Practically measuring the resonance frequency of a LC circuit and a transmitter? - Page 1
Forum section: electronics / projects

author: 6E5
author_rank: Regular Contributor
timestamp: December 07, 2014, 09:01:27 am
content: I have an royer oscillator that has a LC circuit formed by some Wima caps and a single turn of copper coil. Is there a special type of equipment that will allow me to measure its resonance frequency? I need an accurate value, which I won't get by simply measure capacitance and inductance separately and doing the formula. (I'd have to disconnect everything, which would lead to different conditions)Also, what is the preferred method of measuring the output frequency and getting waveforms of the royer oscillator? I can't just directly connect the oscilloscope probe or frequency counter probe, I'd change the frequency and the waveforms.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2014, 09:29:46 am
content: Uh, just what kind of accuracy are you demanding? Parts per million? It'll drift like a raft in that range.If you want waveforms, probe it. Most likely you'll have an error in the < 0.1% range, small enough to ignore. The scope probably can't even resolve a frequency that accurately.If this is like for radio transmitter purposes, you'll get a better read on the frequency with weak coupling, say by holding the scope probe nearby, or an inductive loop. But again, you'll need a frequency counter to resolve it accurately enough, and you'll need regulated voltage supply and temperature control, plus warm-up time, and needless to say you can't really transmit any information that way because you can't get any modulation into it (amplitude or frequency) without upsetting it.That's the main reason why proper transmitters are not oscillators but amplifiers: the frequency generation, modulation and amplification can all be isolated and controlled independently.Tim

author: mikerj
author_rank: Super Contributor
timestamp: December 07, 2014, 10:07:42 am
content: Quote from: 6E5 on December 07, 2014, 09:01:27 amYes, it's called a grid dip oscillator.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2014, 11:14:33 am
content: Depends on how you measure it, though. Powered off, the amplitude can't get very high before the transistors start rectifying the test signal, and the junction capacitance is very high. Tested under bias, but not oscillating (gate drive disabled), the frequency will be high because the capacitance is higher than average. In live operation, it's somewhere inbetween.Also, does a GDO work on a low impedance tank -- as I'm assuming the case is here?Tim

author: richard.cs
author_rank: Super Contributor
timestamp: December 07, 2014, 11:43:31 am
content: I'll add another vote for the grid dip oscillator, or any other weak-coupling method. I would be tempted to measure the frequency of the oscillator when running with an inductive pickup and a frequency counter, I quite often use a scope probe with the ground lead clipped to the tip for this.In a well equipped lab (you don't say if you're a hobbyist, at work or perhaps in a university lab) you can do this with a network analyser. Connect a magnetic pickup loop and measure S11, look for the dip. It's exactly the same principle as the grid dip oscillator, your tuned circuit sucks out more energy at resonance.

author: dannyf
author_rank: Super Contributor
timestamp: December 07, 2014, 12:20:42 pm
content: Quotethe easiest is to apply a variable frequency signal and observe resonance.This can be done a few different ways, depending on equipment you have. The simplest would be a dip meter. or it can be a signal generator + some diodes - your radio is a signal generator, in case the frequency falls into the right band.Another form of signal generator would be a DDS, or a programmable (PLL) oscillator.

author: 6E5
author_rank: Regular Contributor
timestamp: December 07, 2014, 09:41:32 pm
content: Thanks for all the tips guys. I have access to a lab at university, but right now I'm at home. So, I decided to take the advice of Richie, Danny, and the others and measure it using a frequency counter, oscilloscope, and signal generator. My equipment is pretty dated, but I know it's accurate.http://imgur.com/a/C9UL9The first picture is using a signal generator to inject a signal, peaking using the oscilloscope, and reading off the frequency counter. The second picture is measuring the running frequency. I roughly estimated the resonance frequency to be 1.1 Mhz, a reported value of about 1.3 Mhz is fine by me. When I get back to the lab I might use the network analyzer.

author: Richard Head
author_rank: Frequent Contributor
timestamp: December 08, 2014, 06:06:45 am
content: I'd go with Danny f's suggestion. it's easy and allows you to see exactly what's going on.

author: 6E5
author_rank: Regular Contributor
timestamp: December 08, 2014, 09:53:46 am
content: Rich, I did just that in my previous post. It's above your post.

author: krivx
author_rank: Frequent Contributor
timestamp: December 08, 2014, 03:41:56 pm
content: That loop of copper is certainly bigger than I would have expected...

author: richard.cs
author_rank: Super Contributor
timestamp: December 08, 2014, 08:38:13 pm
content: That method of hanging the scope probes on the loop will add rather more capacitance to ground than just putting them near to it, in case you hadn't realised there's no need for the loop to pass through them. As a starting point I would put them parallel to it a few inches away, or off to one side in the plane of the loop. You'll probably find the resonant frequency a little higher without the probes in contact with the loop.In general that big loop is going to be very prone to influence from the surrounding environment, your frequency will change as people walk past, etc. Or is that the idea?