CW Signal Sampling

The simplest type of waveform results from the magnetron anode voltage essentially constant over time or with a relatively small ripple in it. It is also typical for solid-state generators. This type of signal is referred to as CW (continuous wave) or Low-Ripple. The CW mode of sampling is intended for this type of signals.

The parameters governing the CW 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 (Homer|Sampling Setup) 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

Sample count (Ns)

Sample Count edit box. Valid range 1 to 4095. It has the meaning of the voltage averaging number.

Vrpt=201
(Ns = 201)

Sampling rate (fs)

Sampling Rate edit box. Valid range 10 Hz to 200 kHz.

Fsampl_CW=10000
(fs = 10 kHz)

Sampling time (Ts)

Computed internally by Homer; displayed in Sampling Time box.

n. a.
(Ts = 20 ms)

Sampling step (Dts)

Computed internally by Homer; displayed in Sampling Step box.

n. a.
(Dts = 100 ms)

Frequency counting time (Tc)

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

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

In CW mode, the detector voltages are sampled over the sampling time Ts, collecting Ns samples for each of the four Homer detectors. After being collected, the mean values of these Ns voltage samples are computed. The results of interest (reflection coefficient, incident power) are then computed from these averaged voltages. Hence, in the case of true CW signal (i.e., constant power), Ns has the meaning of voltage averaging number, which theoretically decreases white noise floor by

DL = 5 log(Ns)  [dB]

i.e., 1.5 dB for each doubling of the averaging number. Increasing Ns therefore improves measurement precision at low working power levels. Reducing the generator power level to one-half should be theoretically compensated by four-fold increase of Ns.

Since the sampling takes a definite time Ts, it also has a smoothing effect, and, if appropriately configured, can smooth out power supply ripples. Choosing, for instance, Ns = 201 and fs = 10 kHz leads to Ts = 20 ms, which is one period of 50-Hz power line voltage. To cover one period of 60-Hz power line voltage, set fs = 12 kHz instead.

Averaging of detector voltages in the CW sampling mode contrasts with the Rectified and Pulsed modes, where particular results (reflection coefficient, incident power, reflected power) are computed for each individual sample, and then these individual results are averaged to obtain a final result.

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

To select CW mode of signal sampling:

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

To set sampling parameters:

In Homer Sampling Setup window:

  • Edit Sample Count to set the number of samples Ns to be collected for one measurement result.

  • Edit Sampling Rate to set the sampling frequency fs in Hz.

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

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