AM detector IC - available ? - Page 1
Forum section: electronics / rf-microwave

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 06:21:31 pm
content: For my superhet project I learned that a diode causes loss of sensitivity because of the voltage drop and that when biased into conduction it causes distortion.Is there an IC which fixes these problems ?At least between 3-30Mhz . I don't seem to find one.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 31, 2018, 06:29:16 pm
content: You can do synchronous demodulation; like for an SSB signal you add via a BFO the carrier to the signal; this has even an advantage in case of selective fading

author: iMo
author_rank: Super Contributor
timestamp: August 31, 2018, 06:33:31 pm
content: For examplehttps://plus.google.com/102499654761048990073/posts/cEdxAhKBgYf

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 31, 2018, 06:34:23 pm
content: A superhet will have enough IF amplification to overcome the diode loss so no problem.

author: iMo
author_rank: Super Contributor
timestamp: August 31, 2018, 06:42:13 pm
content: Quote from: HB9EVI on August 31, 2018, 06:29:16 pmLong time back I played with TAA661 as the synch detector, afaik.. The limiting IF amplifier re-created the carrier out of the IF and then it was mixed with the IF signal as well (in the same chip). Or something like that

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 31, 2018, 06:54:45 pm
content: Ah yea, those were the days... they're still for sale on ebay.however, PA0PBZ is also right; with a stable signal, there's no issue with the detector diode in a superhet

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 07:02:55 pm
content: I think I saw something called "precision rectifier" and I thought something similar might exist for AM detection.

author: iMo
author_rank: Super Contributor
timestamp: August 31, 2018, 07:04:35 pm
content: Ge diodes are the best for AMOr use an SDR at the 455kHz.

author: iMo
author_rank: Super Contributor
timestamp: August 31, 2018, 07:11:04 pm
content: Quote from: AlexResch on August 31, 2018, 07:02:55 pmThere is no HIFI broadcasted at shortwaves. So no special need for a precision rectifier.See also my above link with a NPN transistor detector (could be an JFET used in so called "infinite impedance detector" setup -http://sound.whsites.net/articles/am-radio.htm).PS: you may try with DMR on shortwaves..https://en.wikipedia.org/wiki/Digital_Radio_Mondiale

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 07:22:39 pm
content: I found thishttp://amwindow.org/tech/htm/alowdisdet.htm

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 31, 2018, 07:25:05 pm
content: wow, that's an effort! I'd call it overkill

author: iMo
author_rank: Super Contributor
timestamp: August 31, 2018, 07:27:10 pm
content: Wire it with an oxygen free copper wire then for the best soundPS: Is there any AM broadcasting at SW? Afaik all stations I've tuned recently are English speaking far-east stations..

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 07:36:01 pm
content: I don't understand your point. I do this to learn , I have no interest in SW "hi-fi". In general, I do not have any interest in HIFI at all.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 31, 2018, 07:41:52 pm
content: I'm also wondering what is your aim.On one side, you want to use an overly complex AM detector, but when we were talking about the IF, you insisted on 455kHz, what is rather poor choice.There would be a big benefit in a higher IF, but a rather low to practically non-existing benefit in that AM detector.You see, what we mean?

author: janoc
author_rank: Super Contributor
timestamp: August 31, 2018, 08:04:01 pm
content: Quote from: imo on August 31, 2018, 07:27:10 pmQuite a bit still. BBC World Service, Deutsche Welle, I have heard quite a bit of US-based talk radio too (mostly religious stuff). Also a lot of middle-Eastern stations can be heard here quite often.

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 08:32:55 pm
content: Quote from: HB9EVI on August 31, 2018, 07:41:52 pmNo, not really. This is a learning project for me . I already have 455Khz filters of various kinds and a couple of ( decent ) 455Khz receivers to compare with. Once done with this project I will move on and do a different IF. Also, 455Khz is easy to track and I have 30-340pf variable caps. I suspect a higher IF will not be as easy to track and I want to keep it simple. So simple that even with some failures and some detours I can do it for sure.I am a lot on an RC heli forum and I saw there the same thing - you ask a specific question and you get a lot of advise on how to do something else.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 31, 2018, 08:38:23 pm
content: Well, keep it simple is nice, but it's not simple anymore with a opamp full wave rectifier at RF.

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 09:07:46 pm
content: Quote from: HB9EVI on August 31, 2018, 08:38:23 pmThat is for a ( much ) later stage. It's just me - I like to plan way ahead. First I'll do a simple Germanium diode detector.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 31, 2018, 09:26:40 pm
content: sounds reasonable...it's a nice idea though, but I've never seen it implemented anywhere; taking into account the possible interferences on AM modulation and the propagation on SW, I'm not wondering

author: BrianHG
author_rank: Super Contributor
timestamp: August 31, 2018, 09:35:51 pm
content: If you have a filter tank input, you can use a TV IF receiver IC. They usually can operate between 15Mhz and 70Mhz, but, maybe you can go lower. Depending on if you are decoding data VS audio, you'll need to adjust the AGC cap value. Many also have a second FM 4-8Mhz internal IF which you wont need.Example, TDA8340 / TDA8341 :https://www.bucek.name/pdf/tda8340,8341.pdfYou also get an automatic fine tuning output on this IC.

