AM Detector Circuit

[n3lrx]

I'm looking at the AM detector located here.
http://amfone.net/ECSound/RFsampler.htm

My question is what sort of resistors should I use? In the picture they look like metal film but who knows. My guess is carbon, but I'm not sure. Does it really matter? I'm planning my trip to Radio Shack in the near future. Hopefully they still have such simple components  as few resistors and a 1n34a diode. I've got the cap already.  I doubt they have the 100uH inductor though I might need to eBay that. The rest of the parts I probably have in my junque boxes.

Thanks,
de Randy, N3LRX

[AB2EZ]

Randy

I think the AM detector design shown in the reference has some serious problems. The peak reverse voltage across the 1N34a will exceed the specified maximum reverse voltage (45V) if the peak RF power going into the box exceeds 20 watts, even if the antenna presents an ideal 50 ohm match.

To answer your specific question:

Any type of non inductive resistor (e.g. metal film, carbon film, carbon composition) will be okay.

Stu

[n3lrx]

Well that bites.. I was hoping for something simple I could dead bug together because I suck at bread boarding anything complex. I've seen a few designs elsewhere on the internet. I'll keep looking.

Thanks

[W1AEX]

I asked a similar question 4 or 5 years ago and Stu suggested a few changes to that circuit that worked out very well for me. I still use the modified circuit in the picture below and it has not failed at AM carrier power levels up to around 150 watts. I have never pushed it beyond that so I have no clue how much smoke would result at higher levels. The detector will drive a pair of headphones, but it sounds much much cleaner with it's audio output feeding into a set of powered speakers (Bose Companion II Series) and the headphones plugged into the speaker's headphone jack. I connect an old Tektronix 485 into the scope port of the detector and it gives a perfectly clean waveform from 160 through 10 meters.

73,

Rob W1AEX

[n3lrx]

Thanks but I need something that will handle around 350w carrier and PEP around 1kw. I'm not having any luck finding something to handle high power signals. Most of the examples I'm finding are similar to the one in question.

I'll keep digging.

Thanks

[Steve - K4HX]

Just add an capacitive coupler to that circuit and your are set.

[W2NBC]

Mark, KA2QFX has some effective designs here:

http://home.comcast.net/~msed01/RFpickup.html

[n3lrx]

Cool, thanks,  I'll look into using one of those.

[n3lrx]

I've got most of my AM Detector wired and ready to install into a nice virgin aluminum project box.

Before I go drilling holes in it can the pickup and the diode detector be in the same box or do they have to be isolated from each other? If I can put them both in one box that would be cool, but if not I've got to find another pair of boxes to put them in. I'd hate to sacrifice this one if something else will be better.

[n3lrx]

Mark, KA2QFX has some effective designs here:

http://home.comcast.net/~msed01/RFpickup.html

In the first example of the RF Pickup on this page what should the resistor rating be for 1KW+ PEP? Will 1/2 Watt be enough? I'm having trouble finding 50 ohm resistors as it is can't find any in half watt.. I've found a few 100 ohm resistors I could use in parallel.  I'm in the process of ordering parts I don't have and a 50 ohm resistor is one of the items.

tnx

[n3lrx]

Oh, almost forgot what gauge wire would be recommended for the wire in the pickup above? I'm gonna need to order some magnet wire for that too.

[W2NBC]

1. It can be one enclosure (pick-up and detector) or two as I did to move the detector around the shack to various pick-ups (more flexible)
2. Radio Shack has 100 ohm one watt resistors.. in parallel and you're all set:
http://www.radioshack.com/product/index.jsp?productId=2062295&filterName=Type&filterValue=1-watt+resistors
3. The amount of turns varies with the core size and the wire.. start with 10 turns and DECREASE as needed for more output.. Use small gauge solid hook-up wire (Radio Shack) or otherwise.
 


 

[n3lrx]

Kewl.. I'll be hitting the local Rat Shack this afternoon then. I've got hookup wire but its stranded. I'll be using a ft-82-43 (T-50-2)
OD = .500 in / 12.7 mm +/- 0.02 in
ID = .303 in / 7.70 mm +/- 0.02 in
Ht = .190 in / 4.83 mm +/- 0.02 in

I'll experiment a little with it to get the output I need. I still have some parts on order so it may be a week or two before I can play.

