VCO design feedback - Page 1
Forum section: electronics / projects

author: pyrohaz
author_rank: Regular Contributor
timestamp: August 08, 2014, 11:36:18 pm
content: Hi All,I never really dabble in analog electronic design because I don't consider myself particularly good at it (I'm not asking for sympathy here, my strengths just lie elsewhere!). I run a blog (www.hsel.co.uk) and I've recently written a post on designing a triangle and square wave VCO from scratch. I'd really appreciate any form of feedback if anyone fancies reading it!The address is:http://hsel.co.uk/2014/08/08/designing-a-trianglesquare-wave-vco-part-1/Hopefully you guys can help me in my general analog electronics design and explanatory skills. Please point out any mistakes and explanation errors present so it will help me in the future! I'm going to be doing a blog on the math behind calculating the frequencies in the future.

author: pyrohaz
author_rank: Regular Contributor
timestamp: August 09, 2014, 12:56:53 am
content: For the theory and math, here is my post:http://hsel.co.uk/2014/08/09/designing-a-trianglesquare-wave-vco-part-2/I don't want it to seem like I'm just wanting to boost views on my blog so I'll post the schematic here too! I'm not going to include the explanation because its really long but if you're that interested, just visit the page!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 09, 2014, 01:55:00 am
content: Pyrohaz and the group,I have had a look at your schematic. It will work as you have drawn the circuit. There are some improvements that you can make.In the current mirror circuit you have base - emitter junctions in series with the resistors that determine the current.I would add emitter resistors to the current mirrors to reduce the sensitivity to mismatch in the transistors. In LTspice the matching is perfect.Here is a (heavily) modified version of your circuit. I have used an active integrator. There is a constant negative current applied to the integrator and a switched positive current twice the magnitude. The result is a switched current +/- I, but only using a single switch. I have removed the base-emitter junctions in the current mirror, by removing the current mirror. I did use symmetrical power supplies.I followed the convention of having the inputs on the left and outputs on the right, signal flow is left to right, feedback signals right to left.Here is the modified model:And the circuit waveforms:I have attached a zip file containing my model.The 555 timer can be replaced by an op-amp Schmidt Trigger.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 09, 2014, 10:26:10 am
content: Hi,The model can be modified to include frequency measurement. This will allow a graph of frequency versus input voltage to be plotted.The model is run in the normal way. The simulation will run several times. When the simulation has finished following the instructions shown on the model to obtain the graph.Here is the modified model:And the results:If I change the model to match Pyrohaz's circuit I get:And the results for Pyrohaz's circuit are:You can see the frequency offset and the non-linearity as the Vin approaches the rail voltage minus Vbe.I have attached a zip file containing the LTspice models.Regards,Jay_Diddy_B

author: pyrohaz
author_rank: Regular Contributor
timestamp: August 09, 2014, 11:45:12 am
content: Hi Jay_Diddy_B,Nice work man! Thats some good work, I didn't know that you could measure the time between two pulses and therefore calculate the frequency in LTSpice, I'll be keeping that in mind in the future.Cool circuit modifications too, it looks a lot like the standard VCO's I've seen on the internet. I couldn't however give a particularly good description, especially of the mathematical side myself so I decided to design my own from scratch and document the thought process that I went through to get the final circuit, your design seems a lot simpler however! Unfortunately, I haven't got a dual railed supply and nearly all my projects are USB powered these days. I will however include a modification to the schematic including the emitter matching resistors.Keep up the good work!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 09, 2014, 03:23:35 pm
content: Hi Pyrohaz and the group,I can derived the equations for the my version of the VCO like this.I = C dv/dtThe thresholds set by the 555 timer are 1/3 and 2/3 VccR4 in series with R5 = 1/2 R1 This is to get currents of -I and +2II = Vin / R1For one half period:t = 1/3 Vcc x C1 x R1 / VinThe full period is 2/3 Vcc x C1 x R1 / VinThe frequency of oscillation is:F = Vin / (2/3 Vcc x R1 x C1)CorrectedWhere Vcc is the 555 power supply.I made a small mistake when modelling the original circuit. I connected the modulation input to mid-rail (Vm) instead of GND. If I correct the mistake:The new result is:This shows the frequency is proportional to Vin - Vbe.For your original circuit the Frequency is given by:F= (Vin - Vbe) / (2/3 x Vcc x C1 x R5) when R5 = R4Regards,Jay_Diddy_B

author: pyrohaz
author_rank: Regular Contributor
timestamp: August 09, 2014, 09:56:41 pm
content: Quote from: Jay_Diddy_B on August 09, 2014, 03:23:35 pmAh yes, thats pretty simple too tbh! Thank you for a simplified equation of the frequency output of my circuit too. Its been a really good exercise for me to go through the steps required to design an analog circuit as its not really something I do particularly often and thank you for the feedback on it too. Your schematic drawing is a lot cleaner than mine!