1382 McMahon Ave., Ottawa, Ont., Canada K1T 1C3

tel: (613) 523-7448

e-mail: morris-instruments@rogers.com

The following application notes describe some of the uses for the R.F. Sweepers other than basic probe and filter tuning which are described in the manual. If you wish to submit application notes please do so by sending them to morris-instruments@rogers.com

- Propagation Velocity of Transmission Line Cable
- Quarter wave cable (no connectors on cable)
- Coil Inductance

- Morris Instruments Inc. R.F. Sweeper (any model)
- 50 ohm through termination (BNC male to BNC female)
- Cable to be tested with at least one BNC connector

- Set the sweeper for one port operation in its highest frequency band, ADC=1, maximum sweep width, RF level = max.
- Attach the 50 ohm termination to the TUNE/DET port on the sweeper. You should observe a trace at the match line right across the screen.
- Connect the cable to be tested to the other side of the termination and leave the far end of the cable open circuit. You should now observe a series of peaks and dips (to the match line) across the screen. Peaks correspond to frequencies where the cable is an odd multiple of a quarter wavelength so the sweeper will be measuring a short circuit. Dips correspond to even multiples of a quarter wave, where the cable looks like an open circuit so the sweeper simply measures the termination.
- Using the cursor (you may zoom in for greater accuracy), measure the separation (S) between successive peaks. For greater accuracy measure the separation between N peaks and divide the result by N.
- Measure the physical length (L) of the cable.
- The propagation velocity is: V = 2SL
- The velocity factor is V/c where c is the speed of light.

- Morris Instruments Inc. R.F. Sweeper (any model)
- small loop antenna (sniffer coil) with diameter comparable to the cable diameter.
- soldering station
- Cable to be tested

- Set the sweeper for one port operation, ADC=1.
- Estimate the quarter wave frequency (It will not be higher than F=c/(4L) where c is the speed of light and L is the cable length.), and set the sweeper band and sweep width to cover the expected frequency range.
- Connect the loop antenna to the TUNE/DET port and then adjust the R.F. Level so that the trace is near but not above the top of the screen.
- at one end of the cable to be tested bend the center conductor around in a loop (diameter about equal to the cable diameter) to the outer conductor and solder it. Leave the other end open circuit
- Couple the loop antenna to the loop you just made and look for a dip to appear in the sweeper trace. Once you've found it zoom in and measure the quarter wave frequency using the cursor. (Note dips will also occur at higher odd multiples of a quarter wave.)

- Morris Instruments Inc. R.F. Sweeper
- small loop antenna (sniffer coil) with diameter comparable to the coil diameter.
- A fixed capacitor of known value and precision. A reasonable choice may be found by estimating the value of the unknown inductor (L) and computing the frequency where its reactance would be 100 ohms. F=16/L Then compute a capacitance value having the same reactance at this frequency. C=1/(630F) Use the nearest available 5% or better, chip capacitor.

- Set the sweeper for one port operation, ADC=1.
- Solder the fixed capacitor across the inductor (the total lead length should approximate the lead length of the inductor in its final application) and mount the circuit on an insulator away from any metal surfaces.
- Estimate the value of the unknown inductor, compute F=1/(6.3(LC)^0.5) and set the sweeper band and sweep width to cover this expected frequency.
- Connect the loop antenna to the TUNE/DET port and then adjust the R.F. Level so that the trace is near but not above the top of the screen.
- Couple the loop antenna to the unknown inductor and look for a dip to appear in the sweeper trace. If you don't see it you may have to open the sweep width and/or increase the coupling. Once you've found it zoom in and measure the resonant frequency (F) using the cursor. This measurement should be made in the limit of loose coupling.
- Calculate the inductance from the following formula (Note that the precision of the measurement is limited by the precision of the fixed capacitor at the frequency of measurement. This may be a looser tolerance than the low frequency value of the capacitor): L=1/(39.48F*F*C)

Morris Instruments Inc. home page

Paul Morris 08/02/19

morris-instruments@rogers.com