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RF alerts traditionally are measured utilizing spectrum analyzers, no less than that was earlier than oscilloscopes supplied adequate bandwidth for these measurements. With oscilloscope bandwidths over 100 GHz, RF measurements are not the unique area of the spectrum analyzer; that is very true for pulsed RF measurements, the place the time area measurements of an oscilloscope have a number of benefits. This text will concentrate on the time measurements of pulsed RF alerts.
Many gadgets use pulsed RF alerts. The plain ones are echo-ranging methods like radar. Moreover, nuclear magnetic resonance (NMR) spectrometers and magnetic resonance imaging (MRI) methods use pulsed RF. Even automotive keyless entry methods use pulse-modulated RF alerts.
Pulsed RF alerts
Pulsed RF alerts are created by gating a steady wave (CW) RF supply, as proven in Determine 1.
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Determine 1 Pulsed RF alerts could be generated by gating a CW RF supply utilizing a change managed by a gate sign pulse prepare. Supply: Arthur Pini
The provider supply is a steady wave oscillator. It’s gated by a change pushed by the gating sign pulse prepare. This can be a multiplication operation with the provider multiplied by the gate sign. When the gating sign is excessive, the change outputs RF; when low, the output is zero. The 350 MHz provider is proven within the higher left grid. A horizontally expanded zoom view (left middle grid) exhibits the small print of the provider waveform. The gating sign (decrease left grid) is a logic sign with a zero state at 0 volts and a 1 state of 1 volt. The gate output (higher proper grid) exhibits the RF bursts at periodic intervals associated to the gate sign state. A zoom view of 1 burst (middle proper grid) offers better element of a single burst. One other view with a better zoom magnification (decrease proper grid) exhibits the turn-on particulars of the pulsed RF sign.
Measurement parameters, just below the show grids, learn the frequency (P1) and amplitude (P2) of the provider in addition to the frequency (P3) and pulse width (P4) of the gating sign.
The frequency spectra of pulsed RF alerts
Wanting on the provider, gate sign, and gated provider within the frequency area offers perception into the modulation course of. Oscilloscopes view the frequency area utilizing the quick Fourier rework (FFT) offering instruments just like a standard spectrum analyzer. The alerts and the FFTs of the three alerts are proven in Determine 2.
Determine 2 The three element alerts provider, gating pulse prepare, and pulse RF output and their FFTs present insights into the modulation course of. Supply: Arthur Pini
The provider (higher left grid), being a sine wave, has an FFT (higher proper grid) consisting of a single spectral line on the frequency of 350 MHz. The gate sign (middle left grid) is a prepare of rectangular pulses. The FFT of the gate sign takes the type of a sin(x)/s spectrum. The utmost amplitude happens at zero Hz making this a baseband spectrum anchored at 0 Hz or DC. The peaks within the spectrum are spaced on the pulse repetition frequency (PRF) of fifty kHz, measured utilizing the relative cursors on the FFT of the gate sign. The cursor readout, below the Timebase annotation field, reads absolutely the cursor positions and the frequency distinction of fifty kHz. The sin(x)/x response has a periodic lobe sample the place the nulls of the lobes happen at intervals equal to the reciprocal of the gate pulse optimistic width. For the reason that optimistic width of the gate pulse is 3.52 ms, the nulls happen each 283 kHz. These nulls are just a little tougher to measure with cursors because the spectral peaks each 50 kHz, which doesn’t have 283 kHz as an integral a number of, are inclined to obscure the nulls.
The gated RF provider outcomes from multiplying the provider by the gate sign. The state of the gate sign determines the output of the gated RF provider sign. When the gate sign is one, the provider seems on the gated provider output. Multiplication within the time area has a corresponding mathematical operation of convolution within the frequency area. The results of the convolution operation on the spectra of the provider and gate sign is proven within the FFT of the gated provider. The baseband sin(x)/x perform of the gate sign is mirrored above and beneath the provider spectrum because the higher and decrease sidebands of the provider frequency.
