Amplification at high frequencies

[SMD1990]

I am trying to read high frequency signals (as in, tens to hundreds of Megahertz) from a magnetic tape. I figure the original preamp will be unable to function anywhere near that range. So, I am trying to find a way to replace it.

I have yet to get an operational amplifier to work. I keep going online, trying to figure out what I am doing wrong. As far as I can tell, everything is correct. Maybe the output from the tape head is too weak for my op-amps?

My multimeter cannot detect any AC voltage from the tape head. It reads 0.1 when I set it to read μA AC. If I connect a piezoelectric earpiece to the tape head's output, and stick the earpiece in my ear, I can faintly hear the playback.

So far, I have only been messing with audible frequencies. Nothing high-frequency yet. My thought is, "First, get the amplification working. Then, move up to higher speeds."

I got to wondering if maybe I could just use the circuitry from an old television antenna. I have one with a built-in amplifier. It does not tune to a particular frequency; it just amplifies. (It leaves the tuning to the TV.)

However, I am not sure how to connect the two wires from the tape head to it. I tried a few ways, but nothing worked. (I also subbed-in the headphone output from a Game Boy. A much stronger signal. Still, I cannot hear anything out of the antenna's coaxial cable. Not even when using my piezoelectric earpiece)

Would an amplifying TV antenna even work for this? I mean, it is supposed to amplify untuned signals in the 54-72, 76-88, 174-216, 470-806 MHz range. To me, it sounds like it should work for my purposes. Then again, I am not sure. And brief testing suggest it will not. (Similar testing says that op-amps cannot amplify the signal, either.)

 

[vk6kro]

Audio tapes and heads will not work much above audio frequencies.

One problem is that the gap in the head is wide enough to have many cycles of high frequency signal in it, on the tape. So, it can't record or recover individual cycles.

Even at audio frequencies, the output of a tape head is very small, probably a few millivolts at most.
You should be able to see this on an oscilloscope, but you can use the amplifier that came with the tape head just for audio.
This amplifier will also provide proper biasing for the head, but none of this will help it work at radio frequencies.

 

[SMD1990]

First of all, I did not specify audio cassettes. I currently am testing with a compact cassette tape, but the final implementation may use a VHS.

I am aware that compact cassettes are limited to about 20 KHz (if that). My thought is that increasing the tape speed will allow for higher frequencies, at the expense of playback time.

 

[Averagesupernova]

To get the desired frequencies out of tape the writing speed needs to be increased. This is why VCRs use a flying head. You cannot record or playback signals onto magnetic tape even with an appropriate read/write head at slow tape speeds. The flying head in a VCR spins at 1800 RPM (I think) and is roughly 2.5 inches in diameter so it goes through about 20 feet per second of tape. Unless you have miles of tape I think you will need to implement a flying head type scheme.
-
I don't think you will be able to read and write speeds onto tape in the hundreds of megahertz range. Most VCRs use signals in the 1 to 10 Mhz range.

 

[SMD1990]

For this project I am working on, all I really need is a few seconds of playback. Just something to quickly demonstrate what I am trying to do.

Certainly, as I have already acknowledged, the tape speed will need to be increased for higher frequency signals.

However, that has nothing to do with my question. I am wondering about ways to amplify the high-frequency signal. Locally, I cannot get transistors (or ICs) with a fast enough switching time. So, I would have to order higher performance parts online.

But, since I cannot seem to get even audio frequencies amplified using either of my two available op-amps (a LM324 and a TL082), I am hesitant to place an online order.

So, basically, I am looking at alternative options. I have this old hard drive which I have taken apart. My understanding is that it contains a preamp. I have even found a PDF containing the pinout. A datasheet would be nice, but the preamp's pinout is certainly next best thing.

Still, it is small and has 32 pins. Not something easy to experiment with. I would much rather use the amplified TV antenna's circuitry. I just do not know how, or if such might even work. To me, it sounds like it should. Then again, I fully expected either of my op-amps to be able to amplify a sound audible through a piezoelectric earpiece. Yet, apparently, neither can.

So one wire should connect where one of the antennas did. Where should the other wire be grounded? Antennas only have one connection. I have tried connecting the other wire to ground, but I cannot hear anything, unlike when connecting directly to the earpiece, when I can.

 

[vk6kro]

But, since I cannot seem to get even audio frequencies amplified using either of my two available op-amps (a LM324 and a TL082), I am hesitant to place an online order.

[PLAIN]http://dl.dropbox.com/u/4222062/LM324.PNG

You can amplify with a LM324 as above. It is shown with one power supply which could be a small 9 V battery.
Alternatively, you could get a LM386 which has enough power output to drive a speaker and it can have a gain between 20 and 200.

However amplifying a signal at, say, 100 MHz is not difficult. You would need to know if it was only 100 MHz or a range of frequencies around 100 MHz that you wanted to amplify.

 

[SMD1990]

That schematic is strikingly similar to what I have tried. The only real difference is the presence of a couple of capacitors in your schematic.