author: BrianHG
author_rank: Super Contributor
timestamp: August 31, 2018, 09:56:58 pm
content: This is a weird one in a TO-92 package which is specked to 3Mhz, but, maybe you can stretch it more:https://www.wiltronics.com.au/product/8202/ta7642-am-radio-i-c/This one should do you entire frequency range: TEA5551Thttp://www.shfmicro.com/TEA5551T.pdfThis one has a complete I2C controlled digital tuner in an IC, minimal parts, 2.3 to 28.5Mhz:https://www.silabs.com/products/audio-and-radio/multi-band-radios/si4822-26-27-40-44-radio-receivers

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 10:38:08 pm
content: Quote from: BrianHG on August 31, 2018, 09:56:58 pmThank you for the first one - news to me. Actually I collect those and have around 10 different radio IC's. Not digital ones - not my interest. Once I am finished with the present project in a couple of weeks I will try the TDA1046 from Siemens. Old IC but I was told it was very good on SW :

author: bd139
author_rank: Super Contributor
timestamp: August 31, 2018, 10:50:40 pm
content: This is all too complicated.The bog standard and boring 2N3904 is a very nice low noise AM detector in the emitter follower arrangement if you stick a small capacitor across the emitter resistor. BE diode is the detector and the really cool bit is the thing is forward biased by the base bias network which means it'll pick up really really low signal levels.I found this out by accident when I was about 12.

author: AlexResch
author_rank: Regular Contributor
timestamp: August 31, 2018, 10:58:59 pm
content: Quote from: bd139 on August 31, 2018, 10:50:40 pmThank you. I will definetely try it.

author: bd139
author_rank: Super Contributor
timestamp: August 31, 2018, 11:07:34 pm
content: Quick LTspice simulation for you...inductor isn't necessary but it kills a lot of the carrier going into the audio stage. Values not optimised. Lots of improvement possible

author: chris_leyson
author_rank: Super Contributor
timestamp: September 01, 2018, 12:28:09 am
content: The emitter follower detector example posted by imo,https://plus.google.com/102499654761048990073/posts/cEdxAhKBgYfhas a lot of advantages over a diode detector, it's a voltage follower and it has current gain ! It's a good high to low impedance match without any voltage loss.

author: bd139
author_rank: Super Contributor
timestamp: September 01, 2018, 08:31:22 am
content: Looks close to mine. I must be doing something right for onceWorked mine out when I was trying to build a follower and amplifier for the tuned part of a crystal set before the detector to increase sensitivity. Originally used a BC109, no decoupling and a mostly flat 9v radio shack (Tandy) batterySide effect was it de-Q’ed the tank circuit badly. This was before I discovered FETs.

author: iMo
author_rank: Super Contributor
timestamp: September 01, 2018, 09:17:27 am
content: Anything with a bit of "diode" effect would work for AMhttps://en.wikipedia.org/wiki/Crystal_detector

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 01, 2018, 10:20:41 am
content: Quote from: imo on September 01, 2018, 09:17:27 amRazor blade anyone?

author: AlexResch
author_rank: Regular Contributor
timestamp: September 01, 2018, 02:12:42 pm
content: Quote from: PA0PBZ on September 01, 2018, 10:20:41 amRazor blade is news to me but years ago a bicycle shop here in Johannesburg still had lead crystals with a steel spring all packed nicely in cotton and then in a glass tube. 30s or so stock. Bought a few, never managed to get one working. Seems one needs to search for a particular contact point.

author: janoc
author_rank: Super Contributor
timestamp: September 01, 2018, 05:33:49 pm
content: Quote from: AlexResch on September 01, 2018, 02:12:42 pmGoogle "foxhole radio":https://en.wikipedia.org/wiki/Foxhole_radioBasically a regular crystal radio with the crystal being replaced by a razor blade with some oxide on it (either rust or from a flame).

author: iMo
author_rank: Super Contributor
timestamp: September 01, 2018, 07:01:41 pm
content: https://rimstar.org/science_electronics_projects/razor_blade_diode_for_crystal_foxhole_radio.htm

author: AlexResch
author_rank: Regular Contributor
timestamp: September 01, 2018, 08:50:26 pm
content: Cute videos !By the way, if the signal is strong enough why would any "detection" be needed ?

author: janoc
author_rank: Super Contributor
timestamp: September 01, 2018, 09:37:59 pm
content: Quote from: AlexResch on September 01, 2018, 08:50:26 pmBecause humans can't exactly hear RF frequency signals?The detector isn't there to amplify the signal. The diode rectifies it (slices off half of the RF waveform) and then a low pass filter removes the rest of the carrier, leaving you with just the audio frequency envelope.

author: AlexResch
author_rank: Regular Contributor
timestamp: September 01, 2018, 10:21:04 pm
content: Quote from: janoc on September 01, 2018, 09:37:59 pmI believe you but I fail to understand why. The audio signal is there anyway and the AM envelope should modulate the flux in the headphone magnets. Interesting - I've never thought about this...Maybe the reason is the magnets are too slow.

