Author Topic: Frequency Divider for older Oscilloscopes??  (Read 459 times)

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Offline GlennSprigg

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Frequency Divider for older Oscilloscopes??
« on: April 02, 2020, 11:16:57 am »
I can't find enough info about doing this. (Excuse my deteriorating mind!  :D)
Is there a 'relatively' simple way to create an external Frequency Divider, to say.. measure frequencies on an old 10-Mhz Analog Scope, such as a 'Divide-By-Ten' circuit to measure up to 100-Mhz??  I've seen some 'Counter' circuits that obviously would not retain anything of the original 'WaveForm', and at best could just count the Frequency, beit a simple SquareWave output. Maybe for the 'too-hard' basket?  :P

Yea, I know. I can hear people screaming... "Just buy a 100-Mhz Scope!"  Sigh..  8)
 

Offline tggzzz

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Re: Frequency Divider for older Oscilloscopes??
« Reply #1 on: April 02, 2020, 11:27:29 am »
You need to define the input you want to observe. Consider especially the amplitude and range of frequencies of interest, e.g. 100MHz+-5MHz.

If you are interested in seeing 0-100MHz on a 10MHz scope, forget it.

If you are interested in 95-105MHz (i.e. 10MHz bandwidth) then you can use RF techniques to mix and downconvert the 100MHz signal to baseband.

Another technique, which only works with repetitive signals, is to sub-sample the input. Many digitising scopes do that, but I suspect building an external sampler will be more than you wish to do.
« Last Edit: April 02, 2020, 11:30:39 am by tggzzz »
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Online David Hess

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Online Ian.M

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Re: Frequency Divider for older Oscilloscopes??
« Reply #3 on: April 02, 2020, 11:51:54 am »
Errr.... *NO* a frequency divider circuit isn't worth doing - just get a 100MHz or better frequency counter.  Its possible to home-brew a 100MHz frequency counter with a PIC18 (using its T0CKI Timer 0 clock input, which is good up to 50MHz if properly handled and can be backed by 24 bits of internal hardware counter - see AN592 for how to get the low byte out of the internal prescaler) + a fast D type flipflop to get it up to 100MHz, guarentee 50% duty cycle at T0CKI and also gate the input and allow AN592 prescaler access.  If you need it to count slow or low level signals you'll need some sort of limiting preamp, and if you need to go above 100MHz, will need a prescaler.   If you aren't already into PICs :( its better to buy a commercial instrument than to build!

OTOH if you need to actually view fast repetitive signals, you could (in theory) build an analog sampling frontend.  You'll need a very fast sample and hold, a fast trigger circuit, and an adjustable sampling clock gated by a monostable.  Its also a bitch to build a properly compensated input attenuator and/or wide band preamp if you want a wide input range.  See https://en.wikipedia.org/wiki/Oscilloscope_types#Analog_sampling_oscilloscope for a very brief intro + a few helpful links.   However, unless you have most of the parts needed already gathering dust , lots of zero cost free time and nearly infinite patience with setbacks, you'd be better off buying a modern 100MHz or better DSO.

Edit: David and Tggzzz are also suggesting analog sampling front ends.
« Last Edit: April 02, 2020, 12:22:07 pm by Ian.M »
 
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Online 2N3055

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Re: Frequency Divider for older Oscilloscopes??
« Reply #4 on: April 02, 2020, 12:08:41 pm »
Yea, I know. I can hear people screaming... "Just buy a 100-Mhz Scope!"  Sigh..  8)

Because what you want is not possible. You have to have 100MHz bandwidth scope, no matter if it is analog, analog sampling, digital sampling or digital real time.

So, yes, you need to get a 100MHz scope to have a 100MHz scope.

You could use current low bandwidth scope as a display for sampling head (that other already pointed to).
Problem is that ANY 100 MHz scope (even the most stupid ones, that you can get for 100 USD new) will be better than any contraption you can make cheaply...

Sorry.
 
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Offline GlennSprigg

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Re: Frequency Divider for older Oscilloscopes??
« Reply #5 on: April 02, 2020, 12:23:35 pm »
To all, point taken and understood.   ;D
 
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Offline tggzzz

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Re: Frequency Divider for older Oscilloscopes??
« Reply #6 on: April 02, 2020, 03:28:07 pm »
You have to have 100MHz bandwidth scope, no matter if it is analog, analog sampling, digital sampling or digital real time.

Not quite true. Back in the 70s I saw 10MHz waveforms captured and displayed on an XY pen chart recorder :)

If you have a sampling scope everything up to and including the sampler has to be full speed, but the rest can be as slow as is convenient.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Offline Kleinstein

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Re: Frequency Divider for older Oscilloscopes??
« Reply #7 on: April 02, 2020, 04:13:25 pm »
Some older scopes used analog CCD like memories to first sample the signal relatively fast and than play it back at a reduced speed for the slower ADC. In principle this could be use, but those CCD memories, especially the faster ones are hard to get parts.
 
