Frequency Doubler - AWR Simulation works, built circuit doesn't. - Page 1
Forum section: electronics / beginners

author: tec5c
author_rank: Frequent Contributor
timestamp: May 07, 2015, 10:28:17 am
content: In one of my classes at Uni we were given a schematic of a diff. amp. design that was being implemented as a frequency doubler.Subcircuit designDUT design (Not sure about the dBW for the source being 20*log10(Vi/10), as dBW is 10*log... I believe this is just a typing mistake and should read dBV.)The AWR file was given to us and it ran the simulation fine. The output was twice the frequency of the input as well as amplified (I can show this if anybody would like to see).We then built the circuit on a breadboard and measured the output. Yet, the frequency of the output was the same as the input frequency.No one's circuit in the class worked as it should have. The circuit on the breadboard was checked and rechecked several times, and confirmed that there were no wiring issues. So, wiring issues are NOT the cause.There is something else happening here and the tutors/instructors have so far been unable to explain why the circuit is not working.Does anyone have any ideas as to why this might be happening?

author: rob77
author_rank: Super Contributor
timestamp: May 07, 2015, 11:55:57 am
content: if am not mistaken frequency mixing/adding/substracting/whatever happens in the nonlienear region of the component's VI chart (e.g. the "knee" on the VI chart of a diode in the diode mixer)... so i would expect the same here - the transistors to be biased into the nonlinear region.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 07, 2015, 12:34:13 pm
content: I would check the DC voltages as a starting point - make sure that it at least biased into sensible range. I would then drive the inputs with appropriate DC levels and check that there is some semblance of multiplication between the two inputs.

author: Rerouter
author_rank: Super Contributor
timestamp: May 07, 2015, 12:35:46 pm
content: What is your signal amplitude? this design seems very amplitude dependant, relying on the phase reversal from over saturating a stage...I tweaked the model in my own little toy simulator that i have found to be better for proof of concept stuff, for a few khz, to a few mhz it gives 2 edges per input edge, just ugly duty cycles, needs between 9V and 30V as the input attenuation, less reduces the second spike, more downs it out by enlarging the duty cycle of the main waveform,

author: Andy Watson
author_rank: Super Contributor
timestamp: May 07, 2015, 12:43:40 pm
content: Quote from: Rerouter on May 07, 2015, 12:35:46 pmI don't think it is that complicated. I think it is supposed to be a simple multiplier, i.e. feed a sinewave into both inputs and get sin² out.

author: Rerouter
author_rank: Super Contributor
timestamp: May 07, 2015, 12:49:22 pm
content: I'm relying on what i am seeing in my simulator as i go messing with values, as opposed to the intended functions, where current is going, what influences a stage more, what actually gets the intended result, etc,i could likely redraw it, but it was a case of making it work, over making a circuit that does its intended job better,

author: Andy Watson
author_rank: Super Contributor
timestamp: May 07, 2015, 12:57:48 pm
content: A good point about the amplitude. If it is producing phase reversals or just saturating I would say that the input amplitude is too much. I would guess that input range for the "RF" input is very limited.

author: tec5c
author_rank: Frequent Contributor
timestamp: May 07, 2015, 01:03:58 pm
content: Quote from: Rerouter on May 07, 2015, 12:35:46 pmI think you're onto something here. Unfortunately I can't recall what input amplitude I was using (this was a week ago, though I know I played around with the amplitude, I can't recall exact values though) but going back to the simulation it is using a 1.48v input signal @ 10kHz. Playing around with the amplitude causes any multiplier effect to be lost when you drop the signal below ~1V. Even slight variances from 1.48v causes some slight distortion to the output.Oh and I use that circuit simulator too. Falstad, right?

author: Andy Watson
author_rank: Super Contributor
timestamp: May 07, 2015, 01:34:33 pm
content: Quote from: tec5c on May 07, 2015, 10:28:17 amI hope you're not paying much for that tuition. I don't think it is unreasonable to expect the class tutors/demonstrators to have prepared the material and ironed-out as many of the wrinkles as possiblebeforethe class. I would, as a minimum, expect them to be competent enough to give you some indication as to why it didn't work.

author: tec5c
author_rank: Frequent Contributor
timestamp: May 07, 2015, 01:43:52 pm
content: I completely agree. The circuit was designed by the lecturer, so I wouldassumethat the tutors would have consulted him since we last met and will have an explanation for us tomorrow. If this cannot be done then I will definitely be taking it to the lecturer who is also the convenor for this subject and lodging a complaint.

author: Marco
author_rank: Super Contributor
timestamp: May 07, 2015, 01:47:57 pm
content: What do you mean with "Yet, the frequency of the output was the same as the input frequency."The fundamental will still be in there, but it should be distorted to hell (for a sine input you get some weird combination of a square wave half cycle, then a half cycle of a positive going sine wave with a negative DC offset).

