Gilbert Cell - RF mixer - Page 1
Forum section: electronics / rf-microwave

author: reimius
author_rank: Newbie
timestamp: May 03, 2020, 06:38:49 am
content: Hello everybody,first I want to apologise if I am making any etiquette mistake as this is my very first post in this forum.So the topic I want to discuss about I would like to experiment with RF mixers for radio frequencies (up to 200MHz). After some research I found that the NXP SA602/SA612 use to be one of the most common ic for this purpose. Since all my work is done in solderless breadboards, my preference is to use a DIP package. However seems that the DIP for that ic is obsolete and only SO8 is available. I would like to know if there are any compatible alternative? I found Gilbert cells like the HFA3101 and the SMA5101, they are not compatible but at least provide matched bjts, but again I have to discard them because only there are surface mounted versions. Should just go for npn transistor array ics (LM3045,ULN2003) or differential pair array ics? or just discrete nps (so unmatched)?All this leads me to another question. Should just forget this kind of mixer and I so just go for a custom double-balance diode ring? (I am always a bit scare about inductors and npns are so friendly ...)Thanks a lot,Javi

author: ogden
author_rank: Super Contributor
timestamp: May 03, 2020, 07:16:31 am
content: Better use SOIC to DIP adapter. SMD soldering is not that hard. If you are not sure about your soldering skills - get some junk ICs and practice.

author: tggzzz
author_rank: Super Contributor
timestamp: May 03, 2020, 07:42:48 am
content: 200MHz on a solderless breadboard? Good luck with that!When designing your circuits, don't forget to take account of the parasitic inductance of the hookup wires (1nH/mm) and parasitic capacitance between socket strips, and variable resistance between contacts. You will end up spending more time debugging the breadboard than debugging your circuit.A better choice is "manhattan" techniques; the masters use it.BTW, welcome, and enjoy your time here. Your first post is better than most

author: ogden
author_rank: Super Contributor
timestamp: May 03, 2020, 07:51:40 am
content: Quote from: tggzzz on May 03, 2020, 07:42:48 amRight. Other option - experiment at lower frequencies like 1MHz or so that are manageable on breadboard. It opens option to use low end scope as well.

author: tggzzz
author_rank: Super Contributor
timestamp: May 03, 2020, 10:38:52 am
content: Quote from: ogden on May 03, 2020, 07:51:40 amIt doesn't matter what yoursignalfrequency happens to be. What is important is whether circuit elements have gain+feedback at any frequency. It is perfectly possible to have an audio circuit that oscillates at 200MHzRemember the old saying: "oscillator's won't, amplifiers oscillate"

author: RoGeorge
author_rank: Super Contributor
timestamp: May 03, 2020, 11:06:31 am
content: Using a breadboard at 200MHz is not the way to enjoy RF experimenting. Too much parasitic capacitance, and parasitic inductance, and parasitic induced RF in the jumping wires, either from the neighbor components or from outside of the circuit.There are some other DIY prototyping techniques known to be working very well for RF, but they involve soldering:

author: profdc9
author_rank: Frequent Contributor
timestamp: May 03, 2020, 03:29:17 pm
content: If you want to do dead-bug style with a DIP-8 package, I will mention that I have successfully purchased NE612 and NE602 DIP-8 chips on ebay that work as advertised. I tend to use the SOIC-8 version just because the frequency response is better and I have my PCBs made by commercial houses so that it's relatively easy to get everything arranged the way I want to without much trouble at all.

author: Kerlin
author_rank: Regular Contributor
timestamp: May 04, 2020, 12:50:19 am
content: Wondering about his original question on Gilbert cell Vs diode ring mixer.The diode ring mixer is supposed to have much better third order harmonics figures.But I what find in practice with the SA602 is it seems the overall RF functionality is ok.Also does anyone find any difference between 602 versions.

author: reimius
author_rank: Newbie
timestamp: May 04, 2020, 05:19:33 am
content: Thanks a lot for your marvellous replies. I really appreciate all of them. So from your feedback I will try to get to a conclusion, or at very least to have a starting point:First, frequencies will determine board construction techniques from your suggestions: bug style, Manhattan, breadboard, soic to dip, etc. (Thanks for the video references, very illustrative). Since my hand skills are really poor (I really try hard, but I am clumsy) and higher frequencies requires more careful construction andawarenessof "hidden" parasitics, I should first start with lower frequencies, let´s say, bellow 1MHz, so I can get comfortable with the common RF building blocks, their characteristics and how different blocks can be chained to get more complex functions.On the other hand, Kerlin raised the question about the performance about 602 versus ring and differences between different versions of the 602, I guess you were thinking on the 612. I was wondering about both questions as well. So about the second I found that nowadays they are the same. When the 612 came out, that was an update of the 602, and 602 was retired. Since people still was trying to get the old 602, Signetics decided to sell the 602 again, but just as a rebranded 612 (Ref.:http://dspview.com/viewtopic.php?f=18&t=166). I am right now inspecting the NXP 602 and 612 datasheets and even if they are different files, I cannot find any difference besides file formatting, characteristic curves are the same (same identifiers) and only spotted a mismatch in the maximum current Icc in the "Static characteristics" table, where 602 draws up to 2.8mA and the 612 up to 3.0. Since the remaining values are the same, can be an errata?About the differences between diode ring and Gilbert cell, Kerlin mentioned diode ring performs better in terms of IIP3. I cannot discuss further on this as I have not enough knowledge, but I would like to crunch some numbers to get that point. On the other hand, all what I can guess to this point is that Gilbert cells should be preferred for system-on-chip since transistors can be done way smaller on silicon that inductances. On the other hand Gilbert cell must work on the linear region so biasing will mean the need of some quiescence current. Last, I draw a ring and a Gilbert cell in LTSpice and the output signal of the Gilbert cell looked fantastic, but the ring one I noticed that signal didn't look nicely rounded at the peaks, but "squared" so tons of harmonics will be generated. Please, note that these are my guesses so I could be totally wrong.By the way, my scope is an old Hameg-205 from my dad, I am at this moment that it will work well no further than 20MHz. Another good reason to stay in the low frequencies.Again, thanks a lot for your help. I hope in the future contribute to the forum, but for the time being I have a long road of learning and take off the fear of troubleshooting my circuits and break things!

