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Sampler (Sampler)

Symbol

Available in ADS

Parameters

Name

Description

Units

Default

Ton

ON-state pulse width, switch in low impedance state

nsec

1

Ron

ON-state resistance, switch in low impedance state

Ohm

1

S11

Input reflection coefficient

None

0

Z0

Input port characteristic impedance

Ohm

50

Notes/Equations
  1. Sampler is a linear behavioral model of a high-frequency sampler. It can also be used to model the sampling efficiency and droop of lower frequency sample-holds.
  2. This model works in transient and circuit envelope simulation. In envelope simulation, the input signal is determined by transforming all the spectral input voltages at the sampling instant to determine the total, real instantaneous voltage. Prior to being sampled, the input signal is first filtered with an ideal sinc( ) filter determined by the Ton pulse width. The input match can be complex, but since it needs to represent a causal response it cannot be complex at dc.
  3. The input impedance looking into the sampler is determined by Z0 and S11 with the standard formula:

    Usually, S11=0 and Z0=50 results in the input impedance of the sampler being 50 Ohms.
  4. The Clock/LO input impedance is infinite, and the sampling instant is defined only by the baseband portion of the input signal and occurs whenever the baseband signal passes through 0.5V with a positive slope.
  5. Sampler has two basic modes of operation. If Ron is equal to 0, then the output is a ideal voltage source with a value equal to the last sampled value of the filtered input voltage; if Ron is not equal to 0, then the output impedance is time-varying. In the quiescent state, it is 1Tohm. However when a sample occurs, the impedance for that time point is reduced to a sampler resistance equal to Ron×timestep/Ton. The actual sampler efficiency is then determined by sampler Ron and Ton values, and the load capacitance on the sampler output.
    If Ton >> Ron×CLoad, the sampler will behave with 100% efficiency. In a microwave sampler, this efficiency is typically much less than 100%. The hold time constant in this sampler mode is determined strictly by the capacitive and resistive load placed on the sampler output by the user.
    The Ron sampler parameter should include all charging impedances, including the sampler switch impedance as well as the effect of any source impedance.
    The output signal in Envelope mode is a baseband-only signal. No RF leakage is included in this model.
    Note that due to the impulse nature of this sampler model output, the analysis integration order should be set to 1 (Backward Euler) when using this model. This is especially true when sampling rapidly changing input signals and when the sampler parameters are set for high sampling efficiency.
  6. In the circuit envelope example in Circuit Envelope Example, High-Frequency Application, the input signal is a high-frequency sawtooth waveform with 15 harmonics. Its frequency is set 10 kHz above 1 GHz. The sampler is being driven at a 500 kHz rate so the output signal should be a replicated sawtooth at 10 kHz. Simulation results for two different combinations of Ron and Ton are shown in Simulation Results. The high-efficiency mode tracks the input. The low-efficiency mode (50 Ohm, 50 psec) shows its lowpass filtering impact. Note the two different time scales: 2 nsec for the input trace and 200 µ for the output traces.

The example in Circuit Envelope Example, Low-Frequency Application shows a low-frequency application that has a significant droop due to the finite output resistance. Simulation results are shown in Simulation Results.

Circuit Envelope Example,

High-Frequency Application

Simulation Results

Circuit Envelope Example,

Low-Frequency Application

Simulation Results
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