For more than 35 years, SHF has been active in the design and production of broadband RF-microwave amplifier. The outstanding performance makes our amplifiers suitable for a wide variety of applications in R&D. This includes not only optical communications, but also satellite communications, high-speed pulse experiments, data transmission, quantum communication, radar and antenna measurements. Above you will find a selection of SHF broadband amplifiers, suitable for your 100G or 400G, 800G or 1.6T applications, fitting different drive requirements in complex signal generation based on Lithium Niobate or electro-absorption modulators. SHF amplifiers has been verified extensively for their excellent performance not only for NRZ but also with multi-level (e.g. for PAM or QAM) or analog signals (e.g. OFDM). In the increasingly important field of quantum technologies, SHF amplifiers are widely used to amplify pulses with extremely short rise and fall times. To transmit entangled photons, some commercially available electro-optical components of ‘classical optical communication’ are being utilized. For example, in quantum key distribution (QKD) experiments SHF amplifiers proved to be well suited as optical modulator drivers as in any other high-speed data communication scenarios.

• Amplifier Overview

• Tutorial Note 1 – Important RF and MW Parameters for Broadband Communication Phase response, group delay, S-parameters, VSWR, noise figure.

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 P_Out/P_In 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. 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

output < P1dB

     

larger signal input

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

symmetrical output with amplitude > P1dB

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.

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).

We do not offer TIAs.

A conventional TIA has an input impedance of 0 Ω, while our voltage amplifiers have an input impedance of 50 Ω.

However, SHF amplifiers can be used instead of TIAs.

An AC input current leads to an AC voltage drop across the 50 Ω input impedance. Therefore, the transimpedance of our amplifiers is 50 Ω multiplied by the voltage gain. If the signal source (e.g. a photodetector) requires a DC path to ground, an internal bias tee or DC-return can be used.

So, all our amplifiers work well with AC input currents. For example, the transimpedance of an amplifier with 20 dB gain (i.e. a linear voltage gain of 10) is

Z = 50 Ω x 10 = 500 Ω

While a conventional TIA has an input impedance of 0 Ω, our amplifiers have an input impedance of 50 Ω.

For basic operation of an SHF amplifier, no software is required. However, the SHF Control Center (SCC) can be used to set the gain, output power and crossing of an SHF D, F and S-Series amplifier.
Please note that older versions of the SHF D-Series amplifiers are to be operated with the “SHF 600 Series Control”.

This SHF software is free of charge for the lifetime of your device. The most current version can be downloaded here.

Apart from numerous modules from our High-Speed Data & Telecommunication Modules section the SHF 600 Series Control can be used to set the parameters of the SHF D836 A, SHF D836 B, SHF D837 A and SHF D837 B amplifiers. All other amplifiers are controlled by the SHF Amplifier Control.

The most current version can be downloaded here.