Critic of circuit before I build it.

On a sunny day (Thu, 02 Apr 2015 09:10:47 -0500) it happened amdx wrote in :

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.

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.

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.

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.

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

That voltage is going to be pretty small. Unless your signal is strong and close, it will be low uVolts.

Critique. Sorry -- I'm critiquing the wrong thing, but there you are.

A little late to the party! I've been allowed to come out of my room now.

I missed that one. Pan or Giganews has started quietly not posting things if there are too many newsgroups, or something. So I was actually fairly early in the process, except for getting cut off at the knees.

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

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

It is easier to compare if you know the curve.

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.

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

| |

Infinite impedance detectors are said to have very low demod distortion by AM aficionados.

piglet

PS so a high Q gives a narrow bandwidth, better 'selectivity'. That may be needed if you receive 2 stations close together in frequency. Especially in the evening on the AM broadcast band you may get very strong signals from far away stations at the same or right next to the frequency of the one you are listening to, you then hear both at the same time, or this gives a few kHz interference.

But, too high a Q (hard to obtain [1]) will attenuate the high frequency sidebands of the station you are listening to, reducing high frequency sound components.

Am stations are spaced 9 kHz or 10kHz apart depending on location, so all sorts of interference from far away stations exists. This site is worth reading:

RR and IF stages with tuned circuits or bandfilters of any kind (mechanical, crystal, piezo, LC, etc) can help create an accurate bandpass curve.

I think you should perhaps get hold of one of the RSGB radio books, and start from there:

...without supper.

Yes, I'm aware of how Q affects selectivity, I can only hope for too high of a Q, easy enough to spoil. I've been following a crystal radio group for a few years, they recently got hold of a low loss ferrite and getting Q's in the 1000 to 1200 range over the broadcast band. Using Litz wire. The material I'm using is some in my stock, and not real good Q at AM band frequencies. What I have is twelve-1/2 potcores stacked as a rod. A coil is wrapped on that and resonated with a cap. With a radio near the core You get an increase in the signal as you tune to resonance. The rod is 67.5mm Dia. x 335mm long, or 2.66 in dia. x 13.2 in long.

I just yesterday, tried a Hartley oscillator on part of the winding in an attempt to raise Q and get more selectivity. It didn't oscillate, but I only spent a few minutes before supper, I'll be trying again.

Oh that one! I have the ARRL Handbook. :-)

Thanks, Mikek

Yes, I can hear that work if I hit resonance quick, the volume jumps up, but then settles back down.

Yes, the long rod I reference before has a wide null off the ends. I have listened to two different stations on the same frequency, just by rotating the rod. I have it on a Lazy Susan.

Yep, I have an old Booton 260A for measuring Q. Thanks, Mikek

Might want to have that looked at!

The last time I posted a schematic of a circuit by the same guy, there were plenty of critics. Someone LTspice'd it, said the input impedance was not as he said, others said there were better ways to do it, etc. So maybe a 'critic' was what I expected! Here's that circuit, I built it, it works, I can't tell that it loads a high Q circuit, so if it does it is negligible.

At AM band frequencies there now exist fast enough OpAmp's to make a nearly ideal diode detector. ...Jim Thompson