[AB2EZ]

Randy

Referring to RF pickup in the circuit you are using [http://home.comcast.net/~msed01/RFpickup.html], which is the type of RF pickup I prefer:

The amount of RF power that will flow into the 50 ohm resistor (and be converted to heating power), if nothing else is connected to the pickup, will be: [the input power to the pickup (i.e. the output power of your transmitter)] x (1/N) x (1/N); where N is the number of turns on the secondary of the transformer (shown as 10 turns in the schematic that is on this web page).

In the example posted on the web page (i.e. with N=10), the power that would flow into the 50 ohm resistor, with 25 watts of RF passing through the pickup, would be 25 watts x 1/10 x 1/10 = 1/4 watt. In the example posted on the web page (i.e. with N=10), the power that would flow into the 50 ohm resistor, with 50 watts of RF passing through the pickup, would be 50 watts x 1/10 x 1/10 = 1/2 watt.

Therefore, the pickup, as shown in the web page will only work properly (i.e. the 50 ohm resistor won't overheat) with a power level of around 50 watts or less.

To make a pickup that would work properly at higher power levels, you should use more turns and a higher wattage resistor. Using only a higher wattage 50 ohm resistor would help, but there are also the separate problems of: having too much peak voltage across the detector circuit when it is connected, excessive core heating, and of saturating the core. If the core material saturates ... e.g. because there are too few secondary turns on the current transformer or too small a core (i.e. the cross sectional area of a single turn is too small)... then the pickup will not produce an output voltage that is proportional to the primary current. Also, if there are too few secondary turns on the transformer or too small a core, the core may get very hot... due to "hysteresis" effects.

[Extra credit for those interested in the physics of what is happening:
To explain the above, note that the voltage across the secondary winding (with the 50 ohm resistor attached) is given by v(t) = N x A x dB/dt, where N is the number of secondary turns, A is the cross sectional area of a single turn (approximately the length of the core x the thickness of the core) and dB/dt is the rate of change of the B-field in the core. Also: dB/dt = u(H) x dH/dt, where u(H) is the differential permeability of the core at a particular value of H, and H(t) is the magnetic field intensity in the core. If N x A (number of turns x the cross sectional area of 1 turn) is too small, then the peak value of H(t) will be too large. As a result, the core will saturate (that is: u(H) will drop to a very low value), the magnetizing inductance of the coil will drop to a very low value, the ratio of a change in primary current to a change in secondary current will drop to a number much less than N, and the core will overheat due to hysteresis effects.]


To accommodate 1000 watts of maximum average power (presumably, for a short time, when tuning/loading the amplifier), you should use
N= 20 turns and a 5 watt 50 ohm resistor (for example, you could use ten (10) 1/2 watt, 470 ohm, metal film or carbon film or carbon composition resistors in parallel = a 47 ohm, 5 watt resistor). The 50 ohm value is not critical. A 47 ohm value is okay. Also, with respect to the core: I think that the core you are planning to use may be a little small (too small a value of length x thickness) for this application. I suggest that you stack four (4) cores on top of each other to obtain a total thickness of 0.19in x 4 = 0.76 in, and wind your transformer using that larger core. This may not be necessary, but if you have the extras cores, I suggest that you use them... particularly for 160m and 75m operation.

You should also use a 2 watt (or larger rating) 75 ohm resistor in the detector module [for example: four (4) 330 ohm, half watt resistors in parallel = an 82.5 ohm, 2 watt resistor). The 75 ohm value is not critical... 82.5 ohms is okay.

From the formula, the RF power that will flow into the 50 ohm resistor (and be converted to heating power) will be:

1000 watts x 1/20 x 1/20 = 2.5 watts (about half of the dissipation rating of the 5 watt resistor)

The peak RF voltage across the 50 ohm resistor, when there is no detector connected to the pickup, will be (1/20) x the square root of: [ 2 x 1000 watts x 50 ohms] = (1/20) x 316V = 15.8V.

This output voltage will be appropriate (not too large and not too small) for driving the detector shown on the web site.

Important: Never apply RF power to this pickup without the 50 ohm resistor across the secondary winding of the current transformer... as shown in the schematic posted on this web site. The value of the resistor is not critical. Forty seven ohms would be okay. But if you applied RF power to this pickup with nothing (an open circuit) across the secondary winding, then the secondary winding could produce a very large voltage across its unconnected ends... depending upon how large a magnetic field (H-field) the core material can support before saturation of the core material occurs.

