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Critic of circuit before I build it.

Started by amdx April 2, 2015
  I'm about to build this circuit to compare output from different 
ferrite receive antennas. I will be using it with the the FET amplifier
referenced at the bottom of the page, so the input impedance is not a 
concern. I'm mostly interested in the DC output to monitor signal 
levels.  AM band frequencies.

  http://www.crystal-radio.eu/diodedetector/endiodedetector.htm

Any fix for the sinewave distortion in the second and third scope traces.
                                Mikek

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On a sunny day (Thu, 02 Apr 2015 09:10:47 -0500) it happened amdx
<nojunk@knology.net> wrote in <mfjijm$4ah$1@dont-email.me>:

> I'm about to build this circuit to compare output from different >ferrite receive antennas. I will be using it with the the FET amplifier >referenced at the bottom of the page, so the input impedance is not a >concern. I'm mostly interested in the DC output to monitor signal >levels. AM band frequencies. > > http://www.crystal-radio.eu/diodedetector/endiodedetector.htm > >Any fix for the sinewave distortion in the second and third scope traces. > Mikek
If you really want to be linear for small voltages at ferrite rod frequencies then a small opamp diode detector would be a lot better. http://en.wikipedia.org/wiki/Precision_rectifier
On 4/2/2015 9:23 AM, Jan Panteltje wrote:
> On a sunny day (Thu, 02 Apr 2015 09:10:47 -0500) it happened amdx > <nojunk@knology.net> wrote in <mfjijm$4ah$1@dont-email.me>: > >> I'm about to build this circuit to compare output from different >> ferrite receive antennas. I will be using it with the the FET amplifier >> referenced at the bottom of the page, so the input impedance is not a >> concern. I'm mostly interested in the DC output to monitor signal >> levels. AM band frequencies. >> >> http://www.crystal-radio.eu/diodedetector/endiodedetector.htm >> >> Any fix for the sinewave distortion in the second and third scope traces. >> Mikek > > If you really want to be linear for small voltages at ferrite rod frequencies > then a small opamp diode detector would be a lot better. > > http://en.wikipedia.org/wiki/Precision_rectifier > > >
So, you think the distortion is caused by the diodes and some gain ahead of them would solve that problem. I don't know how important linearity really is, I just want a relative comparison between antennas. However, if I build it better, it may have more use in the future. He says it listenable at 10mv, which seems high compared to the sensitivity of a good AM radio. I'll be measuring across a resonant ferrite rod, I don't know what a minimum usable voltage would be. I'll go check. Mikek
"amdx" <nojunk@knology.net> wrote in message 
news:mfjr4p$8bi$1@dont-email.me...
> On 4/2/2015 9:23 AM, Jan Panteltje wrote: >> On a sunny day (Thu, 02 Apr 2015 09:10:47 -0500) it happened amdx >> <nojunk@knology.net> wrote in <mfjijm$4ah$1@dont-email.me>: >> >>> I'm about to build this circuit to compare output from different >>> ferrite receive antennas. I will be using it with the the FET amplifier >>> referenced at the bottom of the page, so the input impedance is not a >>> concern. I'm mostly interested in the DC output to monitor signal >>> levels. AM band frequencies. >>> >>> http://www.crystal-radio.eu/diodedetector/endiodedetector.htm >>> >>> Any fix for the sinewave distortion in the second and third scope >>> traces. >>> Mikek >> >> If you really want to be linear for small voltages at ferrite rod >> frequencies >> then a small opamp diode detector would be a lot better. >> >> http://en.wikipedia.org/wiki/Precision_rectifier >> >> >> > So, you think the distortion is caused by the diodes and some gain ahead > of them would solve that problem. > I don't know how important linearity really is, I just want a relative > comparison between antennas. However, if I build it better, > it may have more use in the future. > He says it listenable at 10mv, which seems high compared to the > sensitivity of a good AM radio. > I'll be measuring across a resonant ferrite rod, I don't know what a > minimum usable voltage would be. I'll go check. > Mikek >
If you want more sensitivity, put some (tuned ?) gain in front of the detector. Low noise amplifier is best for the first stage.
On 4/2/2015 10:10 AM, amdx wrote:
> I'm about to build this circuit to compare output from different > ferrite receive antennas. I will be using it with the the FET amplifier > referenced at the bottom of the page, so the input impedance is not a > concern. I'm mostly interested in the DC output to monitor signal > levels. AM band frequencies. > > http://www.crystal-radio.eu/diodedetector/endiodedetector.htm > > Any fix for the sinewave distortion in the second and third scope traces. > Mikek > > --- > This email has been checked for viruses by Avast antivirus software. > http://www.avast.com >
If you look at the paragraph above Figure 8, the distortion you are seeing is for a carrier frequency of 10 kHz. You shouldn't see a problem at AM broadcast band frequencies. And the others are right. You can limit distortion by increasing the input voltage to the detector or decreasing the voltage required for the rectifier to conduct. For instance, a germanium diode has a voltage drop of around 0.3V, while a silicon diode has a drop of about 0.7V. A precision rectifier, as Jan suggested, is even better, with a voltage drop of near zero volts. -- ================== Remove the "x" from my email address Jerry, AI0K jstucklex@attglobal.net ==================
On 2.4.15 17:10, amdx wrote:
> I'm about to build this circuit to compare output from different > ferrite receive antennas. I will be using it with the the FET amplifier > referenced at the bottom of the page, so the input impedance is not a > concern. I'm mostly interested in the DC output to monitor signal > levels. AM band frequencies. > > http://www.crystal-radio.eu/diodedetector/endiodedetector.htm > > Any fix for the sinewave distortion in the second and third scope traces. > Mikek
If you really need to measure a wide range of input signals, get a logarithmic amplifier-detector chip, like AD8307. There are good examples in the data sheet. You may need a tuned circuit at the input to pick your frequency of interest. -- Tauno Voipio
On Thu, 02 Apr 2015 09:10:47 -0500, amdx wrote:
 "Critic of circuit before I build it."

