Rectified Signal Sampling

Many magnetron generators use high voltage which is simply a result of unfiltered half-wave or full-wave rectification of the up-transformed one-phase or three-phase power line voltage. Microwave power envelope then pulses with repetition rate (ripple frequency) fr, which is equal to power line frequency fp or its integral multiple, as shown in the table below:

Rectification Method

Ripple Frequency

One-phase half-wave

fr = fp

One-phase full-wave

fr = 2 fp

Three-phase half-wave

fr = 3 fp

Three-phase full-wave

fr = 6 fp

Three-phase Y+D half-wave

fr = 6 fp

Three-phase Y+D full-wave

fr = 12 fp

This type of signal is referred to as Rectified. Rectified sampling mode is intended for these signal types. It is also useful for handling the Mode Stirrer situation.

The governing parameters of the Rectified sampling are shown in the table below. Refer to Fig. 25 for the symbols in parentheses in the left column. The middle column shows the settings via the Homer Sampling Setup window of HomSoft, plus an alternative (mostly a toolbar button) if one exists. The right-hand column shows examples of Hom.cfg settings for Homer autonomous operation, resulting in sampling time Ts = 20 ms.

Parameter

Setting via HomSoft

Example Setting in Hom.cfg

Ripple frequency (fr)

Ripple Freq edit box. Valid range 1 to 10000 Hz.

Rect_Linfre=50
(fr = 50 Hz)

Number of ripple periods to be covered (Np)

Ripple Periods edit box. Valid range 1 to 1000.

Rect_Periods=1
(Np = 1)

Sample count (Ns)

Sample Count edit box. Valid range 21 to 4095. One ripple period is sampled by approximately Ns/Np samples.

Rect_Nsampl=201
(Ns = 201)

Sampling time (Ts)

Computed internally by Homer as Ts = Np/fr; displayed in Sampling Time box.

n. a.
(Ts = 20 ms)

Sampling rate (fs)

Computed internally by Homer; displayed in Sampling Rate box.

n. a.
(fs = 10 kHz)

Sampling step (Dts)

Computed internally by Homer; displayed in Sampling Step box.

n. a.
(Dts = 100 ms)

Acceptance level
(Lacc)

Accept Level edit box. Valid range 0 to 100% of pulse height. In Mode Stirrer situation, 0 to 20% recommended.

Rect_Acc_Lvl=20
(Lacc = 20%)

Frequency counting time (Tc)

Count Time edit box or  Tc  Standard toolbar button (Fig. 15). Valid range 16 ms to 1 s.

Rect_Tcnt_Us=1000
(Tc = 1 ms; will be truncated to n × 16 ms = 992 ms)

Triggering level
(Ltrg)

Trig Level edit box or Signal toolbar button. Valid range 1 to 99% of the ripple peak-to-peak value.

TrgLevel=50
(Ltrg = 50%)

Free Run

Free Run checkbox or / Signal toolbar button. Normal setting with high-ripple signals is unchecked (). Check the box in Mode Stirrer situation with low-ripple signals.

TrgMethod=1
(Free Run = FALSE)

Notes:

  • If Free Run = FALSE, the triggering timeout is equal to the sampling time (Ts). If no triggering event occurs during this interval, measurement will start automatically. Please also refer to the topic Triggering.

  • Enabling Free Run temporarily instead of triggering may be useful if one wishes to inspect a signal waveform that will not trigger.

  • If ripples are such that the power drops to zero, the frequency counting time Tc should be shorter than one half of the ripple period. When entering Tc via the  Tc  Standard toolbar button (Fig. 15), the software reports the ripple period and recommends a counting time not exceeding 75% of the ripple half-period (an example for 50-Hz ripple frequency is in Fig. 26).

Fig. 26.  Entering counting time for Rectified sampling.

Rectified-type signals are thus sampled over one or more periods of the high-voltage ripple. After being collected, the samples are processed individually to yield particular results for each sample (reflection coefficient, incident power, reflected power). These particular results are then averaged to yield the final results of interest. The rules for averaging are as follows:

  • Incident and reflected powers are averaged over the whole sampling time (i.e., considering all samples). This yields the mean power values.

  • Reflection coefficient is averaged only over the accepted samples. The accepted samples are those with the incident power exceeding the set Accept Level (Lacc).

The Homer must informed of the type of signal waveform it will be confronted with (in this case Rectified). Following this, the corresponding sampling parameters can be modified (most importantly the ripple frequency).

To select Rectified mode of signal sampling:

Open the Homer Sampling Setup window and click the Rectified tab. Alternatively, click the Signal toolbar button (Fig. 15).

To set sampling parameters:

In the Homer Sampling Setup window:

  • Edit Ripple Freq to set the microwave power ripple frequency fr in Hz.

  • Edit Ripple Periods to set the number of periods Np to be sampled.

  • Edit Sample Count to set the total number of samples Ns to be collected over the sampling time.

  • Edit Accept Level to set the acceptance level Lacc in % of pulse height.

  • Edit Count Time or click  Tc  Standard toolbar button (Fig. 15) to set the signal frequency counting time Tc in microseconds.

  • Edit Trig Level or click to set the trigger level Ltrg in % of ripple peak-to-peak value.

  • Check Free Run or click / button to enable/disable sampling without triggering.

  • Check Send Samples or press the sbSamples button to display individual samples. To learn more, go to Displaying Individual Samples.

  • Click the  Refresh  button to check/correct the entered values and compute the derived parameters (Sampling Time, Sampling Rate, Sampling Step).

  • Click the  Apply  button to accept the changes, or click  Cancel  to ignore all changes. (The  Apply  button also automatically performs the Refresh action.)