Author Topic: AM detector circuit not resonating  (Read 1157 times)

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Offline Bancroftjo

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AM detector circuit not resonating
« on: December 30, 2016, 12:50:49 pm »
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 \$\Omega\$, 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

Offline JimRemington

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Re: AM detector circuit not resonating
« Reply #1 on: December 30, 2016, 02:32:41 pm »
Quote
The book shows a very distinct AM signal with a 1Vpp amplitude at this point
A 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

« Last Edit: December 30, 2016, 02:45:03 pm by JimRemington »
 

Offline evb149

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Re: AM detector circuit not resonating
« Reply #2 on: December 30, 2016, 02:50:38 pm »
Hmm I don't know.  Show some pictures of your components.
Maybe measure the components values at some reasonable frequency or at least DC if you can.
Maybe it would be easier to take your function generator and set it to sine wave 50 ohm output and couple the generator through a high valued series resistor (1k? 10k?) to the LC circuit and look for a voltage peak on the oscilloscope.
Maybe best to couple your oscilloscope probe to the LC tank via a series 1k resistor or something so that your probe wire's capacitance has less impact on the LC circuit's Q and frequency also.

My first thought is "1uf Ceramic capacitor" isn't all that specific.  There are lots of bad quality ceramic capacitors out there, Z dlelectric (yuck), Y dielectric (less yuck but still bad), X dielectric (not great but better than Y or Z) and then C0G/NP0 which is theoretically excellent but not likely to be found inexpensively / commonly at such large values of capacitance.  So among the X, Y, Z dielectric types you have a large variation of possible capacitor tolerance vs. DC bias voltage, temperature, and even mechanical strain.  Of course in such a simple circuit I guess you have no appreciable DC bias for a passive circuit so in theory the capacitor's value should be roughly accurate +/- 10% or +/- 20% or something whatever the capacitor is rated for which wouldn't be bad... but then that wouldn't explain why it doesn't work.

So then we have the inductor.  Iron core materials or some other kinds of low frequency materials might be kind of lossy and not exhibit the expected inductance or Q at around 1MHz.  Even ferrites of the wrong type could be not so good.  So I don't know..

Oh wait you said 10nH inductance?!  Heck a 10mm long piece of thin (26-30AWG more or less) solid wire has that much inductance!  10nH and 1uF is way too small of an LC inductance for 1MHz tuned circuits when you could more effectively use something like 10uH air coil and 1nF C0G/NP0 or maybe more likely 100uH air coil and 100pF which would give you a decent sized 'loop' coil that could be the antenna all by itself and let you use a good C0G/NP0 capacitor or even a self made variable plate or gimmick capacitor.  All that assuming you got your LC resonance sums correct and I did too in scaling the values you provided.

The problem with 10nH inductance is even if it is air core (which is likely at that small of inductance) the 10mm length of wire involved and the small inductance value is so small that any parasitic inductance in your wiring or 'capacitor' will be WAY larger than or at least comparable to the inductance of your actual 'inductor' so you stand zero chance of getting an accurate LC frequency.

10nH air inductor is an inductance value you might use in 1 GHz range circuits for tuning, not 1 MHz range due to the small 1MHz reactance and trouble of working with such values.  In fact the reactance is so low at 1 MHz you would have to have quite a good capacitor to even have a reasonable resonance even if the capacitor did not have much more stray inductance than that.

Double check your LC resonance frequency and scale your LC as I said then look for a peak voltage with a sweep of your FG loosely coupled (inductively or through high series R as above) to the LC and with the LC loosely coupled to the DSO.

It'll work better.
 
 

Offline Bancroftjo

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Re: AM detector circuit not resonating
« Reply #3 on: December 30, 2016, 04:24:44 pm »
Thanks for the suggestions, I will try some larger inductance and smaller capacitance.

Offline vk6zgo

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Re: AM detector circuit not resonating
« Reply #4 on: December 30, 2016, 08:56:17 pm »
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 \$\Omega\$, 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

The 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.
« Last Edit: December 30, 2016, 09:10:58 pm by vk6zgo »
 

Offline kerrsmith

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Re: AM detector circuit not resonating
« Reply #5 on: December 30, 2016, 09:46:35 pm »
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 site http://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=2D100314686672015046

This 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=2D100314686672015046

This 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=2D100314686672015046

Using 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.
« Last Edit: December 30, 2016, 09:50:26 pm by kerrsmith »
 


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