Sunday, June 9, 2019

A look at dip oscillators

A dip oscillator (aka grid dip meter or dip meter) is one of the earlier pieces of RF test gear. Partly replaced by antenna analysers and direct reading inductance and capacitance meters, it allowed a wide range of RF tests and measurements to be done. 

It's basically a wide range variable frequency RF oscillator with a meter indicating RF level. The oscillator is made so that its coil (which is normally plug-in) can be held near the tuned circuit or component under test. When the tuned circuit is resonant on the same frequency as the dip oscillator's frequency RF is sucked out of the dip oscillator's coil into the tuned circuit being tested. This causes the meter reading to drop almost to zero. Moving the dip oscillator's variable capacitor either side of the tuned circuit's resonant frequency allows the meter reading to go up again.

Capacitors, especially if fixed, normally have their value written on them. That's not true for many inductors, especially if air wound coils. So what you do is you put the known capacitor in parallel with the unknown coil and hold it near the dip oscillator. Then you sweep the dip oscillator through its range until you see a null on the meter. That's the resonant frequency of the tuned circuit. Now that you know capacitance and frequency you can use a formula to establish the unknown inductor's value.  Now you have that you could then use it to measure capacitor values, particularly variable capacitors that rarely have their value marked on them.  

A dip oscillator can also be used as an RF signal generator. It's output is neither accurate nor stable by today's standards. Nevertheless it's still fine for testing and aligning receivers.  For example you could set the dip oscillator onto the frequency that your receiver is intended to receive and peak tuned circuits in the receiver for maximum signal. If the receiver is a superhet you can move the dip oscillator to frequencies that the receiver shouldn't respond to (ie images) and see if there is good rejection. If there isn't the receiver may be misaligned. 

Antenna tests is another application. Here you couple the dip oscillator to the antenna via a small loop. that can help you find the antenna's resonant frequency. 

Dip oscillators typically come with coils that allow operation from below 1 MHz up into the VHF range. They're simple to build, comprising one or two transistors, a variable capacitor, a meter movement and plug in coils.  Some ideas on them are below: 

Have you built or used a dip meter? Are there things you managed to do with it that I didn't mention above? Please share your comments below.

PS: Want to read more about antennas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


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