What is the Resonance Frequency?

LC-Generator as an Add-On for the Frequency Counter

 

Dr.-Ing.Werner Hegewald – Y25RD

 

Freely translated from the German original (Funkamateur 37, Heft 7, Seite 345-346, 1988)

 

Determination of the resonance frequency of a parallel LC network is one of the most important measuring problems in the ham shack. Not unjustly the commonly used dip meter is nicknamed “sounding-rod of the radio amateur”.

 

A more elegant method for measuring the resonance frequency is doubtless the use of an add-on circuit in front of the available frequency counter. Such a probe contains (among other things) an oscillator with two test pins, to which the unknown LC-network is connected [1]

 

Besides the convenience of not having to find the correct plug-in coil and dip, this bypasses all coupling problems. That advantage is especially important when the magnetic field of the LC network is not directly accessible, e.g. in the case of pot core coils or coils with screening cans.

 

Starting from ref. [1], I tried to extend the frequency range of the employed oscillator – on the one hand to be capable of measuring even in the VHF region, on the other hand to make it possible to determine frequencies below 100 kHz (including the audio frequency region) – frequencies which normally aren’t covered by a dip meter. While the extension to higher frequencies did not pose any problems (by the inclusion of a VHF coil, L1 in Fig.1, and a layout with low parasitic capacitance), the series-switching of another coil (L3) allowed a reliable operation only until about 100 kHz. For lower frequencies, the value both of the coils and of the capacitor C1 had to be altered using a range switch.

 

In tests a DC-coupled oscillator circuit without coils [2] proved unsuitable for frequencies above 0.5 to 1 MHz because of too large frequency errors. These are caused by the capacitances C­CB of the transistors VT4 to VT6 which are parallel to the test object. The solution appeared to be combination of both circuits in a probe-shaped common enclosure.

 

Figure 1: Circuit diagram of the tool for measurement of the resonance frequency of a parallel LC network

 


 

As the circuit diagram of Fig.1 indicates, the low-frequency section employs PNP transistors which allow the use of a common power supply. VD3 limits the operating voltage of this section to about 1.7 V – otherwise an unnecessary high resistance value of RP1 would be required. Since an inbuilt battery is always empty - and has often also become leaky - by the time when one needs the probe, the circuit was designed for an external power supply of 12 V. Of course it is also possible to place a 9-V battery within the enclosure.

 

Figure 2: PC board (copper side, size 119 x 29 mm)

 

 

Figures 2 and 3 show the PC board which has been designed in such a way that (a) various shapes for RP1 and L3 can be accomodated (b) parasitic capacitances are low and (c) all important connections are short. In accordance with these aims, both sections of the circuit have separate inputs. VD1 and VD2 indicate which input has been selected.

 

Figure 3: Parts placement (parts side of board)

 

 

 

The enclosure of the prototype doesn’t have any test pins, sockets and the like but only diminutive “test stumps” to which the unknown LC network can be soldered and a soldering islet for the ground connection. Especially circuits for the higher frequencies can be hooked up with extremely short connections. Using this construction method, I attained additional parallel capacitances of the order of magnitude of 4 to 5 pF, which can be considered a favourable value.

 

In measuring resonance frequencies, one should proceed as follows:

 

(1) From the AF range until about 100 kHz the low-frequency section of the circuit is used. RP1 should initially be set to its largest value of resistance (turned completely counter-clockwise) and slowly be turned back until oscillation occurs. A further decrease causes false frequency readings because of too strong positive feedback.

 

(2) In the range between 100 and 500 kHz, both sections of the probe may be used to test the LC network. Experience has taught that the HF part is to be preferred when the parallel capacitance of the LC network is smaller than 1000 pF.

 

(3) Above 500 kHz one uses the HF part by definition. For resonance frequencies up to about 2.5 MHz, coil L3 should be switched on. Need for this coil is indicated by the fact that its addition leads to lower measured values which are then valid.

 

With this circuit combination, a frequency range of 100 Hz until about 200 MHz can be covered, as indicated during tests with a Y34ZO frequency counter [3]. Measuring errors occur during the assessment of tuned circuits for VHF, but this is not problematic at all since the additional parallel capacitance is known with sufficient accuracy or it can be estimated.

 

It is advantageous that resonance frequencies can be assessed both of “loose” LC networks and of tuned circuits within receivers or transmitters, on the condition that the DC and RF voltages of the probe can be handled. The probe may even be used as a very simple generator for RF/LF signal tracing.

 

In conclusion, I should note that this probe is to be preferred above a dipmeter for specific measuring purposes (assessment of the resonance frequency of an unknown LC network), but it can not replace the dipper because of its universality (modulated test transmitter, absorption frequency meter).

 

 

References:

 

[1] P.Schmidt, Zusatzgerät zum Zählfrequenzmesser, Funkamateur 34 (1985), Heft 3, Seite 131.

[2] K.H.Schubert, Schaltung zur Schwingkreisanregung, Elektronisches Jahrbuch für den Funkamateur 1977, Militärverlag der DDR, Berlin 1976, Seite 87.

[3] H.J.Reichelt, B.Rose, J.Volkstedt, Nachbausicherer Frequenzzähler für den Funkamateur, Funkamateur 36 (1987), Heft 6, Seite 287.

 

 

Additional notes by Aren van Waarde:

 

Last update: January 29, 2006

 

This probe is a nice design and it works very well. I have used it to determine the value of unknown inductors in my junkbox, with a 1% capacitor in parallel and the well-known formula:

 

F = 1 / (2 * PI * SQRT (LC))

 

(F in Hz, L in Henry, C in Farad)

 

Inductors with values ranging from 1 microHenry to 10 H could be measured, i.e. within a range of 7 decades !

 

Please note:

 

1. There is a drawing error in the Funkamateur schematic (Fig.1): the polarity of capacitor C3 is reversed. The polarity indicated in Fig.3 is correct.

2. The board layout is based on dual-pole dual-throw switches from the East German brand Simeto. These can be purchased from Oppermann in Germany.

3. The transistors SC307 (VT4…VT6) can be replaced by BC557, the JFETs KP303 (VT1…VT2) by BF245B,  the KT368 or SF245 (VT3) by a BF199. The pinout of BC557 and SC307 is identical, but the pinout of BF245B and BF199 transistors differs from the Russian originals. Be careful and mount them in the proper way.

4. Coil values are: L1 = 10 microHenry, L2 = 180 microHenry, L3 = 33 mH, L4 = 10 microHenry. I used fixed inductors (Neosid) for L1, L3 and L4, and a variable inductor (Toko) for L2. Since I did not have 10 microHenry inductors in my junkbox, I raised the value of L1 and L4 to 15 microHenries.

 

Questions, comments: aren.van.waarde@hetnet.nl

 

(NB Use my initials instead of my first name if you like an answer to your mail !)

 

 

Figure 4. Photo of my own copy of the probe

 

 

 

 

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