Need signal generator help.

[Explorer]

I've been building an old Rufus Turner "dip adapter" circuit that is designed to be hooked up to a signal generator to inductively measure the resonant frequency of a tank circuit, by watching for a meter spike in the circuit and reading the corresponding frequency off the signal generator display. (The whole thing works very much like a grid dip meter or oscillator, but the signal generator displays the frequency that you're trying to figure out.)

Anyway, I've got a few questions about all of this:

I built the "dip adapter" into a plastic box, because I read that it's not good to have metal around meters; it can impact the accuracy of their reading. (I built a nice old Weston meter into my circuit.) I don't know if I did the right thing or not, because the circuit has a chassis connection and, of course, I can't hook that up to the plastic box. So, I'm wondering about my choice of case...

Second, that chassis connection comes off a bus strip that connects directly to the positive side of the battery--which I find odd. can anyone explain this?

Third, the chassis connection is right by one input in the circuit for the signal generator.

The little signal generator module that I just bought has a positive and negative output. But, doesn't it output an AC signal???

Does it matter how I hook it up to my dip adapter circuit? And, if it does matter, do I want to hook up the negative output of the signal generator to the binding post on the chassis connection side of the circuit, when this feeds directly to the positive side of the battery???

Thanks for any and all help with all of this!

 

[duke37]

A signal generator will put out an AC signal.

I googled Rufus Turner dip adapter and all I got was references to grid dip oscillators. You would not need an oscillator if using a generator.

Show us a circuit diagram (schematic).

 

[(*steve*)]

*wow* thanks for introducing me to Rufus P Turner

 

[Explorer]

I've explained my problems/questions pretty clearly. I'd be happy to post a circuit diagram, as I just redrew it recently and saved it as a PDF file in order to be able to email it to someone, hoping that they would answer all my questions. But, I'm new to this whole forum-thing and have no idea how how I would upload the diagram to this page.

Can someone explain that very simply and clearly?

 

[duke37]

Go to quick reply, then go to advanced.
Click on the paper clip and brouse to find the file, open it and upload it. Close that window (down the bottom).
Preview the post to check that it is right. If so, upload it.

 

[Explorer]

"Dip Adapter" & "Frequency Finder" circuits

OK, Let's see if I can successfully upload the 4-page PDF file that I just created, with 2 circuits that I've built recently. (Paying it forward... Someone was kind enough to email me an article with a circuit diagram a couple of days ago.)

As I said above, the first circuit—the Rufus Turner circuit—is the one I just finished and want to test, but I am concerned about whether or not it is possible to hook up my brand new little signal generator module to it backwards.

I also would like advice as to whether I really should've built it into a metal box, instead of a plastic one.

The simple Robert Brown circuit is very cool. It seems to work. I'm not sure that I can actually pinpoint a resonant frequency with it. I seem to be able to identify a resonant frequency range...i.e., possibly the top of a typical resonant frequency curve. It was a very easy build, as I already had the right "junk" meter that the previous owner had built into a little metal box.

Ahhh...foiled! The file size is too large. My graphics program easily saves documents as PDF files. But, it looks like I may have to save it as 4 separate PDF files in order to upload it. Will try again shortly...

 

[Explorer]

2ND ATTEMPT TO UPLOAD "DIP ADAPTER" & "FREQUENCY FINDER" CIRCUITS.

YIKES! 97 Kb is a small file size. I am going to have to type in some of the explanatory text for the Rufus Turner Dip Adapter circuit, as I can't figure out how to get my scans down to a small enough size to upload.

Please see the post above to offer the help I am looking for. Thanks!
-----------------------------------------------------------------------------------------------------
FROM: 125 One Transistor Projects, Rufus P. Turner, Tab Books, 1972:

5.8—DIP ADAPTER

The circuit in Fig. 5.8 adapts any RF signal generator for use as a dip oscillator. RF output from the signal generator is applied to the RF input terminals, inductor L1 is held close to the circuit under test in the normal manner of using a grid-dip oscillator, and the signal generator is tuned for dip of milliammeter M1. The resonant frequency then is read directly from the signal generator tuning dial. Three plug-in coils provide a test range extending from 100kHz to 250 MHz.... [See below.]

Static (zero-signal) collector current of the 2N107 transistor (Q1) is initially balanced out of the 0-1DC milliammeter (M1) by adjusting the 10,000-ohm wirewound zero-set rheostat (R4) with no signal applied to the RF input terminals. Maximum drain is 6 ma from the 3-volt battery (B1).

COIL A
100 kHz TO 6 MHz
86 turns No. 32 enameled wire closewound on 1-inch diameter form.

COIL B
5 to 35 MHz
11 turns No. 24 enameled wire closewound on 1-inch diameter form.

COIL C
30 to 250 MHz
2-1/2 turns No. 24 enameled wire on a 1-inch diameter form. Space to a winding length of 1/8 inch.

 

[Explorer]

Ahhh...I managed to not pay attention to that last paragraph of Turner's explanatory text until I was typing it in above. I finished building the thing, but never even turned it on, as I was questioning how to hook up the signal generator to it. But, I must've done something wrong. When I turn it on (without the sig. gen. attached), all the needle on the meter wants to do is slam to the left (i.e., below zero). And turning the pot does nothing.

