On a sunny day (Thu, 02 Apr 2015 11:36:27 -0500) it happened amdx
wrote in :

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
wrote in :

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/diodedet...dedetector.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.

No,
diodes need a minimum volatge to conduct
the circuit I referenced does not have that problem,
is simpler, and indeed more linear.


I don't know how important linearity really is, I just want a
relative comparison between antennas.

It is easier to compare if you know the curve.


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.

Yes, AM radios have a lot of IF gain, and real ones (tm) also RF gain.
In those cases, IF a normal diode detector is used, then the signal
at the detector is in the volts range, and the 'knee' in the diode curve becomes less significant.
AGC (Automatic Gain Correction[1]) will keep that voltage high as long as possible with weakening signals.



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

Depends how far you are from any transmitter, and the direction the ferrite rod is pointing.
For maximum signal it should be like this:

|
| ------- 0 transmitter
|
rod

The thing to watch is the Q factor,
the detector circuit can be seen asa resistor parallel with the ferrite rod and tuning cap.
----------
|( | |
|( === [ ] Rp
|( | |
---------

The higher Rp, the higher the Q factor, the more signal, the narrower the bandwidth
B = f / Q

http://en.wikipedia.org/wiki/RLC_circuit
scroll down to 'parallel RLC circuit'.



[1] AGC works on the RF and / or IF gain stages.