I am planning to use the output of the VCO and send it to a freq meter. The output will be in form of a digital pulse, which will be calibrated to some value and will be updated on a screen (digital output using a HMI). Do you think its a right approach.How will you calibrate frequency? Is it analog or DSO?
It should only take a few seconds to measure .
How accurate? Is a freq counter avail, or just for calibration?
I was just being approximate with the values, hence I mentioned rough values for resonant frequency.This is a fairly simple problem made hard by communication here.
the values you chose resonate at 2.91 kHz (1uH 7 mF)
maybe you meant microFarad (7uF !!) in which case that makes more sense 60.1kHz
The impedance Zo at resonance is 500mΩ.
Q= Ratio of Zo/Rs ( including coil) = resonance factor and for Q= 1, 10, 100, Rs= 500mΩ, 50mΩ, 5mΩ ( perhaps the coil Resistance) which results in a step response of <1 , 5 and 50 cycles
You will need 50 cycles for < 1% accuracy unless very precise threshold adjustment on counter.
This also means your connections must use braided wire or Litz wire or be very short.
Once way is to use direct COAX connection to an amplified shunt R of 5mΩ directly across coax and terminated by counter and scope.
Since this now means 5 mV/'per Amp, you can now choose your voltage and amplifier gain at 150KHz BW
I might suggest a pulse circuit that shorts out the coil after energizing it, however the counter AC couple may not response quickly enough and DC couple will saturate counter front end with battery voltage.
1st answer this
What accuracy do you need?
Using the pulse method is not low current.
Using my method with swept F is low current with gain on signal input from shunt in CD4046
View attachment 94289
Tony Stewart I understand and I am sure that this would work. But I realized that this process would always need a scope. My eventual aim is to replace everything with a circuit which is used to plug-in to a tank and give me a number. I would not want to use a function generator or scope during the test (just a circuit which works on its own). Do you have any idea about that. My error range can be upto 5% and I would prefer it in the higher side (more than resonance).If you can isolate coil and cap in parallel, then try this direct to counter.
5W but only for as long as it takes to momentarily charge up the resonator RC= 70us.
Discharge is AC coupled with series cap and probe / counter impedance 1M or 10M with sensitive input on counter.
View attachment 94292
Check sensitivity of counter for zero crossing error and max gain or 1:1 or 10:1 probe,
If you can't get this to work. then we can get fancy with VCO , peak detector m scan ramp and peak and hold circuit with Control Voltage pause on resonance,
Counter must be in gated trigger mode for >100us then tryup to 1ms .
and report accuracy using 3.7V LiPo. If you want it safer, add 1A fuse.The 10 Ohm resistor will draw a few Watts but only for <1ms for each contact bounce. So use a good switch ( momentary ) that is low in contact bounce. If cap is not connected it may arc on switch, so TVS diode may be used for additional safety ( user fault)
This is low voltage, low energy... few mJ (3W x 1ms)
This does not trigger IGBT.
Test time after connected 1 second. Push and read.
On that note, can I create a frequency sweep using a VCO, measure voltage across the series tank. The moment the voltage drops near 0V, it stops sweeping and gives me the value of the frequency.When you specify accuracy , use a ratio then it does not change with frequency. i.e. %
Let me think it over. I'll get back to you.what scope?
I only described the setting for a good low cost frequency counter with a sensitive input. ( in my last couple comments)
By careful selection of passive values and counter trigger thresholds, the test takes 1 second.
Make sense yet? WHich part did you not understand? The probe? I was suggesting using a scope probe for the counter. But as R in raises Q and duration of ringing, it also attenuates input so amplification may be needed. The counter usually has a good limiter inside.
Tony Stewart I read your comments over and over again. Are you suggesting to ring the tank circuit momentarily, for 1 second using a 3.7V LiPo and then measure the frequency using a frequency counter? Will it work?"can I create a frequency sweep using a VCO"
you can but you dont seem to be following my better advice.
So for testing out here will be my values,If you know how to design a limiter and trigger circuit to exclude noise and transients and generate a gated pulse maybe...but you dont.
That circuit is for testing Crystals with a smaller series motional capacitance ( 0.01pF) with a 10 to 20 pF parallel load, yours is 7uF in series and in parallel, both arrangements wont work due to Zo=0.5 Ohm
Tony Stewart
I went through the following document
https://www.ece.utah.edu/~ece3510/Lab_PLL_Basic_S09.pdf
They are sending the VCO output to function generator. I can't understand what is the feedback coming to pin 14 coming from.
Can I use 4046. I really can't see buying a frequency counter.
I thought you knew how the 4046 works... pin 14 is for a low level AC input signal
if you dont understand how to design this, how are you expected to find a solution?
IF you dont have a counter do you know how to calibrate VCO precisely to 1%?
To make the automated circuit you imagine... think of block diagrams then and specify all inputs and outputs.
- VCO: 0-Vcc input, 5:1 freq range output, 300R output (internal ) impedance
- Low impedance Driver: for 5 Ohm load.
- driver to 0.5Ohm parallel resonator with 5 Ohm current sense to ground.
- Precision rectifier Op Amp for resonant peak signal. Vp
- Negative sweep integrator sample and hold circuit for V_ctrl
- 1 shot cct to dump Cap voltage to Vcc with MOSFET switch to restart sweep.
- Sample and hold cct #2 for peak signal. (low leakage switch & low leakage Film cap)
- Comparator between V(t) - V(t-1) between S&H#2 cap and rectifier out , +Ve out if signal > Hold voltage to resample during pulse .. logic level out.
- ! shot timer to stop test if resample pulse has not occurred within 1 ms and hold V_Ctrl
- then use control limits or look up table for Vctrl before it drifts. for V vs F
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