Four cartoony telescope antennas with a clock symbol below the dish. In the middle between them is a large clock that is connected to the four antennas with red wire.

TFR – Time and Frequency Reference

Distributing time and frequency over long distances while providing an option that compensates for offsets.

High precision timing is crucial in radio-astronomical applications. Both the absolute time and, even more importantly, an accurate reference clock are essential, for instance when detecting and long-term analyzing the fast-rotating neutron stars called pulsars. GPS receivers are used as a source for absolute time, while the reference clock is usually generated by a hydrogen maser. Distributing these signals to the receiver systems, in particular with array-based telescopes, poses a major challenge. The synchronization of the single dishes is essential to correlating their individual data-streams.

Figure 1: This block-diagram shows the general concept for a TFR system. The individual components are discussed in more detail in the subsequent paragraphs. — **Figure 1:** This block-diagram shows the general concept for a TFR system. The individual components are discussed in more detail in the subsequent paragraphs.

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**Figure 1:** This block-diagram shows the general concept for a TFR system. The individual components are discussed in more detail in the subsequent paragraphs.

© MPIfR

That is where a Time and Frequency Reference system comes into play. The purpose of this system is not only to distribute time and frequency over long distances but also to provide an option that compensates for offsets caused by different cable lengths to the individual antennas.

TFR-Generator

The TFR-Generator is a unit that generates and distributes high precision reference signals (time and reference) over fiber optical cables. A 100 MHz input signal (usually derived from a hydrogen maser) is used to generate a GHz-carrier tone (typically around 1.75 GHz) on which the time input signal is modulated. Both signals (reference and modulated time) are then distributed via optical fiber .

A TFR-Generator comes as a 19 inch slide-in (see Figure 2) carrying a TFR-Control and up to 9 TFR-TX units (see Figure 3). The purpose of the TFR-Control is to receive the reference signals and to perform all kinds of control and monitoring tasks for the TFR-Generator. The TFR-TX units modulate the time signal on a carrier tone to finally convert and distribute both signals (time and reference) fiber-optically. Each TFR-TX unit provides 8 output pairs resulting in a total of 72 outputs for the whole TFR-Generator.

A photograph of a TFR-Generator as a 19 inch slide-in. It is a rectangular metal box with a number of connectors that are slotted into the front. Each slot says TFR Generator on it. The rightmost slot has a number of ports including usb and hdmi. At the front, on the left and right, there are two grips allowing it to be maneuevered. A fan duct runs below the main object. — **Figure 2:** TFR-Generator as a 19 inch slide-in. It carries a TFR-Control unit (rightmost slot) plus 9 TFR-TX units. Attached to the 3U height main part are a fan array (lowermost 2U units) and an air deflector (uppermost 1U unit).

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**Figure 2:** TFR-Generator as a 19 inch slide-in. It carries a TFR-Control unit (rightmost slot) plus 9 TFR-TX units. Attached to the 3U height main part are a fan array (lowermost 2U units) and an air deflector (uppermost 1U unit).

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A board with a small fan mounted on top of it. In the front, there is a connector. From the board, yellow wires run towards a row of plugs in the back.The wires are bundled together by zip ties. — **Figure** 3: TFR-TX unit providing 8 pairs of fiber-optical time and reference signals on the front panel. Below the heat-sink in the board center, the actual laser-diodes are placed inside a thermally stabilized aluminum block.

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**Figure** 3: TFR-TX unit providing 8 pairs of fiber-optical time and reference signals on the front panel. Below the heat-sink in the board center, the actual laser-diodes are placed inside a thermally stabilized aluminum block.

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TFR-Receiver

An aluminum case with two plugs connecting to the front on the left and right. Yellow wires run from the plugs towards a board in the center. — **Figure** 4: TFR-RX unit providing two fiber optical inputs (time and reference) and three SMA outputs (differential reference and single ended time signals). Since it is located close to the astronomical receiving system, the PCB is placed inside an aluminum housing, blocking RFI emitted by the electronic high-speed circuits.

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**Figure** 4: TFR-RX unit providing two fiber optical inputs (time and reference) and three SMA outputs (differential reference and single ended time signals). Since it is located close to the astronomical receiving system, the PCB is placed inside an aluminum housing, blocking RFI emitted by the electronic high-speed circuits.

© MPIfR

A TFR-Receiver (see Figure 4) is the unit that receives the time and frequency signals fiber-optically and converts them back to a copper-based digital signal. The optical power of the received fiber signals is detected and becomes available as an analog voltage signal. In addition, the TFR-Receiver contains a fiber-retro-reflector. It reflects a small amount of the received time signal back to the sender, which makes round-trip measurements possible.

Round-Trip Measurements

The block diagram below (see Figure 5) shows the basic concept of the round trip system. The idea behind this setup is to determine the propagation-delay that is created when a signal is distributed fiber-optically over a distance d. Therefore a standard TFR-TX unit is used to send the time signal to the TFR-RX 1 unit (usually located inside the astronomical receiver). The light returned by the retro-reflector inside is forwarded to TFR-RX 2 by an optical circulator. TFR-RX 3 directly receives the optical time signal from the TFR-TX unit and serves as a reference. The two signals are then compared by a time-interval-counter, where the calculated offset between the two signals represents twice the propagation-delay for distance d.