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, output power and crossing control of SHF amplifiers actually do?

Each SHF amplifier has a “gain control” function to continuously reduce the gain. Some SHF amplifiers offer further functionality to control the “output power” and the “crossing”. The SHF “S” and “D” series amplifiers (all parts starting with S or D in the product number) even feature a software control for these settings. All other SHF amplifiers have dedicated pin(s) for these controls.

s807b_gui

In case you do not use the control i.e. by not using the pin (leave it floating) or by not modifying the factory settings with a computer, the amplifier always provides maximum gain, maximum output power and ideal crossing.

In order not to mix up these three settings, a little explanation might be helpful:

 

Gain

A gain set to maximum results in the maximum available gain from the amplifier. In other words the amplifier will have the gain stipulated in the data sheet and on the label.

By using the gain control function one can 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 does only provide reduction of the output signal swing in case the amplifier is operated with small signal inputs. In other words, if your amplifier is clipping the signal because it is driven into saturation, there might be limited result from using the gain control as the maximum output power the amplifier can provide is not affected by this setting.

 

Output Power

In case the amplifier is driven into or towards saturation and the output signal swing shall be reduced the output power control shall be used. The output power control lowers the saturation levels (e.g. the 1dB, 2dB and 3dB compression points).  This means, even with the output power reduced as much as possible a small signal gets amplified with maximum gain and is not reduced while larger signals get compressed earlier.

Driving the amplifier into saturation by reducing the output power can have a positive effect on binary signals. As the signal gets clipped on the top line the NRZ signal gets more rectangular. In fact, it may be that the output signal shows faster rise/fall times than the input. Further, unwanted ringing on the top line may be reduced.

If most linear amplification is required, on the other hand, it might be best not to reduce the output power as the linearity suffers from potential saturation effects (please be referred to above FAQ “How hard can I drive an amplifier to maintain amplitude linearity?”). On a PAM signal, for example, one will see a reduced eye opening of the top and bottom eye while the inner eye remains open.

Only with output power set to maximum the amplifier will have the output power as stipulated in the data sheet and on the label.

 

Crossing

All SHF amplifiers are AC coupled at the in- and output.  As a result, in case the crossing control is not used, the output signal swings symmetrically around the 0 V line.  By using the crossing control the operating point of the output stage transistor is modified in a way that the output signal swing is shifted either up (i.e. it swings around a positive value) or down (i.e. it swings around a negative value).

On a NRZ eye diagram one simply sees the crossing (i.e. the point where the rising and the falling edge are crossing each other) modified.

For most applications a 50% crossing is considered as ideal and thus all amplifiers are tuned in a way that if the crossing control is not used they provide 50% crossing. However, in cases where the DUT driven by the amplifier inadvertently shifts the crossing, the control can be used to pre-compensate so that optimum 50% crossing is achieved at the output of the DUT not at the output of the amplifier.

For PAM signals, the crossing control can be used to reduce the eye height of either the top or the lower eye(s).  Again, this can be very helpful to compensate for non-linarites of another part (e.g. a laser).

 

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).