I used a 10K linear pot. Does he mean something different by a zero-set rheostat???

 

[duke37]

The 2N107 is a germanium transistor which will have significant leakage. I presume that R4 is there to pass a similar current so that the resting current through the meter is zero.

Fault could be bad transistor, battery connected the wrong way or wireing error.

Any resistance will do for R4 as long as is can be set to the required value.

 

[Explorer]

It's not a complicated circuit and I can't see what I've done wrong. But, I have seen books that are wrong, including a Google digitized copy of a book by Hertz in which one illustration, for instance, was inserted upside-down.

Does the Turner circuit show the meter connection backwards??? (See Dip Adapter PDF above.)

I'm still looking for advice on whether there's a wrong way to hook up the signal generator to this circuit, as well. Thanks.

 

[Explorer]

Well, I went ahead and reversed the meter connection. Now the needle will move to the upper end of the scale and adjustment of the pot. brings it back down. However, I'm unable to get the needle to go to zero. Does that mean I should use a pot. that's bigger than 10K?

In fact, I'm not sure what this sentence in the text means:

"Static (zero-signal) collector current of the 2N107 transistor (Q1) is initially balanced out of the 0-1DC milliammeter (M1) by adjusting the 10,000-ohm wirewound zero-set rheostat (R4) with no signal applied to the RF input terminals."

Duke37, anyone, can you explain that to me (i.e., how I should adjust the pot. before I hook up the signal generator)?

Also, I'm still looking for advice on whether there's a wrong way to hook up the signal generator to this circuit. THANKS.

 

[duke37]

The potentiometer is there to pass the identical current which is leaking through the transistor.
Measure the voltage across the emitter/collector connections of the transistor.
If the voltage is less than half the supply voltage, then the reistance must be reduced.
If the voltage is greater than half the supply voltage, then the reistance must be increased.
If the voltage equals half the supply voltage, then the mA meter will show zero.
The meter should go up scale when the collector voltage goes down.

The signal generator should be connected to the input terminals.

 

[Explorer]

Duke, thanks. Although, I don't really understand...

Should I measure the voltage, as you suggest, with the pot. adjusted so that the needle is at the upper end of the scale? (The device is supposed to work like a grid-dip meter.)

And, I know the signal generator should be connected to the input terminals, but my signal generator has an output labeled positive and negative. And the chassis/ground connection in this circuit has me utterly puzzled, coming off the positive side of the battery. I'm used to seeing a so-called ground connection coming off the negative side of the power supply. Do I have to worry about polarity, hooking up the signal generator?

 

[duke37]

Just twiddle the pot to get half the supply voltage. You will then find out if the pot is passing too much or too little current.

I do not know what the generator + and - are unless it will give an output with a DC bias. In any case, the circuit uses a capacitor which will remove any DC bias.

 

[Explorer]

Duke, hi,

Well, I'm lost. I did as you advised and got the 3-volt supply down to 1.6 volts. So, it seems as though I could use a slightly larger pot. but, I'd have to order something. The next size I have on hand is 50K. I put that into the circuit, but it seemed too big of a jump. A tiny movement of the knob sent the meter needle all over the place.

I thought I'd be able to use this dip adapter to figure out the resonant frequency of a bunch of coil and trimmer cap combinations that I removed from an old piece of equipment. But, when I put the coil next to any of them (I made coil A, figuring that that had a pretty decent range), the dip adapter dips every time at 4300 KHz.

I made my coil by entering the specs of coil A into this very nifty coil calculator:

http://www.crystalradiosupply.com/1/calculators/Protected_coilcalculator.htm

to figure out the inductance that it should be. I didn't have 32-gauge wire. I only had 30-gauge wire. And my coil form is 1.03 inches. So, the coil calculator above indicated a coil of around 92 turns instead of 86 turns. I got the inductance of my coil VERY close to what the coil calculator above indicated for Turner's specs--within 1 or 2 microhenrys, if I remember correctly.

Can you spot where my thinking might be off or what I may be doing wrong, or what perhaps I've done wrong???

My little sig. gen. goes up to 10 MHz.

THANKS for all your help!

 

[duke37]

You seem to have a good transistor with little leakage (or it is not working). The leakage will vary with temperature.

If you need a bit more resistance, you can put a fixed resistance in series with the pot. The turn sensitivity will then be similar to the original.

I have not used a circuit such as you have but have a grid dip oscillator and it is important to get the coupling correct. At low frequencies it is very difficult to get sufficient coupling.
I do not think the inductance of the coil is critical.

The 4.3 MHz indication that you have may be the self resonce of L1.

 

[Explorer]

Forgive me, Duke, if you're still paying attention. This may be long.

