Using Penelope with a linear amplifier

When using Penelope to drive a linear amplifier it is important to ensure that you use enough drive from PowerSDR, K.I.S.S Konsole etc. to maximize the output Signal to Noise (S/N) ratio.

The Digital to Analog Converter (DAC) used on the Penelope board is a 14 bit device. Hence the maximum theoretical S/N ratio is 84dB and occurs when the board is being driven to its maximum output of approximately 500mW.

If the user interfaces Penelope to a linear amplifier that requires much less than this level of drive then it is advisable to place an attenuator between Penelope and the amplifier rather than reduce the drive level from the PC.

The reason for this is as follows. As the drive level from the PC is reduced the peak power output from Penelope will be reduced in direct proportion. In effect, as drive is reduced the effective number of bits of the DAC is reduced.

However, the noise level from Penelope is independent of drive level, hence the output S/N ratio drops.

In an extreme case, consider if Penelope were to be used to drive a VHF transverter where only say 2mW of drive were needed. If the PC drive were to be reduced to produce this level of output from Penelope then in effect only 6 of the available 14 DAC bits would be used.

This would result in a S/N ratio of 36dB – outside the requirements of the FCC Part 97, subpart D.

Should the user wish to drive a linear amplifier that has a drive requirement of less then approximately 200mW then an attenuator should be used between Penelope and the amplifier to enable Penelope to operate at this (peak) power level.