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Improving my best diode detector

Started by amdx April 19, 2017
On 4/22/2017 2:59 AM, Robert Baer wrote:
> amdx wrote: >> On 4/21/2017 3:33 AM, Robert Baer wrote: >>> Can't get/see that schematic; in any case, for low(er) level >>> sensitivity and "linearity", try adding a small forward DC bias to >>> move the average OP up the curve. >>> >> >> Still works for me. >> https://www.dropbox.com/s/3oyjn2teglg47b7 > * Nope; get: > "Download and save to your Dropbox by creating a free account, or sign in" > I do not have a dropbox...
You don't need a dropbox. Click the X in the upper right corner.
>>> https://www.dropbox.com/s/3oyjn2teglg47b7/Diode%20detector%201N34A%20500ohm%20adjusted%20R1%20and%20R2.jpg?dl=0 >>> > * No schematic..
Because you didn't look... -- Rick C
On 4/22/2017 2:59 AM, Robert Baer wrote:
> rickman wrote: >> On 4/21/2017 4:33 AM, Robert Baer wrote: >>> Can't get/see that schematic; in any case, for low(er) level >>> sensitivity and "linearity", try adding a small forward DC bias to move >>> the average OP up the curve. >> >> The bias would then need to be subtracted out again after rectification. >> > Too complicated??
More complicated than using dropbox. -- Rick C
On 4/21/2017 8:24 PM, rickman wrote:
> On 4/21/2017 1:53 PM, amdx wrote: >> On 4/21/2017 12:25 PM, rickman wrote: >>> >>> BTW, if you are only interested in a given voltage range at the output >>> of the diode circuit, you can connect the meter ground leg to the >>> adjustable voltage source (a pot) and make the zero current point on >>> the meter correspond to a 3 Vrms input and the max reading on the >>> meter face correspond to a 5 Vrms input. Then you have the entire >>> range of the meter to measure the Q more accurately. There is no >>> reason why the low end has to correspond to a Q of 1. >>> >> If biasing the diode detector up by 3 volts will make it more linear on >> the bottom end, That's good. I can bias the other end of my meter up by >> three volts to remove the 3 volts. The Q meter already is setup with an >> adjustable bias on the negative side of the meter. >> I'll keep this in mind. >> I would just as soon keep the zero to 250 scale. > > What I am talking about doesn't affect the diode circuit, it changes the > range of your meter range from 0-5 to 3-5 or whatever numbers you want. > Because you can change your power input level to bring the output of the > circuit to whatever level you want there is no reason to work with the > low end of the output range, so no need to linearize the entire output > range. Besides, your circuit is pretty durn linear as it is. > > If you want to keep the 0 to 250 scale, consider using an adder (+250 > say) before you use the multiplier. That would make the true Q scale > 250 to 500. >
I'll keep that as a possibility. Earlier in our discussion there was a question about why have the termination resistor. I turns out to be important if the circuit is capacitively coupled. It doesn't work without a termination. I'm now using an 11K termination after the coupling capacitor, with a 500 ohm source resistance. I don't know how the termination value affects circuit operation. Mikek --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
rickman wrote:
> On 4/22/2017 2:59 AM, Robert Baer wrote: >> amdx wrote: >>> On 4/21/2017 3:33 AM, Robert Baer wrote: >>>> Can't get/see that schematic; in any case, for low(er) level >>>> sensitivity and "linearity", try adding a small forward DC bias to >>>> move the average OP up the curve. >>>> >>> >>> Still works for me. >>> https://www.dropbox.com/s/3oyjn2teglg47b7 >> * Nope; get: >> "Download and save to your Dropbox by creating a free account, or sign >> in" >> I do not have a dropbox... > > You don't need a dropbox. Click the X in the upper right corner.
* Neat,,,just as expected it closes the site. Very useful.
> > >>>> https://www.dropbox.com/s/3oyjn2teglg47b7/Diode%20detector%201N34A%20500ohm%20adjusted%20R1%20and%20R2.jpg?dl=0 >>>> >>>> >> * No schematic.. > > Because you didn't look...
* But i did, tried a number of things, nogo. However, once in a while it works.
