Radio Design 101 - Finishing the Receiver (Episode 6)

In this episode of Radio Design 101 we finish the receiver design by creating an IF amplifier and demodulator, and then assemble and demonstrate the final product. A limiting IF amplifier is used to address the 100 dB dynamic range of signals typical at a receiver's antenna input. The demod is a classic quadrature architecture that uses a mixer and phase shifter, and leverages what we learned in Episode 5. Details of design, construction, and testing are presented, together with comments on how the overall Radio Design 101 course material is applicable to virtually all radio design work, including modern RF integrated circuits.

[@staviq]

I randomly found this channel, and you just got a sub for the "if they lucky it won't work the first time". People are astronomically undervaluing failure. Repetition is not learning, it's memorising. Failure is where the real learning is.

[@MegawattKS]

Totally agree ! Sorry for the late reply. Thanks for the comments :-)

[@johnwest7993]

It might be a good idea to have each stage of the circuit powered up and tested as it's built. That helps the students learn how to set up the tests of each stage , to see what the signals look like, and to ensure that the entire unit will work when everything is finally wired together. That's what I did when I was helping a group of hams build 40 meter QRP transceivers. Everyone appreciated doing it that way.

[@MegawattKS]

Absolutely agree. Testing is 50% of development work. And of course the university class did that. I think when this series was being put together, I wasn't thinking enough about viewers who might want to actually build their own. Your point is an excellent one. For anyone building their own - they absolutely need to test from the ground up. Later, I did put all the course notes and assignments onto a website for those who wanted a deeper experience. Here's a webpage that has a full set of example project assignments, including directions for testing each. (these don't directly track what's in the videos, but many of the early assignments are close. ecefiles.org/rf-circuits-course-section-0/ Thanks for bringing this up.

[@johnwest7993]

@@MegawattKS, thank you for the link. I'm self-taught in electronics, but by watching your videos I feel like I'm auditing your class.

[@MegawattKS]

@@johnwest7993 Cool ! 73

[@philippeversailles2170]

Thanks a lot for these episodes.This is a great coverage. The mix between theory and application is perfect. I have learned a lot. Again thanks

[@MegawattKS]

Thanks for the comments. Great to hear that it is helpful !

[@tecnomod7]

Thank you for your videos I really appreciate them, from Spain ; saludos!!

[@MegawattKS]

You're very welcome. Best wishes from the US !

[@KerryWiegand]

Excellent series! Thanks for putting this presentation together and sharing it!

[@MegawattKS]

You're very welcome. Thanks for the comment and for watching the series all the way to the end !

[@emailuser3869]

Great video. Very much enjoy learning about RF circuits.

[@MegawattKS]

Thanks for the feedback. I started my journey long ago with a ham radio license and wanted to spread the knowledge I'd gained over the years of how the magic works :-) Glad you found it useful !

[@emailuser3869]

@@MegawattKS Getting started in RF circuit design, when starting with zero information, is nearly impossible. I think this series was extremely helpful. I'm recommending it to the other experimenters in the club. 73

[@VividSolutions]

I’m late to the comments as only just discovered your vids. I’ve been dabbling in RF design for 40yrs, having cut my teeth in Marconi’s microwave labs, here in the uk. Just wanted to say what a brilliant job you’ve done. That’s one of the most accessible, informative and fun intros to RF I’ve seen. Wish you’d been my lecturer all those years ago 😅 Thank you for sharing!!

[@MegawattKS]

Thank you for all the kind words. I started in RF and electronics long ago as well. Worked in a TV repair shop in high school and did CB and ham radio, all of which provided hands-on experience without all the calculus and such. I love the math too (once I got through a few semesters in college), but am grateful to have come at things from that other perspective/lens as well. As it turns out, students love building things too !

[@VividSolutions]

It’s spooky how we’ve got so much in common while living on opposite sides of the pond. I too spent my teen years in my local TV repair shop, watching and learning like a hawk. When the CB radio craze hit the UK in the early 80s I inadvertently became the town’s ‘rig doctor’, fixing radios and SWR’ing whips. Being hands on is definitely the way to learn and it keeps you humble! Always wanted a Corvette too 🤣 Nice to make your acquaintance sir.

