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:
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.
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.
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).