Why does this AM Mixer not work?  - Page 1
Forum section: electronics / beginners

author: Abstr7ct
author_rank: Regular Contributor
timestamp: November 30, 2014, 04:06:30 pm
content: I've an assignment in which I'm required to build an AM mixer circuit. Luckily, a friend of mine, who built the same circuit, still has the schematic of the circuit which is built around a CA3080 transconductance op amp. As shown in the attached schematic, a carrier of 50KHz is supposed to be modulated by a 1 KHz information signal, both of which are sin waves taken from two different function generators. I built the circuit with the same layout of the attached breaboard image, connected everything and there was no output. I kept trying for hours with no positive results.To explain the breadboard image. The black wire is the ground of the whole circuit, that is, the ground to which the ground of power supplies, functions generators, oscilloscope and circuit components are connected. The orange wire is where +6V is connected and the purple wire is where -6V is connected. The brown wire is where the 50KHz carrier is connected and the green wire is where the 1KHz information signal is connected. Finally, the blue wire is where the oscilloscope is connected.Is there anything wrong with the wiring or layout? any tips on why it doesn't work at all?

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 30, 2014, 04:20:35 pm
content: Not to say that it is the case here, but I have seen breadboards where the rails (the top and bottom groups of 5 connections) are not connected to each otherIf that is not you problem, did you measure? Care to share the results?

author: Abstr7ct
author_rank: Regular Contributor
timestamp: November 30, 2014, 04:23:31 pm
content: Quote from: PA0PBZ on November 30, 2014, 04:20:35 pmI considered that, but a voltmeter revealed that they're connected.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 30, 2014, 04:30:57 pm
content: Quote from: Abstr7ct on November 30, 2014, 04:23:31 pmThen why did you connect the ground across the left and right side, but not the + and -6?BTW, you also left out the decoupling caps on the power pins, but that should not make it dead.

author: Abstr7ct
author_rank: Regular Contributor
timestamp: November 30, 2014, 04:35:35 pm
content: Quote from: PA0PBZ on November 30, 2014, 04:30:57 pmI did that just to make sure that the whole rail is grounded, and yet I'm sure that they're connected. In fact, I just did a continuity test for all the 4 rails and found that all points are connected from from far left to far right.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 30, 2014, 04:41:48 pm
content: Quote from: Abstr7ct on November 30, 2014, 04:35:35 pmAllright then, just to make sure...Did you measure the DC and AC on the different pins of the opamp?

author: Abstr7ct
author_rank: Regular Contributor
timestamp: November 30, 2014, 04:48:10 pm
content: Quote from: PA0PBZ on November 30, 2014, 04:41:48 pmI measured the AC voltage on the output, the 4.7K res, and it showed 0. Pin 7 gives +6V and pin 4 gives -6V. I tried to change the resistors and 0.1 uF coupling capacitor, and there was no output. I replaced the op amp by another one of the same model, but it gave no output.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 30, 2014, 05:08:44 pm
content: Please measure all the pins, DC and scope and tell us what you see.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 30, 2014, 05:10:19 pm
content: Is your "information" signal D.C. coupled? These transconductance amps need current flowing into pin 5. Also, be careful not to exceed the specified current (2mA - I think), these chips will self-destruct with incredible ease.

author: Rory
author_rank: Frequent Contributor
timestamp: November 30, 2014, 05:11:43 pm
content: Have you read AN6668?

author: Abstr7ct
author_rank: Regular Contributor
timestamp: November 30, 2014, 05:35:20 pm
content: Quote from: PA0PBZ on November 30, 2014, 05:08:44 pmThe problem is that I don't have function generators in my small workshop at home. I rely on the faculty lab for this experiment. However, I tried something random with what I have right now. I fed the 1KHz test signal of the oscilloscope to both the carrier and information inputs. I probed the output using the oscilloscope and it showed the attached waveform. I'm not sure if that waveform indicates anything positive.Quote from: Andy Watson on November 30, 2014, 05:10:19 pmIt's, isn't? there's no capacitor blocking the path to pin 5. There's only a current limiting 10K resistor. And you're right, the datasheet says that the absolute maximum rating of the bias current is 2 mA, but the 10K will make sure that it won't reach to that value, wouldn't? I'm putting these question marks because I want to make sure that I'm stating factual data. I'm not expert in this especially in this kind of circuits, at least not yet.Quote from: Rory on November 30, 2014, 05:11:43 pmNo. I don't want to show that I'm lazy, but this exact circuit worked for two persons who I know before. There's just something wrong or missing, or something is damaged?

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 30, 2014, 05:52:27 pm
content: Apart from the fact that your 1KHz from the scope can't drive the input 47 Ohm (so you see the 0.1 uF cap integrating the signal) I can't see much wrong with the output.Modulation would be hard to see with the 2 equal signals. You said that there was no output, but there clearly is?

author: Abstr7ct
author_rank: Regular Contributor
timestamp: November 30, 2014, 06:01:29 pm
content: Quote from: PA0PBZ on November 30, 2014, 05:52:27 pmWhy wouldn't it be able to drive the 47 ohm? and isn't the carrier signal going through some type of high pass filter? illuminate me, please. An yes, there's an output now, but I'm not sure why. I kept trying today for hours by feeding a 50 KHz carrier and a 1 KHz modulating signal, and I kept getting nothing at the output. Since apparently there's nothing wrong with the circuit or my breadboard layout, I may have to recheck tomorrow at the faculty lab. I will try to work on another bench or maybe even another lab just in case.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 30, 2014, 06:12:05 pm
content: The test signal is designed to drive a 10M scope probe, so the chance that they created a circuit to drive a 47 Ohm resistor are close to nothing. The 0.1 uF cap will work as a sort of high pass filter, but it is there to AC couple the input (which you could see as high pass...) But yes, the cap has a reactance of about 1K5 at 1KHz so that does not make things any better, in combination with the 47 Ohm resistance that is another 1:30 divider.The best thing to do now is take it back tomorrow and see if there is anything wrong with the input signal(s), or even the power supply there.

