Sometimes you have a shielded transformer in your scrap box, or have acquired some from a surplus distributor, and you’re not quite sure what you have. Even with a marked, currently stocked part, you may want to more closely examine its frequency characteristics or in-circuit Q factor.
There is a very basic way of determining what the intended frequency and passband characteristics were intended to be. For this you will need an oscilloscope and a signal source with a reliably close manual indicator, or a digital frequency display. Ideally the signal source should extend up to 10 or 20 MHz. The idea is to manually dial the frequency adjustment of the generator while driving a sine wave into the primary of the transformer while observing the output across the secondary using the scope display.
The transformers commonly found are going to be adjustable slug tuned or fixed, and will have 4-6 connections. Quite commonly, the primary will be a single, un-tapped winding, with the secondary being center-tapped. In the case of balanced modulator or mixer transformers, this may be immaterial because the two windings can or will serve either role. It’s really the application of the transformer that matters if the turns ratio is something other than 1:1, or a center tap is not required, etc.
You may want to check what you think is a primary or secondary with an ohmmeter just to be sure, but if the piece is really a single adjustable coil, you’ll soon discover it.
Starting at 100 kHz, you increase the frequency of your generator while looking for a peaking effect in the display of the sine wave as seen by the scope across the secondary. If the transformer is broadband, is will be a lower Q transformer that will peak, but very slowly, then decrease in magnitude as you roll pass the center of its passband. If the unit is high Q, and the turns ratio is more than 1:1, you will need to slow your frequency tuning a bit as the peak will be much more sudden as the transformer hits its point of resonance. Note this frequency. If the transformer is slug tuned, try moving the slug up or down and re-check the frequency; you should now see the shift in the passband as a result of this adjustment. Of course if the transformer is not slug tuned, your application circuit may contain a variable capacitor for fine adjustments anyway, or is broadband enough that the isolation across the transformer is all you needed.
You can also make careful measurements to determine the turns ratio of the transformer.
At HF frequencies, it is reasonable to observe the primary and secondary peak to peak voltage readings on your scope and draw a simple conclusion. If the primary P-P voltage is say 1 volt and the secondary is about 4 volts, you have stepped-up the voltage by a factor of 4, so you have a 1:4 ratio. Likewise, if the P-P voltages are about the same, you can conclude there is a 1:1 ratio. Again, the fixed transformers can be tested in the same manner. Obviously if you have some know parts from Mini-Circuits, or your testing a bi-filar wound toroid for example, then the turns ratio will be known.
Testing for Q factor takes a bit more patience and better equipment really; i.e. a scope with cursors that can help you easily find the 3dB point on either side of your transformer. Of course this is not a demanding requirement for a new or used oscilloscope today.
Follow the link for a refresher, but to calculate the Q of the transformer, we are going to need to measure its 3dB bandwidth in addition to the center or resonant frequency measurement we made above. The Q then can be calculated by dividing the 1/2 power or 3 dB bandwidth number into the resonant frequency. In this case we are going to use the (0.707 x the peak voltage) method to determine our 3 dB point in terms of voltage. If you use a convenient secondary output P-P voltage value of 1, then you are looking for the points where the voltage comes down to .707 volts P-P.
For example, if you find your transformer peaks at 9 MHz and your 3 dB point spanned 600 kHz, you would calculate a Q of 15.