Also note: If you build the pickup and the detector in the same box (i.e. the distance between the pickup and the detector is less than 1 foot)... then you don't need both the 50 ohm resistor (in the pickup) and the 75 ohm resistor (in the detector). You can just use the 50 ohm resistor, and omit the 75 ohm resistor.

Stu

[n3lrx]

Wow.. Thanks, that's a lot of information to absorb. It may take a few readings before it all sinks in. I did manage to find a 5w resistor that's not wire wound though. Mouser has them. They are thick film. ( http://www.ohmite.com/cat/res_ta.pdf ) But I doubt they will work for RF. Says low inductance though.. But I'll probably go with the multiple resistors in parallel.

Thanks Stu!

Randy

[AB2EZ]

Randy

Thick film is okay for RF, but parallel resistors would be my choice, as well, in this situation. Please note my recommendation (above) regarding the size of the core you are planning to use.

Stu

Randy author=n3lrx link=topic=35055.msg270427#msg270427 date=1383121464]
Wow.. Thanks, that's a lot of information to absorb. It may take a few readings before it all sinks in. I did manage to find a 5w resistor that's not wire wound though. Mouser has them. They are thick film. ( http://www.ohmite.com/cat/res_ta.pdf ) But I doubt they will work for RF. Says low inductance though.. But I'll probably go with the multiple resistors in parallel.

Thanks Stu!

Randy
[/quote]

[AB2EZ]

Randy

The dimensions of the core that you describe below are significantly smaller than the dimensions of an FT 82-43 (thickness and height).

This, combined with the higher power level (vs the design in the referenced web page) is why I suggest that you use a stack of four of these cores to make your transformer.

But, I'm also worried about the type of material in the core you listed. A T-50-2 is an iron core... not a type 43 ferrite core.

If it is an iron core... it won't work!


Stu

Kewl.. I'll be hitting the local Rat Shack this afternoon then. I've got hookup wire but its stranded. I'll be using a ft-82-43 (T-50-2)
OD = .500 in / 12.7 mm +/- 0.02 in
ID = .303 in / 7.70 mm +/- 0.02 in
Ht = .190 in / 4.83 mm +/- 0.02 in

I'll experiment a little with it to get the output I need. I still have some parts on order so it may be a week or two before I can play.

[n3lrx]

Thanks Stu,
That's fine, I ordered the minimum which is 5 so I'll have 4 to use on the pickup.
I wonder why they are claiming it's an FT-82-43 compatible. Oh well, I'll have plenty and 4 won't go to sit in the parts bins forever.. lol

[AB2EZ]

See my last comment (above).

The T-50-2 has much too low a value of permeability for this application.

You need to buy a type 43 material core.

As long as you are buying a new core, I suggest that you buy a type 43 core having the following dimensions:

OD: 1 inch
ID: 0.5 inch
(I.e. 0.25 inch wall thickness, or more)

Height: 1 inch (or more)

Therefore wall thickness x height will be 0.25 square inches, or more.

Stu

Thanks Stu,
That's fine, I ordered the minimum which is 5 so I'll have 4 to use on the pickup.
I wonder why they are claiming it's an FT-82-43 compatible. Oh well, I'll have plenty and 4 won't go to sit in the parts bins forever.. lol

[n3lrx]

How about this?
http://toroids.info/FT114-43.php

OD = 1.140 in / 29.0 mm +/- 0.65 mm
ID = 0.748 in / 19.0 mm +/- 0.50 mm
Ht = 0.295 in / 7.5 mm +/- 0.25 mm

[n3lrx]

Again, the minimum is 5, so I'll have plenty to double up if need be for the high power.

[AB2EZ]

Okay

Four of those in a stack should be fine.

Stu

Again, the minimum is 5, so I'll have plenty to double up if need be for the high power.

[n3lrx]

I just had a thought... My tuner already has the RF sampler on it for the SWR and output can I tap into it with a diode detector and put an audio out jack on the back of my tuner and pipe it into a headphone amp? Will that work?

That will save a lot of time and take up less space than building the project in a separate box. I can use the cores for something else I'm sure.

[Steve - K4HX]

If you can find the line that drives the meter, you might not even need a diode, just a hi-z amplifier. I've done this with cheap Radio Shack SWR meters and it worked FB.

[n3lrx]

Will that load down the meter though? I was thinking more along the lines of tapping into the coupler itself before the meter circuit.