Your basically on the right track with the GDO Phil. These relativey simple, versatile and easy to use instruments are popular in amateur radio circles but seem to be relatively unknown in radio repair or restoration work – I am not sure why. GDO’s can be used in several ways (I will place a post later describing their use in more detail). In this example, the GDO is allowed to oscillate in the range of the wanted IF frequency. The meter on the GDO shows the level of this oscillation, the frequency of which can be tuned with the ‘tuning’ knob on the GDO (within the range allowed by the GDO coil selected – they are usually supplied with seveal covering a wide range of frequencies). The GDO oscillator coil is then brought close to the tuned circuit for which the resonant frequency is unknown. On tuning through the resonant frequency (by turning the GDO ‘tuning’ knob), a dip will be observed on the GDO meter at the point of resonance of the tuned circuit under test. To get as accurtate measurment as possible, the degree of coupling between the GDO coil and the tuned circuit coil should be as loose as possible to still give a dip on the meter (keep the two coild close at first and once the dip is found, move them apoart and re-tune the GDO if necessary – this is due to the ‘pulling’ effect of the tuned circuit on the GDO. The GDO is calibrated to read the frequency directly for several of its ranges, but the Millen unit shown here uses calibration curves for the ranges below around 1.6MHz (the calibration curves can be seem on the photo). A more accurate frequency determination can be had by coupling a digital frequency meter (DFM) to the GDO coil afer the resonant frequency has been determined as described above (a few turns of wire connected to a coax cable feeding the DFM), reasing the frequency directly on the DFM. I would note though that the simplicity of the GDO circuitry does not allow really precise determinations to be made, but certainly more than enough for the IF transformer example shown here.
The two photos attached here show a similar application – this time to find the approximate tuning range of a radio frequency (aerial) coil from a communications receiver.
Download Test Gear 013 [1024×768].jpg. (Caution: This file may not be virus scanned.)
Download Alignment and Winter [1024×768].jpg. (Caution: This file may not be virus scanned.)