author: tec5c
author_rank: Frequent Contributor
timestamp: May 07, 2015, 01:53:21 pm
content: Driving the circuit with a 10kHz input results in getting a 10kHz signal at the output. There is no frequency multiplication happening.

author: Marco
author_rank: Super Contributor
timestamp: May 07, 2015, 03:08:49 pm
content: Oh, I already see what's wrong I think ...The input signal level is completely drowned out by the Vbe mismatch between Q1 and Q2, one of the transistors is simply always going to be on (in your case Q2).

author: tec5c
author_rank: Frequent Contributor
timestamp: May 08, 2015, 10:37:15 am
content: Right, so I had this class today and still no one got it to work as intended. I'm starting to get quite annoyed at the professor about this. I really want to know why it's not working! We were told to disconnect the buffer portion of the circuit and to just measure the gain of the amplifier on a scope rather than look at the frequency content. Pathetic!We're now done with the circuit in class but I'm not done with it. I have the parts to build the circuit, which I already have and got the same results as in class.Why why why why why!?!?!

author: Rerouter
author_rank: Super Contributor
timestamp: May 08, 2015, 11:33:05 am
content: so to clarify the values i put forward did not help? or even begin to get a secondary peak?

author: Marco
author_rank: Super Contributor
timestamp: May 08, 2015, 11:43:37 am
content: Try adding two emitter resistors to Q1 and Q2, about 10 Ohms should do it.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 08, 2015, 12:17:03 pm
content: Works for me - but not very well. The first pair of traces show the input at approximately 1.2V RMS. The top, blue line is the input, the lower green line is the collector of the second transistor. As somebody has pointed out, the mismatch between transistors leads to an asymetric waveform and it was necessary (in my case) to have an input level of about 750mV before the second peak appears. A large mismatch between the transistors could easily obliterate the second peak.The next pair of traces show the in input level raised to 1.48V RMS.

author: tec5c
author_rank: Frequent Contributor
timestamp: May 08, 2015, 12:45:26 pm
content: Quote from: Rerouter on May 08, 2015, 11:33:05 amI did not try your circuit in class, however I just built up your circuit (using BC547's instead of 107's).At first I thought you had cracked it. I had the output being fed into my spec. an. and I initially saw the fundamental (25kHz) and a second peak at 50kHz. A momentary sigh of relief. Until I increased the span and saw a whole heap of harmonics. So I hooked the output to my scope and saw it was being saturated and was closer to a square wave, which I guess makes sense with all the harmonics that had appeared.I have written an email to my professor and am awaiting his response to see what he has to say about the circuit. I will update once I hear back.Quote from: Andy Watson on May 08, 2015, 12:17:03 pmIs this a simulation or a scope capture?Quote from: Marco on May 08, 2015, 11:43:37 amI'll give this a try and report back.Thanks for the input everyone.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 08, 2015, 12:57:39 pm
content: Quote from: tec5c on May 08, 2015, 12:45:26 pmScope.

author: tec5c
author_rank: Frequent Contributor
timestamp: May 08, 2015, 01:05:46 pm
content: Would you be able to take a few pictures of your circuit please?

author: Rerouter
author_rank: Super Contributor
timestamp: May 08, 2015, 01:10:25 pm
content: So i sat down and tried to reproduce the expected result with a simpler circuit, only limitation is that the resistors must be better than 5% difference, the supplies must remain symmetrical, and the transistor gain must be within the same order of magnitude,Now i admit i could just be hitting the quirks of a simulator, but if you want to play in your own time, try this circuit, once again the signal needs some amplitude, but in this case its more like 3V or greater,Edit: Atleast to me, fitting a buffer looks to remove most of the higher harmonics, so see how doubler3 fares

author: Andy Watson
author_rank: Super Contributor
timestamp: May 08, 2015, 01:15:16 pm
content: Quote from: tec5c on May 08, 2015, 01:05:46 pmOMG! I'll have to tidy the bench so that you can distinguish itI'll be back soon.

author: tec5c
author_rank: Frequent Contributor
timestamp: May 08, 2015, 01:20:51 pm
content: Hmm I'll definitely have a play around with that circuit Rerouter. ThanksQuote from: Andy Watson on May 08, 2015, 01:15:16 pmHahaha, I know the feelingAlso, I thought I should include the output graph that was given to us with the AWR files (probably should have done this from the beginning, my mistake!)

author: Andy Watson
author_rank: Super Contributor
timestamp: May 08, 2015, 02:19:26 pm
content: The circuit is as your first post but without the emitter-follower output stage. I didn't have any 47R so I substituted 56R (and 130R for the 110R). The transistors are BC547B, taken fresh from their tape - so there's a good chance they are from the same batch. The caps are 4u7.On the left, the yellow wire is the input from the signal generator. The bottom rali is 0V. The top rail is +10V (red alligator). The black alligator clipped to the 1k resistor is the -10V. No decouplingAs you can see, I'm not set up for photography!