Baseband vector signal analyzer hardware design - HP 89410A electronic test device - Technical

Hewlett-Packard Journal, Dec, 1993 by Manfred Bartz, Keith A. Bayern, Joseph R. Diederichs, David F. Kelley

The source follower configuration is susceptible to oscillations if driven from a reactive source impedance. Gatestopper resistors (small-value resistors placed right at the FET gate) ensure that the stage is stable for any source impedance presented at the input. 196 ohms was chosen because models showed it absolutely guaranteed stability while negligibly affecting the overall noise figure for the input, which is dominated by the preamplifier.

Preamplifier

Wide-bandwidth current-feedback operational amplifiers are used for all of the remaining signal amplification tasks in the input circuit (see Fig. 3). These very high-speed, high-fidelity devices allow amplification stages to be built with very wide bandwidth, relatively independent of the stage gain, because of their current-feedback topology.

This gain-bandwidth independence is exploited in the variable-gain preamp stage, which can be set to gains of 13.4 dB or 3.4 dB, thus giving the functionality of a 10-dB pad in the circuit for ranging. For the small input ranges, it is important to achieve as much signal gain in this stage as possible to have the best noise performance. The 13.4-dB gain setting is used, providing about a 17-dB noise figure for the stage. This dominates the sensitivity performance of the input. Large gain leads to a quiet preamp stage because the dominant noise mechanism for these amplifiers is inverting input current noise, which generates an output noise for the stage that is independent of gain setting. Referred back to the input, this fixed output noise becomes a smaller equivalent input noise with larger gain.

Signal Return Ground

A special signal return ground is used for the input cable through the preamp stages. This ground is connected back to the main chassis ground at only one point: the point where the input cable shield comes onboard. This is required for reasons similar to those prompting the input connectors to be dc isolated at the front panel, that is, currents generated by other mechanisms in the instrument flow across the board ground planes in this area, generating small but significant spurious voltage drops that cannot be allowed to add to the input signal. Bringing all connections to the signal return ground back to a single chassis ground connection point prevents other currents from flowing across the signal return ground, generating spurious voltages.

After the preamp, the input signal is large enough not to be adversely effected by spurious ground currents, and the board ground planes are used for signal return. The inclusion of this separate signal return ground results in approximately a 10-dB reduction in the level of switching power supply ,'elated spurious signals, adding enough specification margin to guarantee producibility.

DC Offset

A 12-bit digital-to-analog converter sums dc into the dc offset stage, which is a high-speed current-feedback operational amplifier. This is done under control of the instrument CPU and autozero software routines, and compensates for dc offset all the way to the analog-to-digital converter. Care is taken not to introduce noise from the digital side and to maintain frequency response flatness in the summing stage.