One dumb question, though. Should there not be a resistor between the 9V battery and the supply voltage pin?

 

[schip666!]

You don't need a resistor to the power supply pin, in fact if you have one that could be your problem...

Also the unity gain bandwidth of the TL082 is 3Mhz and the full-scale output rolls off at about 200KHz, so those parts are not going to help you at 10-100Mhz. I would look into how a VCR does it's head input amplification.

However, on the plus side, speeding the tape way up does increase the high frequency bandwidth.

 

[SMD1990]

I know that neither of my op-amps can function at the frequency I desire. However, for trying to figure out how to connect an op-amp for amplification, they will do.

I mentioned ordering higher performace components online. I am just hesitant to do so until I can amplify the audible frequencies coming from the tape. First, get it working. Then, move on to higher speeds / higher frequencies.

Really? I assumed the resistor was always just omitted, much like the source of the voltage. I figured since the datasheets always mention a maximum current, that meant I needed to restrict the current to below that.

I will need to try again, sans resistor...

 

[likephysics]

I am trying to read high frequency signals (as in, tens to hundreds of Megahertz) from a magnetic tape. I figure the original preamp will be unable to function anywhere near that range. So, I am trying to find a way to replace it.

I have yet to get an operational amplifier to work. I keep going online, trying to figure out what I am doing wrong. As far as I can tell, everything is correct. Maybe the output from the tape head is too weak for my op-amps?

My multimeter cannot detect any AC voltage from the tape head. It reads 0.1 when I set it to read μA AC. If I connect a piezoelectric earpiece to the tape head's output, and stick the earpiece in my ear, I can faintly hear the playback.

So far, I have only been messing with audible frequencies. Nothing high-frequency yet. My thought is, "First, get the amplification working. Then, move up to higher speeds."

I got to wondering if maybe I could just use the circuitry from an old television antenna. I have one with a built-in amplifier. It does not tune to a particular frequency; it just amplifies. (It leaves the tuning to the TV.)

However, I am not sure how to connect the two wires from the tape head to it. I tried a few ways, but nothing worked. (I also subbed-in the headphone output from a Game Boy. A much stronger signal. Still, I cannot hear anything out of the antenna's coaxial cable. Not even when using my piezoelectric earpiece)

Would an amplifying TV antenna even work for this? I mean, it is supposed to amplify untuned signals in the 54-72, 76-88, 174-216, 470-806 MHz range. To me, it sounds like it should work for my purposes. Then again, I am not sure. And brief testing suggest it will not. (Similar testing says that op-amps cannot amplify the signal, either.)

Try using a differential amplifier and then a head phone driving opamp.
Diff amp has very high input impedance and lots of gain. It can amplify signals in microvolt region.

 

[SMD1990]

Okay... Since I was wrong about using a resistor, let's confirm that I am not making any other mistakes.

I have a resistor (R2) connected between the output pin and the inverted input pin. I have a second resistor (R1) connnected between the inverted input pin and one of my wires from the tape head. The other of the tape head's wires goes to the non-inverted input pin.

I have two 9V batteries and two electrolytic capacitors (35 μF each) configured so as to be a dual power supply. The configuration is just like http://www-ece.rice.edu/~jdw/242_lab4/img6.gif" , only with larger capacitors.

(Edit: Actually, that is not the picture I meant to show. And now, I cannot find the one I intended. Basically, the "power supply" was two batteries, with ground in between. Oh, and the capacitors shown were polarized.)

I connected the positive voltage to the Vcc pin, and the negative to the ground pin.

My piezoelectric earpiece (or, in some cases, speaker) connects to the output pin... and Vcc?

I have tried connecting the other end of it to several different pins. When using output from a Game Boy's headphone jack, that seemed to produce the closest thing to a desired result. Connecting to the inverted input pin was the same as connecting directly to the source, which it basically is.

I could not hear anything when it was plugged into the ground or non-inverted input pins.

Though there seemed to be somewhat of an amplification using the output to Vcc configuration, when using my earpiece with the tape head as the source, I cannot hear anything. Actually, if anything, there is just a hum. Reminds me of a high-voltage hum.

I have tried various resistor values. R2 of 1 Megaohms, R1 of 100K; R2 of 100, R1 of 1; R2 of 10, R1 of 1; R2 of 1, R1 of 1; et cetera. The output voltage of the Game Boy's headphone jack is so high, I could hardly tell much of a difference between values. When trying to amplify that from the tape head, I could never hear anything. Just a hum.

 

[vk6kro]

You probably have something like this:

[PLAIN]http://dl.dropbox.com/u/4222062/LM324-bal.PNG

That seems OK. Note the earpiece should have one side grounded to the center point of your two batteries.

The capacitor in series with the input will stop DC passing through the tape head if the amplifier gets unbalanced.

If possible, get a multimeter and measure the output voltage. It should be at zero volts relative to ground when there is no signal.

If you have a low impedance (like 32 ohm) earphone, then an opamp will have trouble driving it. You need a speaker amplifier like a LM386 to do that.