INTRODUCTION The Heathkit Grid Dip Meter model GD-1U (The GD-1U is the Heathkit G.D.O., the one you are most likeely to encounter) is a very useful instrument. As well as determining resonant frequencies, it is also capable of performing the various tests required in the design, adjustment, and operation of high frequency radio and television equipment. Being basically a variable high frequency oscillator, it can be used as a signal generator or marker generator. As a grid dip meter, it can be used to determine the resonant frequency of a tuned Circuit, methods are outlined for using this information to determine unknown values of inductance, capacity. When used as a diode detector, it checks the frequency of other high frequency RF sources (providing the RF energy is at least 0.5 of a volt). Switched to an oscillating detector, used in conjunction with a pair of headphones, it is even more sensitive for checking the frequency of other oscillators. OPERATION OF THE GRID DIP METER A good understanding of the operation of the grid dip meter will aid materially in the proper use of the instrument. The grid dip meter is a high frequency oscillator operating in the range from 1. 8 MHz to over 230 MHz. Useful range may be extended down to 350KHz with additional coil set 341-U. A microammeter is inserted in the grid circuit of the oscillator valve. Whenever the oscillator is coupled to a load or resonant frequency, a noticeable reduction in grid current takes place. This reduction in grid current is termed the "grid dip". The most common load which will absorb energy from the oscillator is a resonant circuit tuned to the same frequency as the oscillator. The grid dip meter can be changed to a wave meter by setting the bottom switch to the "diode" position. When used as a wave meter, the valve acts as a diode detector. As the instrument is tuned to the frequency of nearby RF sources, the diode current reading on the meter will increase. (The sensitivity control must be advanced to maximum, initially.) Thus the GD-1U can be used to determine the frequency of other oscillating circuits providing there is sufficient RF energy within the oscillator circuit under test. Plugging a pair of headphones into the phone jack and setting the DIODE-OSC. switch to OSCILLATOR position, converts the GD-1U to an oscillating detector. This arrangement can also be used to determine the frequency of other oscillating currents. As the GD-1U is tuned to the frequency of another nearby RF source, a whistle or beat note is heard in the phones. When the GD-1U is adjusted to the lowest beat note (zero beat), the frequency read on the dial scale is the same as the frequency of the other oscillating circuit. (At high frequencies, only a "click" will be heard.) USING THE GRID DIP METER The basic use of the Grid Dip Meter is the coupling of the instrument to the test circuit. It is possible to couple the test circuit either inductively or capacitively. Inductive coupling is usually most convenient. (Figures B & D in diagram 2) Capacity coupling is necessary in some instances, such as in the case of coaxial lines where shielding complicates the situation.(Figures A & D in diagram 2) For the most accurate reading the coupling between the grid dip meter and the test circuit should be as loose as possible (wide spacing between the two units), while yet being able to obtain a readable dip. (Ed. Start with tight (i.e.close) coupling and then reduce coupling until the dip is just detectable) The relative Q of a circuit can be quickly determined by noting the sharpness of the dip. A broad dip in indicative of a low Q circuit. Assembling a few different resonant circuits and determining their resonant frequency will familiarise the builder with the operation of the grid dip meter. Itwill be found that as the dial is rotated from one end of the band to the other, that the meter reading also changes. If the sensitivity control is set for a mid-scale meter reading when the dial is at approximately the middle of the band, the adjustment will probably be sufficient for any frequency setting within the band. DETERMINING AN UNKNOWN CAPACITY Unknown values of capacity between 70 and 2,000 pF can readily be measured with the grid dip meter. The unknown capacitor should be placed parallel with the 14-37 MHz coil, (coil C), the coil and capacitor thus forming a parallel tuned circuit. Depending upon the suspected value of the unknown capacitor, the 2-5 or 5-14 MHz coil, designated as A or B, should be plugged into the instrument. Set the DIODE-OSC. switch to OSCILLATOR position. Couple quite closely the coil of the parallel resonant circuit containing the unknown capacitor and tune the GD-lU through the frequency range. When the dip has been detected, reduce the coupling so that the dip shows up over a very narrow frequency band. At maximum dip, read the frequency indicated on the dial and from the graph, read the value of the unknown capacitor. A glance at the graph (diagram 3) will reveal that capacitors under 65 pF are not covered. To determine values in this range, an extra capacitor of about 100 pF should be used. If it is not a precision capacitor, its value can be determined by the method outlined above. Once its value is known, it should be connected in parallel with the unknown capacitor and the 14-37 MHz coil "C". Using this method, the total capacity of this test circuit is determined. The value of the unknown capacitor is the difference between the total capacity in the test circuit and the value of the known added capacitor. When measuring an unknown capacity, certain errors must be considered. Among these are capacity in the coil, capacity caused by nearby metallic objects, and shift of resonant frequency by inductance within the capacitor. For most applications these may be neglected. MEASURING INDUCTANCE OF RF COILS Unknown inductances can be readily determined by using the Grid Dip Meter and some known capacitor. The capacitor should be a small, low tolerance unit such as a silver mica of about 100 pF. (The capacity of some unknown capacitor may be determined as outlined above and used in this test.) Connect the capacitor across the inductor forming a parallel resonant circuit. Loosely couple the Grid Dip Meter to this circuit and determine its resonant frequency. Using the value of the capacitor and the resonant frequency, the inductance of the coil can be computed as follows: Lx = 1 ΒΈ [ 39.48f2C] The inductance can also be found by referring to a reactance chart such as is found in many radio handbooks. As when measuring capacities, certain errors (usually negligible) affect the results.