Critic of circuit before I build it.
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 --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com |
Critic of circuit before I build it.
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 |
Critic of circuit before I build it.
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. 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 |
Critic of circuit before I build it.
"amdx" wrote in message ... 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. 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. |
Critic of circuit before I build it.
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/diodedet...dedetector.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 ================== |
Critic of circuit before I build it.
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/diodedet...dedetector.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 |
Critic of circuit before I build it.
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. |
Critic of circuit before I build it.
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 |
Critic of circuit before I build it.
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 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. 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 |
Critic of circuit before I build it.
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. |
Critic of circuit before I build it.
On 02/04/2015 15:10, amdx wrote:
Any fix for the sinewave distortion in the second and third scope traces. Infinite impedance detectors are said to have very low demod distortion by AM aficionados. piglet |
Critic of circuit before I build it.
I wrote:
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'. 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. [1] RF feedback can be used in a RF stage to increase Q, 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: http://en.wikipedia.org/wiki/AM_broadcasting 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 the http://www.rsgbshop.org/acatalog/Onl...___CDs_29.html |
Critic of circuit before I build it.
amdx wrote:
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 ....without supper. |
Critic of circuit before I build it.
On 4/3/2015 2:02 AM, Jan Panteltje wrote:
I wrote: 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'. 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. 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. - [1] RF feedback can be used in a RF stage to increase Q, 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. 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: http://en.wikipedia.org/wiki/AM_broadcasting 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 the http://www.rsgbshop.org/acatalog/Onl...___CDs_29.html Oh that one! I have the ARRL Handbook. :-) Thanks, Mikek --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com |
Critic of circuit before I build it.
On 4/3/2015 1:08 AM, Jan Panteltje wrote:
AGC (Automatic Gain Correction[1]) will keep that voltage high as long as possible with weakening signals. Yes, I can hear that work if I hit resonance quick, the volume jumps up, but then settles back down. 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 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. The thing to watch is the Q factor, the detector circuit can be seen as a resistor in 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 Yep, I have an old Booton 260A for measuring Q. Thanks, Mikek --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com |
Critic of circuit before I build it.
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/diodedet...dedetector.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 At AM band frequencies there now exist fast enough OpAmp's to make a nearly ideal diode detector. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. |
Critic of circuit before I build it.
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/diodedet...dedetector.htm Any fix for the sinewave distortion in the second and third scope traces. Mikek At AM band frequencies there now exist fast enough OpAmp's to make a nearly ideal diode detector. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. |
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