RF Over Fiber & Optical Delay Line | FAQ

FREQUENTLY ASKED QUESTIONS

What is RF Over Fiber (RFoF)?

RF over Fiber is a technology used to transmit Radio Frequency (RF) signals over long distances over Fiber.  It is also called RF over Glass, RF Optical Transceiver, and Coax Replacement solution. With RF over Fiber technology the RF signal is converted to an Optical Signal by the laser diode and converted again to an RF signal via a  photo diode.

When is RFoF used?

An RF signal which is received through an antenna needs to be processed by a receiver.  Typically, antennas are connected to nearby receivers using coax cables.  Even though coax cable weakens the strength of the RF signal, for shorter distances, RF power stays within the acceptable input power range of the receivers. In some applications using a coax cable becomes impractical or impossible.

Why switch to RFoF?

The decision to switch to RFoF is mainly based on RF link budget calculation.  Due to coax cable loss typically for distances longer than 150 ft,  RFoF is used and more if high frequency above 10 GHz is used. RFoF is also used where coax cable deployment is difficult or costly due to its size and weight or where the fiber already exists.

How do RFoF Solutions work?

RFoF solutions are made of 2 modules.  The Tx module takes in the RF signal and via a modulation and laser converts it to an optical signal which can have a wavelength of 1310 nanometers or 1550 nanometers depending on the laser type used.  At the far end, the Rx module takes in the optical signal and converts it back to RF signal which then can be connected to a Receiver. This is the direct modulation method.

Another method is the indirect modulation where the RF signal is modulated by an external Mach –Zender modulator and is then converted again RF a receiver. This method is used for high-frequency applications above 8 GHz and up to 40 GHz.

How is the budget of an RFoF link calculated?

In designing an RFoF link one needs to calculate the optical link budget as well as the RF link budget.  The optical link budget is used to calculate the loss introduced by the fiber cable.  Even though the fiber has substantially less loss than the coax cable, it still introduces signal degradation. Fiber cable typically introduces 0.25 dBo (dB optical loss) per kilometer (km).  There is a 1:2 ratio relationship between optical loss and RF loss.  0.25 dBo is 0.5 dB RF loss and the condition of the fiber cable. It could be a huge difference between the theoretical calculation and the real loss due to many parameters so it is recommended to measure the optical power end to end. This can be achieved easily in RFOPtic programmable RFoF series.

Once the optical link budget is calculated, now the end to end RF link budget has to be calculated.  For this, it becomes important to know the specs of the RFoF solution that will be used.  RFoF units based on the frequency range they support can introduce varying degrees of loss or gain.  RFOptic also offers RFoF solutions with nominal gain which means 0 dB S21 system gain.  The gain can be changed easily by using the Tx and Rx 30 dB RF attenuator so in a case of low loss adding gain is done easily and the link budget can be adjusted to the required value.

Example:  When using an RFoF solution that has a nominal gain (S21=0dB) if the input signal, S1, is -10 dBm, then the output signal, S2, will still be -10 dBm.   In this scenario if the Tx and Rx modules are connected with a 10 km single mode fiber, the fiber will introduce 10 x 0.5 dB = 5 dB loss, S2 will be -15 dBm.  The system gain, S21, will be -5 dB.

When calculating link budget, one should also consider if there are any fiber patch panels and/or connectors on the fiber line.  Typically, each connector will introduce about 0.5 dB loss.

How to adjust the System Gain (loss)

The KPI for System gain is S21 and is measured in dB.   It denotes the dB level difference between the output and input of the system.  If the input signal is -10 dBm and the output signal is -5dBm, then the S21 is 5 dB. Even when there is a loss, it is called system gain but noted as negative.  So, for example, if the input signal is -15 dBm and the output signal is -25 dBm, the system Gain is – 10 dB.  Typically the gain of the programmable RFoF is 10dB and 40 dB with LNA and 15 dB without LNA. RFOptic offers units with adjustable system gain.  Through a GUI users can turn on the LNA in the Tx unit and adjust the attenuator to get to the system gain levels needed to achieve a desirable link budget.

Why is RFoF System Gain Flatness important?