After your last remarks, I took out a grid-dip meter I bought a while back that I never figured out how to use (I'd never been able to get the needle to dip anywhere on it except the very top and bottom of each scale) and put a 10 pF cap across my dip adapter coil, and tried to measure it with the GDO. My inductance meter measures my coil for the dip adapter at 153.3 uH. An online calculator ( http://electronics-diy.com/calculators.php ) informed me that the resonant frequency for the combination should be 4.06 MHz. I accidentally stuck the first GDO coil in and got dips in several places--the note I have here says 1.8, 2.1, and 3.2 MHz, which, if I've grabbed the right piece of paper this morning, sounds about right. With the 2nd GDO coil I got a dip around 6.1 MHz, if I remember correctly. I very definitely did not get any dip around 4 MHz, that I can recall. (I gave up in exasperation very early yesterday to go out and enjoy a beautiful spring day.)

The Robert Brown Frequency Finder seems to work great with the signal generator on an old AM radio "Select-A-Tenna" that I took apart. And, I can put an AM radio next to the Select-A-Tenna and hear very interesting touch-tone-type telephone tones across the dial behind the AM radio garbage until I finally get dead silence at about the frequency that the sig. gen. and the Frequency Finder indicate. But the Freq. Finder doesn't seem to work at all with any of the other coil/cap combos I've been trying to measure. (I'll post a photo of them later on.)

And, I think I got a couple of very small questionable dips on a couple of the coil/cap combos with the dip adapter by placing the end of the DA coil right by the end of the test coil. But, then I couldn't get a confirmation with the Freq. Finder.

So, all 3 devices seem to be doing something entirely different! And, I'm not sure that I believe anything that anyone writes about coils anymore! (I saw a comment on another forum by an experience electrical engineer, who said that he thought that all RF work was entirely "black magic"!)

I found an old QST article in my files last night--"Add-ons for Greater Dipper Versatility," Feb. '81--that describes a different way to use a GDO. I'm going to try that today. It describes a way of making a capacitive coupling clip that can be clipped directly to the LC circuit under test:

"The unknown can be a toroid or an LC circuit below a chassis or in a tight place. A single wire is clipped to the unknown and then lightly coupled (a few picofarads) to one side of the dipper tank circuit."

The author goes on to explain that the coils for his Heathkit GDO plug in via a phono plug and that one side of the coil is connected to the outer sleeve of the phone plug, while the other is connected to the center post. He made an aluminum sleeve that fit around the outer sleeve of the phone plug, insulated in between, I assume, to accomplish the capacitive coupling, and wrapped outside in black electrical tape, for exterior insulation. (He describes an earlier version as: "a turn or two of the insulated single wire around the outer surface of the phono plug of the dipper coil.")

The author claims it works quite well.

Anyway, I'm typing all of this and trying to be very clear about what I'm doing, in case my exploits are at all helpful to any other poor soul.

Duke, if you have any more thoughts or comments, I'm appreciating them all.

 

[Explorer]

Photos of what I was hoping to measure and what I'm working with...

 

[Explorer]

Duke37, are you still out there?

I spent yesterday re-doing my circuit board in the dip adapter, because I felt I hadn't laid out the first one very neatly. I also took the time to line the whole plastic box with aluminum tape, as I read another text that says don't EVER build any RF project in anything but a metal box. But, after I got all done doing that (which was pretty time consuming) I wondered if, at some frequency, the box wouldn't become a resonant cavity. Wouldn't it???

I also tried a couple of chassis connections in different places (now that I have metal inside my box), but, I didn't feel that they helped at all. In fact, I ran a wire from the point that Turner indicates in the circuit diagram to a ring around the shaft of the on/off switch, so that it would be snug up against the metal tape, but then the needle on the meter just wanted to slam back to the left again, when I turned the device on.

So, I got done with all of that and removed my 2nd attempt at a chassis connection and decided to keep my testing to something I thought I could be fairly sure about--the AM radio range. I tuned the dial of a little AM radio until I kept hearing "ABC radio 77, ABC radio 77." And I placed the dip adapter's coil so that it lined up with where I believe the antenna coil to be in the radio and I got a good dip right at 770 KHz! But, then I kept turning the knob on the signal generator and I got at least 3 small dips in other places and a HUGE dip at 1250 KHz. If I hadn't listened to the radio and identified the station, or had covered up the dial, I would've thought the radio was tuned to that frequency.

770 and 1250 are not harmonics of each other. Any idea why I'd get such a big dip in a totally unexpected place?

And, do you know enough about this stuff to suggest any improvements to Turner's circuit diagram, which I posted above?

THANKS!

 

[duke37]

I am still here - just.

Lining the box to screen it is not a bad thing but I think it not necessary at low radio frequencies. If the box is fully screened and the detection coil is inside, then nothing will happen.

The little radio you have will be a superhet with an intermediate frequency of about 465kHz (not KHz). To translate an incoming signal of 770kHz to 465kHz the oscillator would run at 1235kHz not far from what you measure.

1250 - 770 = 480 which is higher than a normal IF but is possible. Try a modulated signal from your generator to find the actual IF, it will doubtless come through loud and clear.

The resonant frequency of a tuned circuit will be affected by any adjacent capacitance or inductance. To get reasonably accurate readings the coupling should be a little as possible subject to sufficient sensitivity.