author: tggzzz
author_rank: Super Contributor
timestamp: May 04, 2020, 07:15:28 am
content: It is good to see a posterlistening toandthinking aboutthe repliesYou mention being clumsy; unless there is an underlying condition, it is worth realising that we all started out being clumsy, and only improved with practice. Sometimes the improvement was rapid, e.g. when we instinctively tried to catch a falling soldering iron.You may need sight aids for small components. Those areverypersonal and you will only find out what suits you by trying them. I'd start with something cheap like a head-mounted visor with multiple interchangeable plastic lenses.Finally, in terms of mixers you may like to look at "Tayloe mixers", since they can be homebrewed.

author: ogden
author_rank: Super Contributor
timestamp: May 04, 2020, 09:02:28 am
content: Quote from: reimius on May 04, 2020, 05:19:33 amDon't bother. They are the same. Buy new SA612 from reputable supplier, avoid random eBay/ali sellers because you may get "refurbished" crap desoldered using open-flame torch.QuoteIf you use square wave for LO, you will get tons of frequency mixing products - as expected. Proper filtering of mixer output is as important as mixing itself. Further info:https://www.electronics-notes.com/articles/radio/rf-mixer/rf-mixing-basics.php

author: mio83
author_rank: Regular Contributor
timestamp: May 04, 2020, 07:52:42 pm
content: Hi all, interesting thread!@reimius,I am in a very similar situation (I think): I am a weekend hobbyist who is trying to learn about RF and mixers.After years of very basic things (and lots of youtube videos, especially of W2AEW who was linked above) I have decided to step up a little bit in the hobby: my goal is to build a basic etherodyne radio from bottom up, designing and testing everything by myself.This said:I have chosen, to experiment a bit, the MCL SRA-1+ mixer from Minicircuits because it has easy to manage pinsI also have a very basic 20 Mhz analogue oscilloscope like yourself.I decided to invest some money on a 2nd hand spectrum analyzer because I think it's a very useful tool for learning and experimenting with mixers (HP 8591E)I also decided to invest on a waveform generator with 2 outputs (up to 30Mhz): the idea is to use one ouput for the LO and the other ouput for the RF, and anlyzer the IF with the spectrum analyzer. I bought the Siglent 1032X.I got my HP 8591E about one month ago and I received my Siglent 1032X only 2 days ago and today I finally could start my experiments!!!!I am quite excited. My background is in Computer Science, and I find electronics (and RF) much closer to Physics, so everything feels like a Lab experimentI share some of what I do in my channel if you want to interact:have fun!

author: Bud
author_rank: Super Contributor
timestamp: May 04, 2020, 08:58:45 pm
content: Get a Minicircuits SBL-1 passive mixer, it is a beast. Requires higher LO level but also can work with higher input signal levels than SA612.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 04, 2020, 09:49:21 pm
content: Why not just wire your own diode ring mixer?Almost any recovered toroid will do, including the ones from PC power supplyies, and it doesn't even has to be a toroid. Any transformer will work, especially at low frequency and strong signals. Even a thick rusted iron nail should work as a core. Any four diodes will work as ring detectors, too, not as good as identically paired and thermally coupled U-I characteristic diodes, but good enough to see the expected results.If no signal generator is available, any PC with a sound card can generate two frequencies, in order to simulate the signal (with or without modulation) on one channel, and the local oscillator on the other channel, but at much lower frequencies, in audio. Using audio instead of RF is also interesting because the signals can be heard, so no oscilloscope required, only some headphones and good ears (or maybe the audio in of the same sound card and a visualization/oscilloscope or FFT/SA software).The less we have, the more we learn.

