General > General Technical Chat
Frequency Divider for older Oscilloscopes??
(1/13) > >>
GlennSprigg:
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)
tggzzz:
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.
David Hess:
https://www.eevblog.com/forum/projects/diy-ghz-sampling-head-for-lt100mhz-scopes/
Ian.M:
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.
2N3055:

--- Quote from: GlennSprigg on April 02, 2020, 11:16:57 am ---Yea, I know. I can hear people screaming... "Just buy a 100-Mhz Scope!"  Sigh..  8)

--- End quote ---

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.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod