Exploring the Possibilities of the INA Inline Amplifier

Following Meinberg's announcement of our new Kybernion initiative several weeks ago, it's time to go a little deeper into the possibilities afforded by the various products in the Kybernion product family - starting with Meinberg's new INA inline amplifiers. These compact devices provide a cost-effective and practical means of extending the transmission route of a GNSS signal to ensure reliable reception. INA amplifiers are patched directly into the antenna line and require no separate power supply - the connected Meinberg receiver provides the operating voltage required to increase the signal gain.

As devices in the Kybernion family, Meinberg's INA amplifiers have been designed from the ground up for optimum operation in GNSS-synchronized timing applications. This means that:

• they are engineered specifically to pass primarily known GNSS frequencies for all four of the main constellations and
• they have been engineered and tested to carry mbgARC communication signals without any disruption.

In addition to this, Meinberg's INA units are manufactured at Meinberg's factory in Germany for maximum security of supply and assured quality control.

When Do You Need an Inline Amplifier?

Without inline amplification, the signal from a GNMANTv2 antenna can be carried along up to 70 m (230 ft) of Speedfoam HFJ240 coaxial cable to the point where it no longer satisfies the input requirements (front-end gain specifications) of the receiver. For many applications, 70 m of cable may be enough, but for large scale installations in which large distances need to be covered, additional range may be required. Being able to extend the antenna route to lengths of several hundred meters also provides additional flexibility when it comes to positioning the antenna in the best possible location.

Enter the INA-20 and INA-30: these inline amplifiers take the GNSS signal from a GNMANTv2 antenna and boost the level enough for it to satisfy the gain requirements of a Meinberg GNS, GNM, or GXL type receiver. INA units can also be cascaded - meaning that they can be connected at intervals of around 70 m along an antenna line to provide signal gain increases where the signal level would diminish to unusable levels.

To this end, there are two variants of the INA: the INA-30, which provides 30 dB of gain, and the INA-20, which provides 20 dB. The selection of which of the two amplifiers to use in each segment will depend on the signal level to be expected at the end of each cable segment.

Illustration: An example of a 200 m (650 ft) antenna line enabled by INA inline amplifiers

In the example above, the antenna is connected directly by a 60 m length of HFJ240 cable to the first INA-30 amplifier, and then by two 70 m lengths of HFJ240 cable separated by an INA-20 amplifier. The arrangement of the amplifiers and cable lengths generally have no impact on the gain at the receiver end of the antenna line - you could also have the 70 m cable as the first segment and place the 60 m segment at the end. In either scenario (70 m → 70 m → 60 m or 60 m → 70 m → 70 m), you would have a gain of around 21 dB at the receiver end, assuming the use of a GNMANTv2, which outputs 37 dB.

However, because the first segments in a line typically have the greatest impact on signal-to-noise ratio, it is advisable to keep the first segment somewhat shorter in order to reduce the noise figure of the cable length somewhat and improve the overall signal-to-noise ratio slightly.

This approach, however, results in a rather high overall gain at the output of the third inline amplifier, which is why a cable measuring a full 70 m is connected between the final INA-20 and the receiver - to ensure that the signal is attenuated enough to prevent the receiver from being overdriven.

If local installation conditions dictate that individual segments must be shorter than 70 m or if the final cable segment leading to the receiver is too short and the input signal exceeds the specified front-end gain, it may be necessary to use individual INA-20 amplifiers at certain other points in the chain to accommodate this. Meinberg's Technical Support team can assist in planning the antenna route in this case.

How Do Inline Amplifiers Affect Timing Accuracy?

Typically, we assume that each meter of HFJ240 cable introduces a consistent 4.01 ns of delay into the antenna line. We compensate for this delay at the time server end by multiplying this value by the length of the antenna cable and having the server apply the result as an offset to the received time. Accordingly, we would have an offset of 280.7 ns applied for an antenna line (without amplifiers) that is 70 m in length.

For timing applications that require nanosecond accuracy, it is helpful to consider that each amplifier adds between 1.9 and 2.6 ns of propagation delay from antenna to receiver. By way of example, we will consider the well-established 1.5 GHz bands (GPS L1, Galileo E1, BeiDou B1, GLONASS G1), which incur 1.94 ns of delay.

If only one or two amplifiers are integrated, the impact on timing relative to the accuracy of the overall timing infrastructure is typically negligible, even in applications that require nanosecond-level accuracy. With several amplifiers patched in, however, the effect is cumulative, and in this case it can be beneficial to accommodate the timing delay introduced by the amplifiers into the delay offset of your time server.

If we assume, as in the example above, that we have an antenna line measuring 200 m, comprising a 60 m cable segment followed by a INA-30 amplifier and two 70 m segments separated by an INA-20 amplifier, the basic propagation delay of the cable is 802 ns, but the two INA units each introduce 1.94 ns of delay, resulting in a total delay of 805.88 ns, assuming that no other splitters or devices are patched in. While the impact of the amplifiers appears negligible in this context, this small step can help to slightly improve synchronization with other timing-critical devices in the network.

For timing applications that require no more than microsecond accuracy, the impact of the amplifiers on timing accuracy can be disregarded.

Which Additional Factors Need to be Considered?

It is important to ensure that the 70 m threshold is not exceeded between cable segments. If an INA is patched into a route too far along a cable segment, the signal output from the amplifier will have the requisite gain, but the signal-to-noise ratio will likely have degraded to the extent that the cable is only transmitting boosted noise from this point.

Also, each amplifier introduces its own noise into the signal, typically degrading the signal-to-noise ratio with each amplification stage. This limits the number of amplifiers that can be cascaded on a single antenna line.

Finally, there is also the question of current draw - Meinberg GNS receivers typically provide a maximum of 120 mA and 5 V and the GNMANTv2 draws 30 mA, leaving 90 mA for additional devices to be patched into the cable without an external power supply. With each INA unit drawing 8 mA, this places an absolute limit on the number of inline amplifiers to eleven (88 mA), regardless of signal-to-noise ratio issues.

Where Can I Get Advice?

Meinberg's Technical Support team is on hand to provide expert advice on how to cascade the inline amplifiers in your antenna installation, including assisting with the calculation of suitable cable lengths and the appropriate amplifier to use at each point in the route.

Meinberg's INA inline amplifiers are the perfect solution for ensuring reliable signal transmission over longer routes to your Meinberg GNSS-synchronized reference clock and all for providing greater flexibility in the positioning of your antenna for optimum reception.