RFoF system gain depends on the RF frequency signal.  If the RFoF system gain doesn’t change much for the whole bandwidth the unit supports, it is said to have a flat response.   System gain is important for deployments where the input signal frequency changes.  RFOptic units are very flat meaning there is not much variation in system gain.  For example our 3 Ghz RFoF solution has a flatness of ±1.5 dB between 0.5MHz to 3 GHz and ±2.5dB between 0.5MHz to 6GHz.

Can the P1dB Compression Point be adjusted?

This parameter is especially important to determine the max linear RF input power into an RFoF solution.   A signal that is stronger than the P1db point of the RFoF system will be compressed and distorted.  This will cause the RFoF system not to be able to produce the same RF signal at the far end.

RFOptic provides an option to its customers to adjust the P1dB compression point to fit their needs,  In the GUI configurable units P1dB point can be adjusted by modifying the S21 gain levels.  In higher frequency units (8 Ghz – 20 Ghz) RFOptic can still adjust the parameter by incorporating a pre and/or post LNA (low noise amplifier) based on the customer needs.

How can I measure the RF link without connection to the input before installation?

One of the challenges of installers is to know that the RFoF link is that the optical and RF functions are in order. This imposes to bring RF measurement tools such as a signal generator and spectrum analyzer and optical power meter. RFOptic solves this issue of fast diagnostic by adding an injection of the pilot signal from the Tx to the Rx. The installer can check the functioning of the transmitter alone, the receiver alone and the link (transmitter and receiver) by injecting 15MHz pilot signal and measuring the output signal in comparison to the input. This of course not accurate like RF measurements tools but it ideal for field installation without the need to bring heavy and expensive equipment.

What is Optical Delay Line - ODL?

The ODL is an electric-optic-electric instrument. It performs fixed time delay(s), between a few nanoseconds up to several hundred microseconds, for RF signals from 0.1 up to 20 GHz and more (there are low-frequency ODL versions 0.1-5 GHz, and high-frequency ODLs versions: up to 8GHz, 15GHz, 18 GHz and 20 GHz. The RF input signal is converted into an optical modulated signal. The optical signal is transmitted into a long single mode fiber, usually at a 1.55-micron wavelength or similar. Passing the fiber, the optical signal is converted back into an electrical RF signal. The electrical control on the ODL elect optical system is done automatically, with no need for tuning by the operator.

An Optical delay lines system (ODL), incorporates high-performance lasers such as DFBs, optical modulators for high operation frequencies, photodiodes, and optionally other components such as optical dispersion compensators, optical switches, optical amplifiers and Pre and Post RF amplifiers, to provide exceptionally high performance. The ODL optical system supports very high bandwidths of analog signals, high sensitivity with wide dynamic range, for various delays.

What is Light coefficient?

Light coefficient refers to light traveling at a different velocity in the glass fiber and its index of refraction is 1.5. That means that 10 Km range in the air is equivalent to 6.666 Km in fiber and vise verse. Usually, RFOPtic requests data from the user to be provided in microseconds from the fiber so it will easy to prepare the spools.

Can an Optical Delay Line (ODL) support more than 1 Delay Line?

Yes, RFOptic standard Optical Delay Lines can support up to 8 distinct delay lines.

RFOPtic has a range of products that support more than one delay line and also the customer can extend it later by adding additional spools. Eight (8) delays lines that can create 256 combinations of delay lines are standard.

(ODL) Optical Delay Line Investment Protection

As the most expensive portion of an Optical Delay Line (ODL) is the transceiver, it is important to design an ODL that captures the customer needs in the future and/or with different applications.  Therefore, if the customer is not sure of a number of delay lines that may be required, we provide specific ODL’s that allow for the possibility to add more delay lines as needed in the future.

What is a Progressive  Optical Delay Line?

Our Progressive ODL’s can combine distinct delay line to create additional delay lines.  Progressive delay lines are used if the customer needs more than eight (8) delay lines.  Progressive Optical Delay Lines can be used to provide up to 255 delay lines.  One classic application is for phased radar arrays where on top of a base delay line, the delays have to be entered in equal steps.  For example, this solution would allow creating delay lines from 1 microsecond to 255 microseconds in 1-microsecond steps.

What are variable delay lines?

Variable delay lines are of considerable interest in a variety of applications including radar range simulation and signal processing. There are two basic techniques to consider; Switched RF and Switched Fiber.