heh...  
This place is infested with critics.
What you want is a critique.
On 4/2/2015 3:43 PM, Allodoxaphobia wrote:
> On Thu, 02 Apr 2015 09:10:47 -0500, amdx wrote: > "Critic of circuit before I build it." > > heh... > This place is infested with critics. > What you want is a critique. >
I'll do better next time. I'm going to my room now. Mikek
On 4/2/2015 12:36 PM, amdx wrote:
> On 4/2/2015 9:23 AM, Jan Panteltje wrote: >> On a sunny day (Thu, 02 Apr 2015 09:10:47 -0500) it happened amdx >> <nojunk@knology.net> wrote in <mfjijm$4ah$1@dont-email.me>: >> >>> I'm about to build this circuit to compare output from different >>> ferrite receive antennas. I will be using it with the the FET amplifier >>> referenced at the bottom of the page, so the input impedance is not a >>> concern. I'm mostly interested in the DC output to monitor signal >>> levels. AM band frequencies. >>> >>> http://www.crystal-radio.eu/diodedetector/endiodedetector.htm >>> >>> Any fix for the sinewave distortion in the second and third scope >>> traces. >>> Mikek >> >> If you really want to be linear for small voltages at ferrite rod >> frequencies >> then a small opamp diode detector would be a lot better. >> >> http://en.wikipedia.org/wiki/Precision_rectifier >> >> >> > So, you think the distortion is caused by the diodes and some gain > ahead of them would solve that problem. > I don't know how important linearity really is, I just want a > relative comparison between antennas. However, if I build it better, > it may have more use in the future. > He says it listenable at 10mv, which seems high compared to the > sensitivity of a good AM radio. > I'll be measuring across a resonant ferrite rod, I don't know what a > minimum usable voltage would be. I'll go check. > Mikek
That voltage is going to be pretty small. Unless your signal is strong and close, it will be low uVolts. -- Rick
On Thu, 02 Apr 2015 09:10:47 -0500, amdx wrote:

> I'm about to build this circuit to compare output from different ferrite > receive antennas. I will be using it with the the FET amplifier > referenced at the bottom of the page, so the input impedance is not a > concern. I'm mostly interested in the DC output to monitor signal > levels. AM band frequencies. > > http://www.crystal-radio.eu/diodedetector/endiodedetector.htm > > Any fix for the sinewave distortion in the second and third scope > traces. > Mikek
Critique. Sorry -- I'm critiquing the wrong thing, but there you are. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com