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Online 2N3055

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Re: Frequency Divider for older Oscilloscopes??
« Reply #8 on: April 02, 2020, 05:10:57 pm »
You have to have 100MHz bandwidth scope, no matter if it is analog, analog sampling, digital sampling or digital real time.

Not quite true. Back in the 70s I saw 10MHz waveforms captured and displayed on an XY pen chart recorder :)

If you have a sampling scope everything up to and including the sampler has to be full speed, but the rest can be as slow as is convenient.

That was only display. You had 10MHz scope (sampler type) in front of it. I thought I explained what I meant. It was a 10 MHz sampling scope assembled from two devices, sampling head with S/H and pen chart recorder as display....
 

Offline tggzzz

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Re: Frequency Divider for older Oscilloscopes??
« Reply #9 on: April 02, 2020, 07:16:48 pm »
You have to have 100MHz bandwidth scope, no matter if it is analog, analog sampling, digital sampling or digital real time.

Not quite true. Back in the 70s I saw 10MHz waveforms captured and displayed on an XY pen chart recorder :)

If you have a sampling scope everything up to and including the sampler has to be full speed, but the rest can be as slow as is convenient.

That was only display. You had 10MHz scope (sampler type) in front of it. I thought I explained what I meant. It was a 10 MHz sampling scope assembled from two devices, sampling head with S/H and pen chart recorder as display....

The OP wants to see a 100MHz signal on his 10MHz scope. That is possible with a sampling front end.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Online 2N3055

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Re: Frequency Divider for older Oscilloscopes??
« Reply #10 on: April 02, 2020, 07:31:16 pm »
You have to have 100MHz bandwidth scope, no matter if it is analog, analog sampling, digital sampling or digital real time.

Not quite true. Back in the 70s I saw 10MHz waveforms captured and displayed on an XY pen chart recorder :)

If you have a sampling scope everything up to and including the sampler has to be full speed, but the rest can be as slow as is convenient.

That was only display. You had 10MHz scope (sampler type) in front of it. I thought I explained what I meant. It was a 10 MHz sampling scope assembled from two devices, sampling head with S/H and pen chart recorder as display....

The OP wants to see a 100MHz signal on his 10MHz scope. That is possible with a sampling front end.

That is what I'm saying too. And that comprises a sampling scope with required bandwidth.
His original question whether he can down-convert the signal like it is done in direct heterodyne receiver,  and look at it on 10MHz scope and preserve pulse shape of original signal. He can't.

It wasn't discussion of how sampling scopes work.
 

Offline tggzzz

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Re: Frequency Divider for older Oscilloscopes??
« Reply #11 on: April 02, 2020, 08:35:45 pm »
You have to have 100MHz bandwidth scope, no matter if it is analog, analog sampling, digital sampling or digital real time.

Not quite true. Back in the 70s I saw 10MHz waveforms captured and displayed on an XY pen chart recorder :)

If you have a sampling scope everything up to and including the sampler has to be full speed, but the rest can be as slow as is convenient.

That was only display. You had 10MHz scope (sampler type) in front of it. I thought I explained what I meant. It was a 10 MHz sampling scope assembled from two devices, sampling head with S/H and pen chart recorder as display....

The OP wants to see a 100MHz signal on his 10MHz scope. That is possible with a sampling front end.

That is what I'm saying too. And that comprises a sampling scope with required bandwidth.
His original question whether he can down-convert the signal like it is done in direct heterodyne receiver,  and look at it on 10MHz scope and preserve pulse shape of original signal. He can't.

It wasn't discussion of how sampling scopes work.

In the first post he is after frequency, not wave shape.

In the second post I noted you can simply downconvert to below 10MHz.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Online 2N3055

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Re: Frequency Divider for older Oscilloscopes??
« Reply #12 on: April 02, 2020, 08:46:34 pm »

In the first post he is after frequency, not wave shape.

In the second post I noted you can simply downconvert to below 10MHz.

I went back an read it again. And now I must say you are 100 % correct about that. I was confused..
Sorry about that and thank you for being patient with me and setting me straight..
All the best,
Sinisa
 

Offline tggzzz

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Re: Frequency Divider for older Oscilloscopes??
« Reply #13 on: April 02, 2020, 09:20:01 pm »

In the first post he is after frequency, not wave shape.

In the second post I noted you can simply downconvert to below 10MHz.

I went back an read it again. And now I must say you are 100 % correct about that. I was confused..
Sorry about that and thank you for being patient with me and setting me straight..
All the best,
Sinisa

No problem. We all make mistakes, but too few of us admit it :)

Besides, some mistakes are more interesting than others :)
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
The following users thanked this post: 2N3055


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