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Bi-Directional Amplifier FAQs


What is the P1dBm Compression Point and why does it matter?

Every amplifier has gain and a compression point. The TX gain on an amplifier is a measure of the amount of amplification and is measured in (dB). This gain holds true for RF input power up to a certain point. By increasing the input power more and more the amplifier starts to become non-linear and compress, and the amplifier starts to actually lose gain. When the input RF power is high enough to cause the TX gain to drop by 1dB, this is the point called the P1dBm compression point. This is also where an amplifier is typically rated for maximum usable power. When selecting a WLAN amplifier one should always consider the P1dBm compression point of the amplifier; it's a measure of how much horse power is under the hood.

When using any type of fixed gain WLAN amplifier one needs to pay close attention to not exceed the rated P1dBm point. Compressing amplifier beyond the P1dBm point can potentially decrease the operational life and damage the device. In addition overdriving and compressing an amplifier well beyond its P1dBm point will cause unwanted distortion products and harmful interference to both in-band and out-of-band signals. GEM Microwave offers both fixed-gain and AGC style amplifiers, whereby they automatically adjust their TX gain to maintain linearity and thus retain a fixed average RF power output at the Antenna Port for a wide RF input range. GEM Microwave Amplifiers are rated at both an Average and Peak value, and thus work optimally with OFDM modulations.

Advertised 802.11g / 802.11a output power and how do I compare from one manufacturer to another?

By definition the IEEE 802.11g / 802.11a standard uses OFDM signal modulation. This type of modulation called OFDM requires addition headroom or Peak Envelope Power (PEP) for field usable output power. Many other manufacturers are advertising only the peak power of their 802.11g / 802.11a amplifiers. This means that your field usable or average RF output power could be significantly much less than advertised. It also means you don’t know what the Peak to Average power rating is and this could have a dramatic impact on data throughput and range.

We advertise and rate our amplifiers for both average and peak power, we have nothing to hide from the customer and believe they should know what performance they paid for. In addition our advertised peak power is rated at the amplifiers P1dBm point and many other manufacturers are rating their amplifiers at saturated power sometimes at it’s P3dB point. Thus, a competitors amplifier might say “saturated TX power= 2W (33dBm)” and not give the specifics of compression point or average usable power (dBm). If RF Power is measured at the P3dB point it’s about 2dB less or in this chase 1.25W(31dBm). This is false advertising, don’t be fooled ask for both the amplifiers P1dBm point and Average Power.

Should I mount the amplifier at the tower top or tower bottom with the radio?

This is an often asked question and there are several correct answers. In a typical cellular telephone industry active RF equipment is mounted at the base to provide ease of access to components like amplifiers and radios. However it requires very expensive low loss RF cable to preserve RF performance. In most WiFi applications the cost of developing a base mounted system is sometimes cost prohibitive due to high RF losses that’s where a split system where the radio is at the base and the amplifier is at top of the tower close to the antenna is the favored approach.

A split system uses a radio and DC Injector, located at the tower bottom, that injects DC power and Radio RF energy onto a single coax cable. The other end of the coax, located at the tower top, consists of an amplifier and antenna. The benefit of this system is that it allows one to use a cost effective indoor commercial radio and yield the high performance of an outdoor solution. In this case a weatherproof amplifier sets the critical RF performance at the antenna (Low Noise Figure in RX and maximum boosted TX power). It is at the antenna where you want to preserve these important system parameters.

The traditional cellular approach limits all outdoor components to passive elements (non powered devices). This is what cellular companies do, yet it requires very expensive low loss coax to be used from the amplifier to the antenna. Every dB of loss associated with this cable is directly added to your critical RX Noise Figure and robbing your TX power. We recommend no more than 3dB of loss in these applications, since 3dB is 1/2 your TX power and equates to about 25% reduction in your overall system range.

How is my radio’s range affected by TX output Power.

A radios operating distance is determined by many factors, frequency, path loss, NF, Fresnel effects, cable losses, and antenna gain; all of these parameters effect the overall system link margin and S/N Signal to Noise. In establishing a good system link margin one must overcome all these RF losses and arrive at a desired S/N ratio. Raising a systems TX power in a spectrally clean fashion can overcome these RF path losses. As a rule of thumb every 6dB increase of TX power doubles one’s free space operating distance.

Another method to increase one’s operating range is to provide a low system receive NF followed by an appropriate amount of RX gain to preserve the NF and present a clean signal to the radio receiver. A radios receive performance is rated by many factors, but typically the most important being RX sensitivity. Using an amplifier mounted close to the antenna with a low NF will provide a spectrally clean signal to the receiver.

If your system RF performance (TX Power and RX NF) is being robbed by high cable losses from your radio to antenna you’re getting hammered with a double whammy hit from a link margin standpoint. Improve you link margin significantly by inserting an amplifier mounted near the antenna to provide spectrally clean TX output boost and highly selective RX gain.

What does Automatic Gain Control (AGC) really do and why should I use it?

Automatic Gain Control or AGC as it is commonly called offers a plug-and-play type functionality. The AGC feature of the amplifier will adjust the gain up or down so that it always maintains a steady average RF output power equal to it's rating regardless of changes in the input power or cable loss between the radio device and the amplifier. Please note for the AGC to function properly you must have an RF input signal at the amplifier between +2dBm to +23dBm to switch from receive to transmit mode.

Example: If you use a standard 32mW (+15dBm) radio device you have +15dBm of Radio signal strength. If you have 7dB of cable loss (equal to 17 m of RF240) between the tower bottom mounted radio and the tower top mounted +30dBm (1W) Amplifier you would be providing the amplifier with about 8dBm of signal. This is more than is required to switch the amplifier from RX to TX mode. The amplifier will sense the signal strength of +8dBm and then add +22dBm to it to bring the signal power output to +30dBm or 1W as rated at the antenna port of the amplifier.

You should use an AGC amplifier when you want the ease of designing your system power level or Effective Isotropic Radiated Power (EIRP) based upon the constant AGC controlled output. You may also find that having an AGC amplifier offers a nice standardization where you can mix and match radios and cable types / losses and still use the same amplifier from installation to installation. This allows you to keep a spare unit that will work as a drop in replacement for all or many of your systems depending on the antenna used and the EIRP expected.

When you say the unit has an average power limit and a peak power limit when using OFDM what does that mean?

OFDM (Orthogonal Frequency Division Multiplexing) is the type of modulation used in 802.11g and 802.11a devices. OFDM modulation with its multiple carriers requires a high degree of Peak to Average RF power to perform properly. This wide power range is for the multiple carrier bursts and often described as headroom and measured as Peak Envelope Power (PEP).

When using OFDM modulation we recommend a 4 to 6dB back-off from the amplifiers P1dBm point. This will provide the additional headroom for the OFDM modulation and yield maximum data throughput.

Does it matter what radio power I use with a fixed gain amplifier?

The radio power, as well as the cable loss between the radio and the amplifier, is critical when using a fixed gain amplifier. You must be aware of and consider the RF radio power, pigtail losses and amplifier TX Gain before selecting a fixed gain amplifier. If you provide the fixed gain amplifier with too much power you will overdrive it potentially damaging the unit and creating system performance issues. Too little power and you will not obtain the intended results. Our AGC amplifiers handle this in the background by reading the radio input power and adjusting the amount of TX gain applied to ensure that the rated average RF output is reached with a wide RF input range.

Can you change an amplifier from AGC to Fixed Gain?

It is possible to change anyone of our AGC amplifiers to Fixed Gain. We simply disable the AGC circuit and tune the amplifier to a customer’s custom specifications. Caution should always be used when adjusting an amplifiers TX gain, since it’s easily to compress an amplifier beyond it’s P1dBm compression point.