Tektronix P7313SMA, Техническое описание

Страница: 16 / 62

Operating Basics
Matched-Delay Cables
The standard delay-matched cables for the P7313SMA differential probe have been carefully designed to provide guaranteed
probe performance at the SMA connector interface on the end of the cable. The delay between the two matched cables in
the standard cable assembly is adjusted to provide an initial skew of less than 1 ps. Cable skew this small can be degraded
by cable
fl exure and through other environmental factors. Care should be taken to minimize physical mishandling of this
quality cable assembly to preserve probe performance.
The cable used in the standard cable assembly has also been selected for its low-loss characteristics, and the cable length
was selected to match the cable loss compensation designed into the probe differential ampli
fi er. If an alternative cable
assembly is used in measurements with the P7313SMA differential probe ampli fi er, this loss compensation characteristic
must be considered. The following approximate equation for cable loss compensation can be used as a guideline in custom
cable designs and is valid over a frequency range of about 1 GHz to 8 GHz:
Loss = –[0.5 dB + 0.15 dB * (F ⁄ 1.25)], where F is frequency in GHz.
Custom cable pairs must also be designed with very low skew or the skew must be minimized using a pair of adjustable
phase trimmer adapters like those listed in the
Optional Accessories . (See page 48, Optional Accessories .)
Input Termination Network
The input termination network in the P7313SMA differential probe includes a pair of attenuation resistor networks laser
trimmed to 50
Ω terminations, connected together at a common-mode voltage node, labeled V
T
. (See Figure 2 on page 3.)
The common-mode termination voltage node, V
T
, is designed to provide a broadband, low impedance termination for
input common-mode signals. The probe termination voltage can be adjusted using several different modes that will
be described later.
The termination voltage range is +3.6 V/–2.5 V. For DC-coupled serial data signals, the termination voltage, V
T
, should
generally be set to equal the input signal common-mode voltage, V
CM
; for AC-coupled serial data signals, the termination
voltage, V
T
, should generally be set to 0 V.
The adjustability of the termination voltage also provides measurement fl exibility for characterizing or stressing serial data
signal drivers. Because of the low impedance of the input termination and attenuator network, the signal termination currents
can become quite large. The following table can be used to calculate the DC common-mode voltages and currents at the
probe inputs and termination voltage driver under several common source impedance conditions. (See Table 1.)
Table 1: Common-mode voltage and current formulas
. Source impedance
1
Common-mode term 0 Ω 50 Ω
. V
I
V
CM
0.5 x (V
T
+ V
CM
)
. I
I
40.00 mA x V
T
- 40.00 mA x V
CM
20.00 mA x V
T
- 20.00 mA x V
CM
. I
T
40.00 mA x V
T
- 23.33 mA x V
CM
28.33 mA x V
T
- 11.67 mA x V
CM
1
When inputs are AC coupled: V
I
= V
T
, I
I
= 0, I
T
= 16.67 mA x V
T
4 P7313SMA Technical Reference