[@soulrobotics]

"Feel like champion" haha, who else want to build this experimental radio? Thank for stimulate my curiosity!

[@jessishandsome]

thanks for giving us this amazing class

[@MegawattKS]

Thanks for the feedback. This was always my favorite course, with a strong hands-on component to bring out things that the real world throws at us :-)

[@ernestb.2377]

Great stuff. This invites to be studied and built 🙂

[@dixsusu]

Realy great work , i was building radios in my 17-20's years old but with no complex math .

[@MegawattKS]

Thanks. Similar here. I started working in ham radio in my teens, and working a TV repair shop as well. IMO that kind of early experience (without the math) lays a great foundation - an ability to understand things deeper due to the multiple perspectives, even when the math is added later :-)

[@rafusy]

Thank you!

[@cholan2100]

SA615 did perform better(relatively better sensitive) than my discreet build, although not to its full potential without all matching involved. It looks like i somehow broke the IF_AMP part of the chip(can't see anything on IF_AMP_OUT, but once connected to IF_AMP_OUT=>LIMITER_IN, there is output on LIMITER_OUT), and so only getting gain from LIMITER part. Looks like SA636 can handle 10.7 low Z filters directly, that should give compact build without the dang matching Inductors(risks of magnetic coupling).

[@BaldurNorddahl]

Thanks for the video series. I am going to try building my own but adjusted for 145 MHz 2 meter HAM band.

[@MegawattKS]

Nice. Thanks for the comments. For 2m work (narrowband FM), you may want to try one of the SA604A FM IF subsystems as shown at the end of the video (timestamp 32:40). Figure 6 shows this, preceeded by a second downconversion to 455 kHz so a narrowband filter can be used. It will also give the gain needed (FM stations are often strong enough to get away with what we did in the discrete IF amp, but 2m not so much :-) 73's

[@cholan2100]

Good one. Thanks. Fun with my demodulator built with 10.7 ceramic discriminator, it works if i power down VCC and drive input signal with 2Vpp. But collapses when powered up. Haven't had a chance to debug yet. Hopefully this weekend.

[@longjohn526]

That resistor and capacitor series thing on the output of the LM386 is a Zobel Network and very useful to add to a speaker crossover network to make sure that 8 ohm impedance speaker is actually at 8 ohms at the crossover point because if it's not it will crossover at another frequency and/or spoil the shape of the crossover rolloff

[@MegawattKS]

Interesting. I had not heard of Zobel networks. My understanding from the LM386 audio amp datasheet is that the series RC to ground is recommended to mitigate oscillation in the MHz range. I think what happens is the speaker inductance above audio frequencies causes instability in the opamp architecture used in the 386. So it helps maintain a good 8 (or 10) Ohms not only at the crossover point, but also at MHz frequencies 🙂

[@princetn]

Very Nice presentations. I really enjoyed learning quite a lot as I have been very interested in emitter but the demodulation receiver side was quite a mystery. I feel the most difficult part of this approach is really getting the whole thing to tune properly to your low frequency 10.7MHz. All of this is now sold in a single chip TEA----, AR1010 to name a few.

[@MegawattKS]

Thanks. Glad it was helpful! Stay tuned for the "Epilogue" episodes (hopefully soon). The epilogue 1 episode will look at performance, and go into some of the limitations of low cost FM radios including fully integrated designs. (Front-end filtering is relaxed in these and weak stations get blocked by strong ones...)

[@souravpanchal7268]

Thanks a lot sir very honestly

[@MegawattKS]

You're certainly welcome. Thanks for the comment.

[@user-pi1ih6wn3j]

Thank you for this series of videos, this material on RF design is one of the best I've come across. Can I ask you a question about the design of the IF amp? How did you calculate the values of the coupling and bypass capacitors C6, C3? Thank you and congratulations for such good videos.

