a sine or square wave around 0V and not a TTL signal. In this case, there are two possibilities to convert this signal to the required input level.

  1. Use a comparator to convert the sine/square wave to a digital TTL/CMOS output.
  2. If the output amplitude of the 10 MHz reference is at least 1.6 Vpp (against high-Z, without 50 Ω load), then it is possible to use AC coupling and level-shifting. The corresponding schematic is shown in the figure below. Please note, that a 5 V supply is needed in order to adjust the common-mode level at the clock input to approximately 1.6 V. A 5 V signal from the DIO 0 connector can be used to provide this 5 V.


This setup was successfully used with an Agilent 33250A arbitrary waveform generator.

It is worth mentioning here, that the HF2LI uses an internal clock cleaner. Therefore, jitter on the 10 MHz signal will be filtered out internally. Important is, that the 10 MHz signal is stable and drift-free