>
rickman wrote:
> On 4/22/2017 2:59 AM, Robert Baer wrote: >> rickman wrote: >>> On 4/21/2017 4:33 AM, Robert Baer wrote: >>>> Can't get/see that schematic; in any case, for low(er) level >>>> sensitivity and "linearity", try adding a small forward DC bias to move >>>> the average OP up the curve. >>> >>> The bias would then need to be subtracted out again after rectification. >>> >> Too complicated?? > > More complicated than using dropbox. >
Actually, i think the bias does not need subtraction; in the "proper" circuit, it helps.
amdx wrote:
> On 4/21/2017 8:24 PM, rickman wrote: >> On 4/21/2017 1:53 PM, amdx wrote: >>> On 4/21/2017 12:25 PM, rickman wrote: >>>> >>>> BTW, if you are only interested in a given voltage range at the output >>>> of the diode circuit, you can connect the meter ground leg to the >>>> adjustable voltage source (a pot) and make the zero current point on >>>> the meter correspond to a 3 Vrms input and the max reading on the >>>> meter face correspond to a 5 Vrms input. Then you have the entire >>>> range of the meter to measure the Q more accurately. There is no >>>> reason why the low end has to correspond to a Q of 1. >>>> >>> If biasing the diode detector up by 3 volts will make it more linear on >>> the bottom end, That's good. I can bias the other end of my meter up by >>> three volts to remove the 3 volts. The Q meter already is setup with an >>> adjustable bias on the negative side of the meter. >>> I'll keep this in mind. >>> I would just as soon keep the zero to 250 scale. >> >> What I am talking about doesn't affect the diode circuit, it changes >> the range of your meter range from 0-5 to 3-5 or whatever numbers you >> want. Because you can change your power input level to bring the >> output of the circuit to whatever level you want there is no reason to >> work with the low end of the output range, so no need to linearize the >> entire output range. Besides, your circuit is pretty durn linear as it >> is. >> >> If you want to keep the 0 to 250 scale, consider using an adder (+250 >> say) before you use the multiplier. That would make the true Q scale >> 250 to 500. >> > I'll keep that as a possibility. > > Earlier in our discussion there was a question about why have the > termination resistor. I turns out to be important if the circuit is > capacitively coupled. It doesn't work without a termination. I'm > now using an 11K termination after the coupling capacitor, with a 500 > ohm source resistance. I don't know how the termination value affects > circuit operation. > > Mikek > > --- > This email has been checked for viruses by Avast antivirus software. > https://www.avast.com/antivirus >
Value should affect the observed linearity, especially if too high.
On 4/23/2017 3:03 AM, Robert Baer wrote:
> rickman wrote: >> On 4/22/2017 2:59 AM, Robert Baer wrote: >>> amdx wrote: >>>> On 4/21/2017 3:33 AM, Robert Baer wrote: >>>>> Can't get/see that schematic; in any case, for low(er) level >>>>> sensitivity and "linearity", try adding a small forward DC bias to >>>>> move the average OP up the curve. >>>>> >>>> >>>> Still works for me. >>>> https://www.dropbox.com/s/3oyjn2teglg47b7 >>> * Nope; get: >>> "Download and save to your Dropbox by creating a free account, or sign >>> in" >>> I do not have a dropbox... >> >> You don't need a dropbox. Click the X in the upper right corner. > * Neat,,,just as expected it closes the site. Very useful.
Not sure what you did. I can make the signup overlay go away and then I see the full page.
>>>>> https://www.dropbox.com/s/3oyjn2teglg47b7/Diode%20detector%201N34A%20500ohm%20adjusted%20R1%20and%20R2.jpg?dl=0 >>>>> >>>>> >>>>> >>> * No schematic.. >> >> Because you didn't look... > * But i did, tried a number of things, nogo. > However, once in a while it works.
What did you do differently to make it work? -- Rick C
On 4/23/2017 3:05 AM, Robert Baer wrote:
> rickman wrote: >> On 4/22/2017 2:59 AM, Robert Baer wrote: >>> rickman wrote: >>>> On 4/21/2017 4:33 AM, Robert Baer wrote: >>>>> Can't get/see that schematic; in any case, for low(er) level >>>>> sensitivity and "linearity", try adding a small forward DC bias to >>>>> move >>>>> the average OP up the curve. >>>> >>>> The bias would then need to be subtracted out again after >>>> rectification. >>>> >>> Too complicated?? >> >> More complicated than using dropbox. >> > Actually, i think the bias does not need subtraction; in the "proper" > circuit, it helps.