[@MegawattKS]

Thanks! Glad it is helpful. The short answer for those capacitors is to just select a "large value" like 1 or 10 nF. That's "large" in the sense that at 10 MHz, 10nF is on the order of about an Ohm of reactance. So it makes a good low impedance connection. The longer answer involves looking at the signal impedances on the nodes it connects to. For example, the input coupling capacitor sees about 300 Ohms source, and 300 Ohms load impedance - so about 600 Ohms total. As long as it's reactance is much less than that, its good. So even Xc = 10 Ohms that the 1nF gives will work OK. The bypass capacitor is typically bigger because it sees an impedance of 1/gm looking into the emitter. So that's maybe 30 or 50 Ohms (depends on bias current/etc). So that's why 10 nF was used there. NOTE: I just noticed the schematic doesn't show a bypass on the power supply node. A bypass is needed there too. I think I just put a 10 nF there (again - the value is not critical - as long as Xc

[@fjs1111]

Awesome, maybe I should have gone to school at KS

[@MegawattKS]

All schools are good - but I especially like K-State's focus on the teaching side of things. Did research too, but teaching seems to be the most important mission and I always tried to give the students priority...

[@zuhairhasan8392]

great thanks

[@MegawattKS]

You are welcome!

[@jlog7395]

Thank you for a great series of videos. A very unique balance of instruction and theory on what can be a complex topic. Are you still teaching at KSU? I received my BS and MSEE from there back in the mid 80's. Great school.

[@MegawattKS]

You're welcome. Did you take EECE 662 when you were there? I'm retired now, but did the course for 20 years. Sadly this course is not being done there now - although the microwaves and antennas one is. So I thought it would be good to memorialize it, by putting this together for when it or something like it is offered again. With the NanoVNA and TinySA, it's finally possible to do this level of design (up to 300 MHz or so) at home. A good ending hopefully to a career that was inspired by CB and ham radio long ago :-)

[@jlog7395]

Unfortunately I didn't take EECE662; I wish now that I would have added your class. I'm also recently retired from a career as a digital / analog hardware designer then later more DSP. I always wanted to add RF design to my repertoire hence finding your channel. Thanks again for the great content.

[@kassisaba]

Hi, thanks for the series! Well laid out and presented. About the phase shifter network, correct me if I'm wrong, but it seems to me you calculated the values for L1 & C2 based on reactance of ~26 ohms instead of R/Q = ~56 ohms, giving the circuit a BW of ~188kHz instead of 400kHz. Possibly a wee brain fart, mixing the X with the Q? I guess the correct values would be roughly 830nH and 260pF? Cheers!

[@MegawattKS]

Good catch! Yes - I think you're right. The components shown do give 26 Ohms rather than the 56 mentioned in the video. However, there is a fortunate extra factor that does bring it back closer to a bandwidth of 400 kHz (I can't say I did it intentionally ;-) ). If we assume the Q of L1 is 50, then it presents another 1300 Ohms in parallel with the 1.5K, so the Q is cut in half and we're back to 400 kHz :-) Again - not sure if I did that intentionally or not. Thanks for the careful check !

[@rjordans]

Thanks for the great series! Any plans for the tx side as an extension?

[@MegawattKS]

You're very welcome. Not sure on the transmit thing. I've mostly stayed away from that since not everyone watching has a license - and even low-power unlicensed FM transmitters often violate the rules on output power/etc. Plus they could potentially tune outside the FM band - which is definitely not a good thing. Good idea though. Transmitters are an important part of ham radio. Maybe I could do something as part of an amateur radio focused video on frequency synthesis ? Will give it some thought.

[@BaldurNorddahl]

@@MegawattKS +1 to a video on tx. Some of us _are_ licensed HAMs :-)

[@PepeAlipido-ph9dz]

very good class. I am sorry that the video does not have the translation

[@MegawattKS]

Thank you. Were you able to do translation on Episode 5 (the previous episode)? I see that I cannot select "CC" on this Episode 6, but I can see it on Episode 5. I will try to fix this on Episode 6. But I do not understand why KZbin isn't showing them. All the settings seem to be the same on Episodes 5 and 6. Thank you for letting me know.