author: Rory
author_rank: Frequent Contributor
timestamp: November 30, 2014, 06:36:20 pm
content: I don't mean to be offensive when I ask, what is the purpose of this exercise? Is it to demonstrate a non-specific circuit that amplitude modulates a 50Khz signal, or is it an exercise to educate you on the operation of the CA3080 OTA?Please read AN6668. How can you build a circuit and expect it to work without understanding how the active device works?Here is a link:http://www.intersil.com/content/dam/Intersil/documents/an66/an6668.pdfIn it, you will find a nearly identical mixer circuit with less complexity.

author: Yansi
author_rank: Super Contributor
timestamp: November 30, 2014, 06:40:32 pm
content: Quote from: PA0PBZ on November 30, 2014, 05:52:27 pmIt is a derivative. Not an integration!There should be a voltage divider, to smallen the input signal for the OTAmp, to lower the distortion. This C-R connected input was done wrong. Please, search for and read application notes about using otamps first.And also note, that the bias modulation input is referenced to V-, not actual ground.

author: Abstr7ct
author_rank: Regular Contributor
timestamp: November 30, 2014, 06:53:43 pm
content: Quote from: Rory on November 30, 2014, 06:36:20 pmThe purpose is merely to demonstrate that you were able to build a circuit that does AM mixing. However you do it, isn't in anyway important. Understanding how it works isn't required either! just build it, show the output at the scope and you're done! that's why I'm trying not to concern myself with the application notes of the device.Quote from: Yansi on November 30, 2014, 06:40:32 pmHow is the bias modulation input referenced to V-, which is in this case -6V? where's that shown? if there's something wrong with the breadboard layout, please pinpoint it to me.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 30, 2014, 07:03:19 pm
content: Quote from: Abstr7ct on November 30, 2014, 06:53:43 pmAm I the only one not seeing the value of that exercise?Quote from: Yansi on November 30, 2014, 06:40:32 pmAllright, allright, but it looks like an integral

author: Yansi
author_rank: Super Contributor
timestamp: November 30, 2014, 07:12:44 pm
content: And also, this is not a MIXER it is MODULATOR. These two are different circuits for different purpose. If you need to build a mixer, go for NE602.If you are asking me, where did I get where is the bias input referenced, you have not even opened the datasheet for CA3080.http://pdf.datasheetcatalog.com/datasheet/intersil/fn475.pdfInternal schematic digram on page three. Did you see that? Now do you know where is the bias input referenced? goodAnd also, you completely missed to use any decoupling capacitors in your breadboarding schematics. Due to natural behavior of OTAmps, lack of decoupling is an issue. OTAmps are usually quite fast acting circuits, the cany easily start to self-oscillate.

author: Rory
author_rank: Frequent Contributor
timestamp: November 30, 2014, 07:13:14 pm
content: Quote from: PA0PBZ on November 30, 2014, 07:03:19 pmLooks rather pointless to me. Maybe to introduce the student to breadboarding techniques?Personally I think OP is getting cheated out of an education.

author: Abstr7ct
author_rank: Regular Contributor
timestamp: November 30, 2014, 07:55:26 pm
content: Quote from: Yansi on November 30, 2014, 07:12:44 pmThanks. But I would still connect the 1 KHz modulating signal between the 10K and the common ground of the circuit, or I'm making the mistake again? that's what the circuit schematic, even in the application notes, show. I will add these decoupling capacitors next time.

author: Yansi
author_rank: Super Contributor
timestamp: November 30, 2014, 08:02:07 pm
content: That is actually correct, because it gives you half the maximum bias at 0V input. I only pointed that out, so you should know now, that the circuit is supply voltage dependent in this configuration.

author: Abstr7ct
author_rank: Regular Contributor
timestamp: December 01, 2014, 12:03:40 am
content: Maybe one thing that I could have done to diagnose the circuit, is to check with an ammeter if there's current being drawn by the power supply inputs of the op amp. If the circuit inside the IC, a group of BJTs and diodes, is doing some work and is activated by the input signals going into the bases of the inner transistors, then there would definitely be some current going into the supply pins. I did this just now with the inputs fed from the test signal of the oscilloscope and I found that the supply pins draw a current on the range of a few milliamps.I'm excited to try this again tomorrow. I hope it works.

author: Abstr7ct
author_rank: Regular Contributor
timestamp: December 01, 2014, 04:41:12 pm
content: I tried the circuit today and it worked. I attached three images taken from the output on the oscilloscope. They show different waveforms due to the changing modulation index. I kept varying the 1 KHz modulating signal amplitude to change the index. The peak of the positive envelope of the first image reveals, from what I understand, a saturated op amp, doesn't it?Also, when I kept varying the 10K pot, I didn't notice any difference on the output. I started to question its use in this circuit. What's it supposed to do?Quote from: Yansi on November 30, 2014, 07:12:44 pmIsn't a mixer and an AM modulator essentially the same? They both multiply the carrier and the modulating signal to produce the difference and the sum of their frequencies.