AM detector circuit not resonating - Page 1
Forum section: electronics / rf-microwave

author: Bancroftjo
author_rank: Contributor
timestamp: December 30, 2016, 01:50:49 am
content: Hello,To preface this I should say I do not have any real experience in radio, so I don't doubt I'm making a simple mistake.I am working through the book "Learning the Art of Electronics" and am on lab number 3L.4, designing an AM radio receiver. This is a design exercise and therefore there isn't a schematic exactly but it does lay out the building blocks and gives oscilloscope screenshots of what to expect after each stage in the detector. The circuit blocks are a 30' piece of wire as an antenna going directly into a LC resonant circuit tuned for an AM station (I chose 1500kHz to match a fairly local station), then rectify that signal through a schottky diode to a leaky peak detector to yield an audio frequency signal.My circuit takes the antenna directly into a parallel LC circuit, with a 10nH chip inductor and a 1uF ceramic capacitor (which I believe should resonate at 1590kHz theoretically). I have the circuit built with a 1N5817 diode rectifying that signal into a parallel RC 'leaky peak detector' of 1nF and 30k, but I am not even getting anything resembling the sample screenshots shown in the book at the LC point. My screenshot is attached. The book shows a very distinct AM signal with a 1Vpp amplitude at this point...I assume my problem lies here.I tried a different resonant frequency by adding a second 1uF cap in parallel with the first to lower the resonant point to 1130kHz since I also have a fairly strong signal on 1120kHz here, but no difference. I have attached a photo of my construction (ignore the L and C being in series in this photo, I was just experimenting). I also disconnected the diode just to see if the LC circuit would resonate by itself, it did not. All that said, I can get a signal to couple into the antenna from my function generator but the amplitude at the LC point is just 100mVpp.I was able to build and test the resonant circuits earlier in this chapter, in which I used a 10mH toroidal inductor and a 10nF ceramic disk capacitor to resonate around 16kHz. When I built this new circuit with the 10nH chip inductor and 1 or 2uF ceramic SMD capacitor is there some detail I am ignoring that is preventing it from resonating?Any insights would be appreciated!- Jon

author: JimRemington
author_rank: Regular Contributor
timestamp: December 30, 2016, 03:32:41 am
content: QuoteA 1Vpp RF amplitude is extremely unlikely with a 30' antenna, no Earth ground and using a poor quality inductor and capacitor as you are doing. The demodulated AM will be even lower in amplitude.Basically you need a very high Q resonant circuit to see voltages like that, unless you are very close to a powerful AM transmitter. Typically you might use a high quality ~200 uH inductor and ~400 pF (variable, max) capacitor for AM radio reception.Check out the "crystal radio" websites for better choices for resonant circuit inductors and capacitors. Example:http://makearadio.com/crystal/crystal-schematics.php

author: Bancroftjo
author_rank: Contributor
timestamp: December 30, 2016, 05:24:44 am
content: Thanks for the suggestions, I will try some larger inductance and smaller capacitance.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 30, 2016, 09:56:17 am
content: Quote from: Bancroftjo on December 30, 2016, 01:50:49 amThe resonance formula will quite happily accept all sorts of values & spit out a resonant frequency,but in the real world things are very different.A classic medium wave radio receiver normally uses a capacitor with a range of from about 15pf to 415 pf,to tune between the highest limit of the band (around 1600kHz),& the lowest limit.(around 500 kHz).Apply those figures to the resonance formula.to find the required inductance.---it should be around 250 uH.

author: kerrsmith
author_rank: Regular Contributor
timestamp: December 30, 2016, 10:46:35 am
content: I really enjoy making radios - I always like to try out new circuits. A good one I made a while ago is shown in the attached image (I am not sure where I found this exact one but it seems to be the broadcast band original of the one shown on the following sitehttp://kd1jv.qrpradio.com/swbcrx/AMSWBC.HTM)If you are looking for other interesting radio projects (other than your current one) then I suggest trying a few radio kits such as the ones shown below, I have made all three and they all work really well.This is an AM radio and the simplest one I tried and has two tuned stages:http://www.banggood.com/9018-2AM-AM-Radio-Electronic-Kit-Electronic-DIY-Learning-Kit-p-946169.html?p=2D100314686672015046This is also an AM radio but is slightly more complex with three tuned stages:http://www.banggood.com/Seven-AM-Radio-Electronic-DIY-Kit-Electronic-Learning-Kit-p-946167.html?p=2D100314686672015046This is an FM radio that uses a CD2003GP IC to do all the work:http://www.banggood.com/DIY-FM-Radio-Kit-Electronic-Learning-Suite-p-1017325.html?p=2D100314686672015046Using kits is a great way to get started as you have all the bits you need as well as a circuit board - once it is made and working you can probe it and make notes on all the different voltages and signals you find. The is really handy when you are making up other circuits from scratch as you have a good set of data to use so you know you are on the right track.I also find it fun to take ideas from the circuits I find online as well as these kits and merge bits of them together to come up with new circuits.My current project is making a Bitx (http://www.phonestack.com/farhan/bitx.html) but using a Si5351 for the VFO and BFO (I made the original and it worked really well so thought I would have a go at making the tuning Arduino controlled). So far I have it all breadboarded up and working and my next step is to make the PCB, build it up and get it put in to a case.