[@PepeAlipido-ph9dz]

​@@MegawattKS Hello, thanks for answering. Certainly in all other episodes except 6, translation can be activated. Although you pronounce very well, my English is not enough to understand in detail everything you explain. I sincerely believe that the series of videos on radio design that you have shared is something very good for those who like to design and are not satisfied with just building other people's designs. Thanks again for sharing your knowledge.

[@danielrodriguez1224]

Hi! great video series. Sorry for the ignorance, but I wanna know if it would be too difficult to build an am receiver working for 146MHz using all the information you provided along the playlist, thank you!

[@MegawattKS]

(edited) Technically, the main changes/additions I can think of would be the need for AM demodulation of course, adding some form of AGC, and you probably need a more narrow IF filter. A few years in the university course we did some variations on the FM broadcast theme to be able to receive NOAA weather radio (at around 151 to 156 MHz). The NOAA signal bandwidth was also only about 5 kHz, so we had to find a suitable IF filter - and I think we went with a dual-conversion receiver architecture. But the signal was still FM (narrowband). For AM, a different demod would be needed. The demod shown in this series wouldn't work because it's specifically for FM. I'll look for options and add a second reply.

[@MegawattKS]

This is a follow-up to the reply above (now edited). I forgot to mention the need for auto-gain-control (AGC) initially as another factor, and when I went looking for an AM demod I realized we were still doing (narrowband) FM. Since that reply was written, I've searched for commercial ICs that might be useful for AM demod. Sadly, I haven't found any. I'm starting to think that AM has been left behind in more ways than one. In the course, we start with AM, but then moved to FM and digital modulations that are more commonly used now. For AM, we discussed the classic envelope detector, but also "synchronous" detection using a mixer. Alternatively it might be fun to try using a modern "linear detector" IC like this as a demod www.ti.com/lit/ds/symlink/lmh2120.pdf (just a wild idea as that is not the intended market for this little guy). Now-adays, I think most people would opt to build the receiver out as far as the filtered IF and then digitize that and do the AM demod function in software, perhaps.

[@sullivanzheng9586]

Signal split - frequency dependent phase delay - phase detection sounds like an general idea. Possible to use a "digital way" to do it? like: Signal split to two - use two or more NAND gates to delay one split of the signal - compare phase difference of these two signals, using XOR gate.... If I use logic gates with short rise time (like 74LVC series, rise time ~2ns) I think it can handle 10.7Mhz FM IF signal.

[@sullivanzheng9586]

OK I found a bunch of FM demodulators based on logic gates. Some designs are really simple and really suitable for downstream digitization.

[@MegawattKS]

Nice ! Yes - that sounds like it should definitely work. The XOR is acting as a 1-bit digital multiplier, I guess :-)

[@sullivanzheng9586]

@@MegawattKS Indeed. It is amazing to see how analog circuit can be connected to digital circuit without ADC -- just by introducing a variety of 1-bit computations.

[@sullivanzheng9586]

​@@MegawattKS I just realized some interesting equivalence: if you consider XOR as a 1-bit digital multiplier, then it is a mixer, and the whole NAND phase delay -> mixer is a quadrature detector. But you can also consider NAND phase delay -> XOR phase comparison as a pulse generator, which generate pulses equivalent to those with a "zero crossing detector". Therefore this NAND-XOR FM detector is equivalent to quadrature detector (a digital one tough) and zero crossing detector (which is digital by nature) at the same time. Is that a coincidence or there is some deep link underneath?

[@robballantyne3]

Thank you so much for putting in the effort to put this series together. It can't have been a little amount of work, even if you taught it as an EE course at University. It is clear you viewed your audience as being at least slightly adverse to covering the mathematics involved. What would you suggest to someone who has the requisite mathematical skills? Is there anything like this course with the math that you're aware of? It's probably not easy to go and find a course at the local University to sit in on. I would expect that also knowing things mathematically would give one a much deeper understanding of whats going on. Mathematics comes in many levels but I would think the requisite math would be: differential equations (ODE, including Laplace transforms), Fourier Series and Transforms, and Complex Analysis. Linear Algebra is important in basic circuit theory but I didn't seen much of a requirement in what you did present. Would those be the list of requirements for your University course? Thanks again!