author: mio83
author_rank: Regular Contributor
timestamp: May 05, 2020, 12:01:53 am
content: Hi RoGeorge, thanks for sharing these nice videos! W2AEW is my favourite channelI agree that doing things with less is better!I will try to build thisQuote from: RoGeorge on May 04, 2020, 09:49:21 pmFor some results above sound frequencies you probably need fast switching diodes.In the video W2AEW is using Schottky diodes (1N5711).One point he makes towards the end is interesting to me.He says that we need to be careful when filtering the IF because filters reflect rejected frequencies back to the mixer, and this makes sense to me.He says that, due to the above reason, we need to "properly terminate" the RF before applying a filter.I imagine i could do that with a opamp (as voltage follower) in parallel with a 50Ohm to ground.Buthow todo that with just resistors/capacitors/inductors?

author: Kerlin
author_rank: Regular Contributor
timestamp: May 05, 2020, 12:07:39 am
content: Building your own DBM works well.It is best to have a large selection of diodes on hand so you can match them, for HF 1N4148s work very well.The matching is done buy getting a large selection of diodes and testing their forward drop voltage and selecting ones that are the same.The commercial ones such as the SBL-1 provide better figures. There is even the TAK-3H which has a larger dynamic range.It is interesting that an excellent performance receiver can be built using only a DBM and putting all the gain in the audio section.See this pdf-https://www.arrl.org/files/file/Technology/tis/info/pdf/9208019.pdfThey can be even simpler do a search on Youtube and check out "VK3YE Beach 40".You can see in that he uses only a DBM , two transistor oscillator and two transistors in the audio stage.He demonstrates using it over vast distances receiving low powered radio stations, it is even doubles as part of the transmitter section.This is one of thoseIf you are interested in building such things and testing them I would highly recommend getting a Ham radio license, it enables you to experiment live on air.There is help available from local radio clubs and online.I work as a radio engineer in Defence, Avionics, and in Asia designing consumer products and I find it gives me a great advantage over others.There are many projects and work with Ham radio and Arduino, so as a programmer I am finding it a hobby with no end and combined with continuing study with a few courses at at College it really boosts my abilities and career.It does however compete for my time with my other hobby of sports cars.One one recent course the I was lucky to have the head of the electronics department and when asked about Gilbert Cell Vs Ring mixers he had an almost violent reaction and said that the Gilbert cell was far inferior receiver front end to to using a DBM .This is because one of the key things about a receiver is its dynamic range and the DBM is far better.

author: mio83
author_rank: Regular Contributor
timestamp: May 05, 2020, 08:13:22 am
content: Quote from: Kerlin on May 05, 2020, 12:07:39 amWow this was a fantastic read, it also addresses the question I made earlier (how to properly terminate the IF).He uses a "multiplexer" kind of solution. Very nice indeed, thanks for sharing.

author: Kerlin
author_rank: Regular Contributor
timestamp: May 05, 2020, 08:35:30 am
content: Yes he not only did a good design but also wrote a great article.Read it quite a while ago and though I would post it as it answers most questions.The reason I also posted VK3YE effort was it is very good for its simplicity.He just wants a minimalist tranceiver that doesn't draw much current that he can take on his beach outings.He does tend to spend most of the time covering the transmit and then races through the receiver at the very end, recommend you need to replay that last part on the receiver a few times.

author: ogden
author_rank: Super Contributor
timestamp: May 05, 2020, 04:17:12 pm
content: Quote from: mio83 on May 05, 2020, 12:01:53 amIf you want filter with good match over wide frequency range - use broadband lowpass/hipass diplexer as filter, terminate port where unwanted band goes.

author: w2aew
author_rank: Super Contributor
timestamp: May 07, 2020, 01:47:18 am
content: If you're interested, I did a couple of videos on the Gilbert Cell:and

author: profdc9
author_rank: Frequent Contributor
timestamp: May 07, 2020, 02:55:46 am
content: One question I have had about the Gilbert cell is how the transition frequency of the transistors used to make the Gilbert cell relate to the maximum usable frequency. For example, the 2N3904 you use have a transition frequency of 300 MHz. Did you test the conversion gain of the mixer to see what frequencies you could get with it?

author: reimius
author_rank: Newbie
timestamp: May 07, 2020, 12:40:19 pm
content: All of you are amazing, thanks a lot for all your support. You really created and great community. Sorry for no answering sooner. All your answers are full of information and takes a lot for me to digest and I feel that in comparison my replies are empty of interesting stuff. Still, I wanted to give some reply.First, I tried Rick Campbell article, but is far beyond my knowledge.Quote from: Kerlin on May 05, 2020, 12:07:39 amW2AEW videos and mio83Quote from: mio83 on May 04, 2020, 07:52:42 pmthat you referenced are quite educative so I was setting my LTSpice diode ring to those freqs (7MHz and 10MHz) and I was getting pretty similar results, included harmonics. But after playing with inductance values I realised that (For 50ohms source series resistances) transformers must be set up in some (still unknown for me) specific ratio, otherwise the LO, RF inputs after the series resistors will look attenuated at the peaks (is that maybe somehow related with the P1db compression point). Furthermore, I was playing with other input frequencies (actually further apart and I was getting a totally different IF shape, actually what I think was double side band carrier suppressed signal. So I have to put more thinking.Quote from: w2aew on May 07, 2020, 01:47:18 amThanks w2aew, but too late, those videos about the Gilbert Cell are actually the ones that lead me on the track of the 602 ics!