If you look at the schematic he is using it is already doing a pretty good job of biasing the diode. He doesn't need the low end, so there is no reason to bias the measurement out of the low end, it's already doing that just by the way it is used. -- Rick C
On 4/22/2017 2:01 PM, amdx wrote:
> On 4/21/2017 8:24 PM, rickman wrote: >> On 4/21/2017 1:53 PM, amdx wrote: >>> On 4/21/2017 12:25 PM, rickman wrote: >>>> >>>> BTW, if you are only interested in a given voltage range at the output >>>> of the diode circuit, you can connect the meter ground leg to the >>>> adjustable voltage source (a pot) and make the zero current point on >>>> the meter correspond to a 3 Vrms input and the max reading on the >>>> meter face correspond to a 5 Vrms input. Then you have the entire >>>> range of the meter to measure the Q more accurately. There is no >>>> reason why the low end has to correspond to a Q of 1. >>>> >>> If biasing the diode detector up by 3 volts will make it more linear on >>> the bottom end, That's good. I can bias the other end of my meter up by >>> three volts to remove the 3 volts. The Q meter already is setup with an >>> adjustable bias on the negative side of the meter. >>> I'll keep this in mind. >>> I would just as soon keep the zero to 250 scale. >> >> What I am talking about doesn't affect the diode circuit, it changes >> the range of your meter range from 0-5 to 3-5 or whatever numbers you >> want. Because you can change your power input level to bring the >> output of the circuit to whatever level you want there is no reason to >> work with the low end of the output range, so no need to linearize the >> entire output range. Besides, your circuit is pretty durn linear as >> it is. >> >> If you want to keep the 0 to 250 scale, consider using an adder (+250 >> say) before you use the multiplier. That would make the true Q scale >> 250 to 500. >> > I'll keep that as a possibility. > > Earlier in our discussion there was a question about why have the > termination resistor. I turns out to be important if the circuit is > capacitively coupled. It doesn't work without a termination. I'm > now using an 11K termination after the coupling capacitor, with a 500 > ohm source resistance. I don't know how the termination value affects > circuit operation.
I can't say I follow the need for a termination resistor after the coupling cap. What does it do for you other than raise the corner frequency of the high pass filter? -- Rick C
On 4/23/2017 2:35 PM, rickman wrote:
> On 4/23/2017 3:05 AM, Robert Baer wrote: >> rickman wrote: >>> On 4/22/2017 2:59 AM, Robert Baer wrote: >>>> rickman wrote: >>>>> On 4/21/2017 4:33 AM, Robert Baer wrote: >>>>>> Can't get/see that schematic; in any case, for low(er) level >>>>>> sensitivity and "linearity", try adding a small forward DC bias to >>>>>> move >>>>>> the average OP up the curve. >>>>> >>>>> The bias would then need to be subtracted out again after >>>>> rectification. >>>>> >>>> Too complicated?? >>> >>> More complicated than using dropbox. >>> >> Actually, i think the bias does not need subtraction; in the "proper" >> circuit, it helps. > > If you look at the schematic he is using it is already doing a pretty > good job of biasing the diode. He doesn't need the low end, so there is > no reason to bias the measurement out of the low end, it's already doing > that just by the way it is used. >
I'm putting a cap between the diode. Biasing is an option, but. I'm off the detector for a while, I need to get a flat response out of my amplifier. Here's the amp as wired, measurements are without the diode detector connected. https://www.dropbox.com/s/qsu1olsvza878t6/Dagmar%27s%20complete%20tube%20replacement.png?dl=0 Up until recently, all my testing has been at 1MHz, yesterday I ran the SigGen up to 10 MHz. At 10Mhz the detector had much more output than at 1MHz. I decided the 27PF need to be increased. After increasing it to 0.1µf it was much better, but it was still about 35% higher at 10MHz than at 1MHZ. I have seen circuits where they use an inductor where the termination resistor is. (68 ohm in the article)> Maybe this would block the high frequency and equalize the response. I have another problem, my amplifier is down about 17% at 10MHz. So, I'd like to make that flat before I get involved working on the detector. I just swapped in a 2N3866 for the BC547b, it didn't help. Or maybe degrade the amp further {by decreasing the 10µf cap) to oppose the increase of the detector. Mikek --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus