FAQ – RF Amplifiers

Frequently Asked Questions

For any questions regarding our RF Broadband Amplifiers, you are always most welcome to contact us. In addition, you might already find an helpful answer by clicking one of the questions below.

Is the heat-sink included?

Whether an amplifier is provided with or without a heat-sink depends on the power consumptions of the device.  If no heat-sink is included, the amplifier is safe to be operated at room temperature in a typical laboratory environment. Amplifiers requiring a heat sink (e.g. the SHF 810) will have one attached while amplifiers with less power dissipation which do not require a heat sink (e.g. the SHF M827 A) come along without it.

The heat-sink of an SHF amplifier can easily be removed by the customer. However, in that case additional cooling measures will be required. For example, the amplifier can be mounted onto a common metal plate with other modules, to provide the required cooling. If you find a SHF amplifier in your shelf, please assure that nobody else removed the heat-sink.

Can I amplify very low signal amplitudes?

Yes. However, the minimum signal level has to be above the equivalent input noise floor.

Every signal within the operating frequency range and below the input saturation voltage will be amplified with the gain specified in the data sheet.  There is no such thing as an input threshold or similar.

For low signal amplitudes, however, another factor has to be considered: Noise.

The noise power Pnoise at a temperature T on every RF channel with a bandwidth of ∆f can be calculated with help of the Boltzmann constant k by:

Pnoise@input = k · T · Δf

This means, the noise power in dBm at room temperature (T=300 K) can be approximated by:

Pnoise@input = -174 + 10 log(Δf)

This is the noise at the input of the amplifier. This noise (as well as the signal) will be amplified by the amplifiers gain G. Further, the amplifier adds noise (this is given by the noise figure NF):

Pnoise@output = -174 + 10 log(Δf) + G + NF

To properly use the amplifier, the signal power at the output Psignal@output must be higher than the noise power Pnoise@output :

Psignal@output > Pnoise@output
 Psignal@input + G >  -174 + 10 log(Δf) + G + NF
 Psignal@input
>  -174 + 10 log(Δf) + NF

 

Let’s assume a typical SHF amplifier with ∆f= 50 GHz and NF= 6 dB. For this amplifier one would need to provide a signal of more than -61 dBm (0.6 mV). Everything lower will vanish in the noise.

Typical noise figure of SHF families of amplifiers is approximately 5 dB at mid-band, unless otherwise specified.

How hard can I drive an amplifier to maintain amplitude linearity?

The gain given by the data sheet of the SHF amplifier is only valid for the linear region of the amplifier. Above certain input power levels the amplifier gets into compression and the gain POut/PIn is actually less than for input signals with lower power level.

To quantify this amplitude non-linearity, SHF specifies the compression points. The ‘x dB compression point’ is the output power at which the output level is x dB less than of an ideal (linear) amplifier.

Practically, the 3 dB compression point is very close to output saturation. In other words, the output power of the amplifier will not get higher even if the input power is further increased.

gain of an amplifier

For binary signal, there is no problem driving the amplifier into the non-linear, saturated regime. The output signal simply gets ‘clipped’. In fact, sometimes this effect is beneficial as it can reduce the over- and under-shoot of the signal, resulting in a more rectangular output eye and improved rise and fall times.

For analog signals, however, amplitude non-linearity degrades the signal quality. In such case the amplifier has to be driven within its linear region.

For PAM signals, for example, driving the amplifier into compression will result in a reduced eye height for the outer eyes. If this is not wanted, the amplifier has to be driven within the linear region. As a rule of thumb, one should stay below the 1dB compression point P1dB.

small signal input

small signal input

output smaller P1dB

output < P1dB

larger signal input

larger signal input

output larger P1dB

output > P1dB (outer eyes compressed)

 

If higher output amplitude level is required, it is possible to pre-compensate for the amplifiers non-linearity.  If, for example, the PAM signal is generated by an SHF DAC, it is possible to provide a signal with increased outer eye openings (pre-distortion), so the PAM signal at the output of the amplifiers, or even the E/O converters, is perfectly equal shaped.

Larger signal input pre- distorted

larger signal input pre- distorted

Symmetrical Output with amplitude

symmetrical output with amplitude > P1dB

What does the gain control function of SHF amplifiers actually do?

Each SHF amplifier has a gain control function to reduce the gain continuously by up to 3 dB. This is intended to be used for fine tuning, as external attenuators in the range of 1 dB to 2 dB are not available with good performance at high frequencies. For attenuation ≥ 3 dB an additional external attenuator would be the recommended choice.

The gain control is not to be mixed up with the output power control available for some amplifier modules. The gain control reduces the gain, i.e. a small signal gets amplified less. The output power control lowers the saturation level, i.e. a small signal gets amplified with maximum gain while larger signals get compressed earlier.

s807b_gui

GUI of the SHF S807 B control

The SHF “S” and “D” series amplifiers (all parts starting with S or D in the product number) feature a software control to set the gain, output power and crossing. All other SHF amplifiers have a dedicated pin for the gain control. If this pin is left floating, the amplifier provides maximum gain. In case a negative voltage is applied, the gain is reduced.

 

What is the option MP (matched pair)? Can I use two amplifiers for a differential signal?

For all SHF amplifiers we offer the option matched pair (MP).

When choosing this option the two amplifiers of the pair will be matched to provide identical gain, output amplitude and propagation delay within the tolerance range. As a minimum, we guarantee the difference between two amplifiers to be smaller than shown in the table below:

10/20G amplifiers  40G amplifiers
Gain Difference ≤ 1 dB ≤ 1 dB
Propagation Delay Difference ≤ 5 ps ≤ 3 ps
Output Amplitude Difference ≤ 0.5 V ≤ 0.5 V

This is used very often, for example to transmit differential signals, but also for other applications where two signals have to drive a DUT synchronously (e.g. when driving an I/Q modulator).