Figure 1.
Using the Wavetek and Oscilloscope to Tune and Match a Probe
The Wavetek is a frequency sweeper. It can be used in two different modes. In the continuos wave mode the instrument puts out a continuous radio frequency wave of constant frequency to be observed with 50 Ohm (turn button "sweep width" to "cw"). In the swept mode with larger sweep widths the radio frequency covers a frequency range of your setting. The center frequency of the window is set roughly with the left most big knob. For finer adjustments the "Vernier" button is available. In either mode, the rf is emitted through the BNC labeled "RF out" suitable for 50 Ohm applications.
Principle
To use the Wavetek in the frequency sweep mode to tune and match a probe you basically pass the swept rf through a directional coupler into the probe. The directional coupler allows you to observe how well the probe absorbs the radiation. Well tuned and matched the probe absorbs all input at the desired resonance frequency. The response of the directional coupler is fed back through the Wavetek and put out again as x / y information. This you can display on the Oscilloscope.
Set Up
In practice you assemble the system as follows (see figure 1). You connect the "RF out" BNC to the "CPL" input of the Directional coupler. I labeled it "RF". You connect the "IN" BNC at the directional coupler to the Probe (or filter, or 1/4 lambda cable, what ever you want to test). I have labeled that one with "PROBE". The "OUT" ("back") BNC has to be connected to the "DETECTOR in" BNC at the Wavetek. The "VERT" and "HORIZ" outputs on the Wavetek are attached to the Channel 2 and Channel 1 inputs, respectively, at the Oscilloscope.
Operation
After connecting all parts you want to turn on the Wavetek and Oscilloscope and observe the response.
On the oscilloscope, you choose first off all the display mode to xy by turning the centrally located timing button (labeled SEC/DIV) to the left until the ASWP light turns off. The BNC inputs for both channels need to be set to 1M( under the Volts/DIV buttons. Also you want to choose to observe CH2, because CH1 is only used for the x-axis. Now you should start to see something on the scope. You can use that to set the scales with the Volts/div buttons correctly. Useful values are 1V for the CH1 (x-axis) and 50mV for the CH2 (y-axis) as indicated in the display window.
On the Wavetek you use the Frequency and Vernier buttons to set the center frequency of the sweep window correctly and the "Sweep" width button to open up or narrow the frequency range to your needs. A big dip down indicates zero frequency. The response to the left of it is just a reflection of the positive ones.
Tuning and Matching
Goal of tuning and matching of a probe is to get the probe to absorb as much rf at the desired frequency as possible. Good resonant absorption corresponds to a sharp line on the scope at the correct frequency that touches the top line. Use the tuning and matching capacitors on your probe to bring the resonance line to that frequency and shape. Realize that when you change the capacitors over large ranges, both tuning and matching capacitors change the tuning and matching.
To finely set the matching after you set the frequency correctly you can play a trick. (However, the final tuning and matching has to be done on the spectrometer itself, because two frequency sources are never identical. Therefore being too picky here is not worth it and you may actually skip this part.) Unplug the cable from the "Detector in" BNC at the Wavetek and plug it into the CH2 input into the Oscilloscope replacing the cable coming from "Vert". Now you see a V shape on the display and its image reflected through the center line. Unfortunately you lost your frequency labeling. So shift the frequency to a marker on the Oscilloscope. A well matched line is obtained, when the V and its image meet at the center line forming an X. You will have to use the tune to undo the frequency change introduced through the matching to get the X at the correct frequency.
Good resonating!
Figure 1.