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 01, 2018, 10:41:47 pm
content: @BD139:On your simulation , the signal going into the AM port of the modulator should have a DC offset , the waveform displayed is a double-side-band modulated signal, these are fairly unintelligible when demodulated with an envelope detector.Typical AM waveforms are always less than 100% modulation.Also I would be interested to see the results of your sim when the collector was disconnected , i.e. just a simple biassed diode.In practice you would normally want to use a transistor, so as not to load down the IF transformer, and provide a low impedance output e.g. a volume pot.

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 01, 2018, 10:48:08 pm
content: You can also make a "synchrodyne receiver" similar to what other posters have mentioned, here is an example using a NE612 / SA612https://www.mikroe.com/ebooks/radio-receivers-from-crystal-set-to-stereo/ne612-synchrodyne-am-receiver.Just google "synchrodyne receiver" or "NE612" "SA612" "NE602" "SA602"Also have you checked out the ZN414https://en.wikipedia.org/wiki/ZN414?

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 01, 2018, 11:42:24 pm
content: QuoteYes , with sufficient amplification you could hear the signal directly, it's not that dissimilar to class-D "inductorless" headphone amplifiers.In practice it won't work, as the amplified RF signal will get radiated from the speaker/headphone wiring back to the antenna, and cause the radio to oscillate.Even with a superhet, amplifying the 455kHz signal enough to work the headphones/speaker will cause that signal to get coupled back into the radio too and make it unstable.In the early days of radio they had "reflex receivers" where a small fraction of the RF was fed back to the input to make it marginally unstable, this greatly increased the RF gain.Another aspect to consider is automatic gain control , incoming radio signals can vary over 1000:1 from different radio stations. You need a detector to be able to control the gain of the IF stage, this needs to make a DC voltage to change the bias on the transistors in the IF stage. Without AGC , the RF signal to the headphone speaker will either be saturated or too small to be audible much of the time.

author: boB
author_rank: Frequent Contributor
timestamp: September 01, 2018, 11:45:07 pm
content: Wonder what the I-V curve looks like of these diodes ?boB

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 02, 2018, 08:07:23 am
content: Quote from: salbayeng on September 01, 2018, 10:41:47 pmAM modulation always has 2 sidebands, or do you mean DSBSC (Double Side Band Suppressed Carrier)? It looks like a normal 100% AM modulated signal to me. You would not be able to see if it is double or single sideband in the time domain anyway, you need frequency domain for that (eg spectrum analyzer).QuoteTrue.

author: janoc
author_rank: Super Contributor
timestamp: September 02, 2018, 01:10:47 pm
content: Quote from: AlexResch on September 01, 2018, 10:21:04 pmIgnoring the inductance of the coils for the moment, if you don't rectify the signal, the AC carrier signal will cause the magnetic flux to change directions too rapidly for the diaphragm to follow (mechanical inertia due to the non-zero mass of the moving parts and all that). That will cause the signal to effectively cancel itself so the diaphragm of the headphone/speaker won't move. You wouldn't hear anything. It is not the magnet that is "slow" but the entire electromechanical system.The inductance of the coils will also have effect, preventing the AC current from the high frequency carrier from flowing (all that about inductors resisting changes in current ...) - and with no current you don't have any magnetic flux and membrane deflection neither.If you extract the envelope using a detector, you still have an AC signal but a low frequency one, so impedance of the coils is much lower, you can get meaningful electric current to flow. The diaphragm of the headphones is also able to follow the (slow) changes in the magnetic flux and you hear something.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 02, 2018, 01:47:10 pm
content: bd139's signal is clearly no AM, but either DSB/SSB with suppressed carrier, what is not supposed to be decoded with a detector@salbayeng: did you mean this link here:https://www.mikroe.com/ebooks/radio-receivers-from-crystal-set-to-stereo/ne612-synchrodyne-am-receiver? your one is not working.other words for this receiver concept are monodyne or direct conversion receiver. It's mixing the reception frequency down to 0Hz, so directly from RF to AF.It has the advantage, that it can demodulate SSB emissions, but because of its concept it misses image rejection completely.Modern variants of it are found in SDR receivers as I/Q or Tayloe mixers

author: AlexResch
author_rank: Regular Contributor
timestamp: September 02, 2018, 02:58:25 pm
content: Quote from: janoc on September 02, 2018, 01:10:47 pm...works fine in my headphones with 50Khz modulated with 1000Hz.

author: bd139
author_rank: Super Contributor
timestamp: September 02, 2018, 03:24:15 pm
content: Quote from: HB9EVI on September 02, 2018, 01:47:10 pmYou are correct. Only because I couldn’t work out how to change the modulation depth in Ltspice. Works in real world

author: janoc
author_rank: Super Contributor
timestamp: September 02, 2018, 11:29:22 pm
content: Quote from: AlexResch on September 02, 2018, 02:58:25 pm50kHz is not exactly RF ... Try a 1MHz or more.