Cardinal表征门控振荡器

Cardinal Components Inc Characterizing Gated Oscillators

Measuring Stability And Envelope Shape In Gated Oscillators

Gated or keyed oscillators are turned on and off rapidly. This type of operation often causes frequency changes or “chirps”. Characterizing the keying characteristics and frequency stability of gated oscillators is an easy job for LeCroy oscilloscopes with the jitter and timing analysis package.

Figure 1 shows a typical measurement on a 315 MHz keyless entry system transmitter. This type of device uses a keyed continuous wave (cw) transmitter to send digital information to the receiver located in a vehicle. Data is encoded by controlling the width of the burst.

Of interest to the designer is the shape of the gated burst envelope and the frequency variation as the oscillator is keyed. The shape of the envelope affects the spurious content of the signal. Likewise, frequency or phase modulation of the carrier caused by the gating the oscillator affects signal purity. Either of these effects can result in out of band signal components and a potential for failure to meet emissions certification requirements.

In figure 1 the leading edge of the transmitted waveform is captured in channel 2 shown as the top trace. Trace A contains the extrema math function called roof, which extracts the peak values of each cycle of the transmitted signal. This trace is used to evaluate the risetime of the burst. The risetime parameter appears on the first line of the parameter readout located directly below the displayed traces.

Trace B, third from the top, contains the JitterTrack function of frequency and shows the variation in frequency as a function of time. Note that the frequency converges to the nominal 315 MHz in about 250 ns. The bottom trace contains the JitterTrack of cycle to cycle period variation. Both of these functions allow the user to specify the amplitude threshold and hysteresis level. This allows the user to start the measurement only after the signal has achieved a reasonable level. The threshold level appears as a dotted horizontal line of the source waveform(CH2). Note that the JitterTrack function do not start until the threshold level is achieved. This avoids errors caused by reading noise at the beginning of the waveform. 

Cardinal表征门控振荡器，测量门控振荡器的稳定性和包络形状

门控或键控振荡器快速打开和关闭。 这种类型的操作通常导致频率变化或 “唧唧喳喳”。对于带有抖动和定时分析包的勒克罗伊示波器来说，表征门控石英贴片晶振的键控特性和频率稳定性是一项简单的工作。

Cardinal表征门控振荡器

迹线B，从上面数第三个， 包含抖动轨迹 频率和的函数 显示了中的变化 频率是时间的函数。 请注意频率 收敛到标称值315 大约250纳秒。这底部轨迹包含 周期到周期的抖动跟踪 周期变化。这两个功能允许用户 指定振幅阈值 和滞后水平。石英晶振这允许用户启动 测量仅在 信号已经达到合理水平。阈值水平 显示为水平虚线 来源行 波形(CH2)。请注意 抖动跟踪功能不启动 直到阈值水平被 已实现。这避免了错误 由读取噪音引起 波形的开始。