-Switched RF uses multiple delay lines and RF switches to select various delay values. This technique has a good performance but is relatively expensive because multiple delay lines are required.

-A second approach is: Switched Fiber delay system which is more cost effective, it consists of  an ODL system with include several different delay lines, where two optical matrixes (e.g. 1:2, 1:4 or 1:8) that select (either manually or through PC) the desired delay line (i.e. DL 1 to DL 8) - see below in Figure 4 ODL system with up to 8 delays that can be selected by optical switches matrix. The disadvantage of this approach is that the switches are relatively slow, with switching time in the order of milliseconds.

ODL Block Diagram

ODL Block Diagram including two Optical switched for multiple delay lines.

  • A third approach for a variable delay system is an ODL system configuration which includes cascaded 1:2 and 2:2 optical matrixes with several different delay lines in between (replacing the above two optical switch matrix 1:8). The cascaded switch matrix - Progressive Delay Configuration which is shown in Figure 5 below, selects the desired combination of delay lines to define the desired delay. See below in Figure 3 a schematic picture of four progressive delay lines cascaded switch matrixes. With such configuration the user may select any of the 16 combinations of possible delay values (16=24): for example can select a Delay which is equivalent to  Dtot= D1+D2 +D4, or Dtot= D3+D4 etc.)

Progressive Delay Configuration

Figure 5: Progressive Delay Configuration consisting of four 2:2 optical switch, providing 16 different delay lengths.

Progressive ODL is also used when adding small increments on top of a base delay line.

What are the minimum and maximum delay amounts?

RFOptic created optical delay lines solutions that range from a few nanosecond up to 1000 nanosecond and more.

When  Dispersion Compensation needed?

As the signal frequency and the delay line length increase, the optical signal can be dispersed and weakened significantly. As needed we incorporate DCM (Dispersion Compensation Module) to its ODL solutions.

  1. Optical Dispersion of long fibers at high RF frequencies causes additional insertion loss at specific frequency range per defined delay line length/s, where the insertion loss deep can reach 20 dB and more. The optical dispersion loss can be eliminated by using an Optical dispersion unit connected to the long delay line to compensate the undesired dispersion loss (see the optical dispersion effect in Figure 7 below).

ODL System Gain

Figure 7: System Gain (S21) of a 100 μsec delay ODL system up to 20 GHz. The ‘deep’ around 15GHz is due to the ~20.7 km SM fiber dispersion effect at 1.55 mm wavelength. The dispersion effect can be eliminated by adding a DCM unit with negative dispersion.

ODL System Gain

Since ODL typically involved fairly long delay lines, the link budget calculation becomes important.  Based on customers need pre and post LNA are used.

Automatic Gain Control

When the customer requires in an ODL solution delay lines that are significantly different in length from each other, the output signal will also vary significantly.  While a 5 km delay line will introduce 2.5 dB RF loss, 100 km delay line will introduce 50 dB loss.  An AGC mechanism can be added to ensure that the output signal strength is steady regardless the delay line used. Gain control is used in very long and short delay lines in the same ODL so that the output signal will be the same.

How do I control the ODL?

The delay line can be controlled manually using the push button on the unit, or remotely through an RS-232 or Ethernet connection.

What is the delivery time of the purchased items?

Usually, all RFoF products are sold from stock so delivery time is about 2 weeks. An ODL which is a customized product can be less than 4 weeks depending on the complexity of the system.

We support customized solutions and are open for changes in the system architectures upon customer request.

What enclosures does RFOPtic provide?

RFOptic provides stand-alone solutions so the customer can install his RFoF link in his own enclosure.

In a case of specific requirements for outdoors,  RFOptic provides IP-65 enclosure for High-Frequency RFoF above 8 GHz. The solution incorporates metal robust enclosure together with N-type RF connectors, waterproof power and data connector and the optical connector. The system can carry 2 RFoF and can use one or 2 fibers utilizing the CWDM method. It has threads for wall installation or poll installation.  There is similar enclosures for programmable RFoF but much smaller in size.

In a case that the customer requests for multiple link solution, RFOptic can provide 1 U removable panel solution that supports up to 4 units (Tx, Rx or mix) or 8 units by 2 U removable panel solution. Both enclosures have double power supply and HUB to control the RFoF units through one or two ports.