Pulsed RF timing measurements
The timing measurement of pulsed RF alerts begins with the heart beat bursts. In a lot of the purposes cited, the PRF, pulse width, and obligation cycle are of curiosity. The traits of the burst envelope, together with the rise time, overshoot, and flatness, may be desired. These measurements can’t be made straight on the heart beat RF sign. To make measurements on the gated provider, the sign needs to be demodulated to extract the modulation envelope and take away the provider. The demodulation course of varies from oscilloscope to oscilloscope, relying on the mathematics processes accessible. This instance used a Teledyne LeCroy oscilloscope which affords 3 ways to demodulate the gated provider sign. The primary methodology is to create a peak detector utilizing absolutely the worth math perform and a low move filter. The second methodology is to make use of the non-compulsory demodulation perform. This math perform offers demodulation of AM, FM, and PM alerts. The ultimate method is to make use of the oscilloscope’s means to embed a MATLAB script into the mathematics processing chain and use one of many MATLAB demodulation instruments. That is additionally an non-compulsory characteristic within the oscilloscope.
Evaluating demodulation processes
Evaluating the outcomes for these three strategies is fascinating. For the reason that first methodology can generally be completed with most oscilloscopes that supply an absolute worth math perform and low move filtering. The height detector methodology was used on this instance and the outcomes are proven in Determine 3.
Determine 3 Comparability of the amplitude demodulated sign of the gated provider and the gated provider, with measurements of the demodulated envelope from the height detector primarily based on absolutely the math perform. Supply: Arthur Pini
Utilizing the twin math perform, absolutely the worth of the gated provider was calculated. The second math perform is a low move filter. The low move filter cutoff frequency needs to be lower than the 350 MHz provider frequency sign, and the filter roll-off needs to be sharp sufficient to suppress the provider. On this instance, a Sixth-order Butterworth low move filter with a cutoff frequency of 125 MHz and a transition width of 100 kHz was used. This oscilloscope has low move filters accessible as enhanced decision (ERES) used for noise suppression in addition to a digital filter choice. Both low move filter supply can be utilized. The objective of this operation is to have the demodulated envelope monitor the peaks of the gated provider.
The detected envelope of the RF pulse is proven as hint F3 within the decrease left grid. Horizontal zoom shows within the higher and decrease proper grids present the match of the demodulated envelope (blue hint) to the RF burst at two completely different horizontal scales. The overlaid traces within the decrease proper grid present one of the best view for evaluating the efficiency of the demodulator. Regulate the low move filter cutoff to acquire one of the best match.
Measurement parameters P6 by P10 learn the PRF, width, obligation cycle, optimistic overshoot, and rise time of the demodulated envelope.
The identical measurement made utilizing the oscilloscope’s demodulation perform is proven in Determine 4.
Determine 4 Measurement of the pulsed RF modulation envelope utilizing the oscilloscope’s non-compulsory demodulation math perform and comparability with the pulsed RF sign. Supply: Arthur Pini
The demodulation perform was arrange for AM demodulation. The provider frequency and measurement bandwidth must be entered. The end result proven right here is for a bandwidth of 100 MHz.
The identical measurements are carried out with superb settlement with the height detector methodology. Vertical scales differ because of the completely different processing operations. For the reason that parameters being measured use relative amplitude measurements, no effort has been made to rescale the vertical information to a standard scale.
The third methodology talked about was using a MATLAB script within the oscilloscope’s sign path to demodulate the RF pulse sign. That is proven in Determine 5.
Determine 5 Instance of utilizing a MATLAB script to demodulate the Pulsed RF sign. The MATLAB script used is proven within the popup. Supply: Arthur Pini
The MATLAB demod perform, accessible within the MATLAB sign processing toolbox, is used to demodulate the pulsed RF. It’s a quite simple two-line script requiring the entry of the provider frequency and oscilloscope sampling fee. The outcomes are in step with the opposite strategies the place the first distinction happens within the rise time measurement is because of the completely different filters utilized in every of the completely different processes. Evaluating the rise time measurements of the demodulated envelope to the rise time of the gate sign, the utmost variation is about 1 % from the rise time of the gate sign. The variation among the many three demodulation strategies is about 0.2 ns of the nominal 22.67 ns rise time. These three accessible demodulation strategies produce almost an identical leads to studying the timing parameters of a pulse RF sign.
Characterizing pulsed RF alerts
The oscilloscope is properly matched to the duty of characterizing pulsed RF alerts. It will probably render the alerts in both the time or frequency area allowing evaluation in each domains. The flexibility to precisely demodulate the pulsed RF alerts permits measurement of the timing traits of the pulsed RF alerts.
Arthur Pini is a technical help specialist and electrical engineer with over 50 years of expertise in electronics check and measurement.
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