[@MegawattKS]

Thanks for the comments. Yes - I was trying to minimize the math as much as possible - partly because of a varied audience, but also because much of RF design doesn't require it. In my design career before returning to school for the PhD so I could teach, I generally viewed circuits and their functioning using "compiled knowledge" as presented. Indeed, my research was even done this way. But you're absolutely right - with math, a much deeper understanding of why things work is obtained, and for some areas is arguably necessary. And some sub-fields (pun intended) like EM theory are difficult to do without it. I like your list of requisite math for the underpinnings. Everyone comes at stuff differently, and for my own studies, I had Complex Analysis in my MS program and enjoyed it, but honestly never used the deeper aspects. Of course complex numbers to represent and simplify analysis of linear systems is key due to the 2-D basis functions involved in things like Fourier theory. And as a circuit designer, I would actually maybe elevate Linear Algebra in importance, as it underpins the analysis techniques including superposition that is at the core of breaking the solutions into things like DC and AC parts. And also for solving/understanding/designing circuits with multiple inputs.

[@cholan2100]

any easy way to say if SA612 has been burnt? The attenuation through SA612 in demodulation circuit is so high, i am only able to see a tiny demodulated signal(frequency matches modulation frequency). Don't have much of spare SA612s at the moment to try around.

[@MegawattKS]

Its probably something in the build. Have you tried checking the DC bias voltages on the pins? I have the SA602, but suspect they are about the same. Here's what I measured for the DC voltages on the pins when powered by 5.0V: Pin1 and pin 2 DC bias should show about 1.4V (I got 1.38). Pin 3 should be zero (ground). Pins 4 and 5 should be around 3.9V. Pin 6 = 5.0V, Pin7 = 4.3V, and Pin 8(Vdd) should be 5.0. If those DC values look reasonable, then might have to inject some signals and see if signals are getting in as well. What frequency deviation are you using? Also - have you tried changing the frequency away from 10.7 center to see if maybe the quadrature network is resonanting somewhere else? In the class, we would often find it resonant a few MHz away and have to retune. (the phase slope is very small when off resonance and so is the output amplitude if it's not matching the center freq of the FM test signal).

[@cholan2100]

@MegawattKS yeah, it looks the biases are off. Pin1 at 500mV. I am using a 10.7 Ceramic discriminator salvaged from old radio, so resonance shouldn't be off much. It appears variable inductors are impossible to find anywhere around here :( So have to try the toroid t50-6 with 10 turns, but ceramic discriminator is probably alright.

[@cholan2100]

@@MegawattKS frequency deviation= 75khz, but i am still just testing with 5khz tone for modulation, not real audio signals.

[@MegawattKS]

Any DC path to ground external to the chip on pin 1? Also - is this without an input signal (input signal could influence DC measurement). Yeah - I was lucky to find a variable inductor in my box. In class we used 9 to 50 pF variable caps, but I didn't have any of those - and that range is not really enough sometimes anyway, given the 600'ish pF in the LC resonator. Your ceramic resonator sound like a more robust solution (assuming it matches the IF filter frequency well :-) )

[@cholan2100]

@@MegawattKS I did once wire it directly to function generator without DC block capacitor, but not sure if it got ground path. Now i swapped SA612 from another old prototype board, but the output pin's bias are wrong. Hard to beleive that such simple IC wiring like SA612 gets breaking. Wait for my new SA612's to get delivered, to redo the build. It works fine on LTSpice simulator with SA612 model though.

[@pilobond]

Thank you for these great videos. I get inspirations for further experiments and learning. Only if I had some time though... :) VA3GPJ

[@MegawattKS]

Thanks. I know what you mean about the time issue - although I'm retired now, so I do have some finally ;-) 73's