Test Equipment: Oscillocopes

[Brian516]

For decent coax and sinewaves:  20MHz*meters will be OK.  IE if you have a 10MHz signal a 2 meter unterminated coax won't cause much distortion from the reflections.  For squarewaves, 0.4ns/cm = 1ns/inch rise or fall times will be reasonably clean.  Modern logic like DDR3 has rise and fall times down to around 50ps - 70ps which demands termination in almost all applications.  Stuff like common CMOS logic is much slower and can go a couple to several inches without termination cleanly.

MarkE -
So if I want to test the max output of my oscillator (50Mhz), I should use the shortest possible coax direct into my scope (I do not have any in-line attenuators yet), preferably around 15.6 inches or less? (Just a guess here, but preferably I would use a tee connection with one side going directly into a 50ohm terminator and a 10:1 or 20:1 attenuator between the tee and the scope, so I don't have to max out my time/div to .05us .  Am I correct?)   
What you are stating with square wave with fast rise/fall times applies when I am using a direct connection to scope and not a probe?

Once I actually receive at least the terminations, and make some decent patch cables, I will experiment further with these setups and compare probe use to direct connections with the PC signal generator and the oscillator.  Should I find a pair of, or at least one 20:1 attenuator as well, or is that not necessary for the time being?

TK (and all),

What are your thoughts on this GR974 - BNC adapter?
http://www.ebay.com/itm/BNC-TO-GR874-COAXIAL-CONNECTORS-WITH-16-BELDEN-WIRE-RG58A-U-/291176479245?pt=LH_DefaultDomain_0&hash=item43cb787a0d

I personally do not like the crimp connection at the BNC, but the main thing here is just getting a GR874 connector. I can always replace the BNC fitting, shorten the cable, or possibly even replace the RG-58 cable with something better if it becomes necessary at some point.  I would rather get a GR to BNC direct one piece, but everyone is wanting upwards of $30 for a single unit..

[MarkE]

MarkE -
So if I want to test the max output of my oscillator (50Mhz), I should use the shortest possible coax direct into my scope (I do not have any in-line attenuators yet), preferably around 15.6 inches or less? (Just a guess here, but preferably I would use a tee connection with one side going directly into a 50ohm terminator and a 10:1 or 20:1 attenuator between the tee and the scope, so I don't have to max out my time/div to .05us .  Am I correct?)   
What you are stating with square wave with fast rise/fall times applies when I am using a direct connection to scope and not a probe?

For sinewaves the signal is narrow band and we can talk about the signal frequency.  Yes, if you do not want to see significant distortion from an unterminated 50MHz signal then you don't want a propagation path longer than about 1/10th the period: 1/10th * 20ns = 2ns ~1.25' or about 15 inches.  It is not as though at 15.1" you will have a problem.  But you will get a very rude surprise at a quarter wavelength.

Things won't be too bad using a BNC T right at the scope that has a 50 Ohm terminator on one leg.

Once I actually receive at least the terminations, and make some decent patch cables, I will experiment further with these setups and compare probe use to direct connections with the PC signal generator and the oscillator.  Should I find a pair of, or at least one 20:1 attenuator as well, or is that not necessary for the time being?

Attenuators have limited usefulness.  They can be useful for limiting reflections.  Your 465 limits you to 100MHz, so I don't think reflections are going to be your biggest headaches.  Sometimes a 2:1 power divider is a handy thing to have around.  With one of those you can fire off a signal to a DUT and monitor the original signal on your scope without distorting it.

TK (and all),

What are your thoughts on this GR974 - BNC adapter?
http://www.ebay.com/itm/BNC-TO-GR874-COAXIAL-CONNECTORS-WITH-16-BELDEN-WIRE-RG58A-U-/291176479245?pt=LH_DefaultDomain_0&hash=item43cb787a0d

I personally do not like the crimp connection at the BNC, but the main thing here is just getting a GR874 connector. I can always replace the BNC fitting, shorten the cable, or possibly even replace the RG-58 cable with something better if it becomes necessary at some point.  I would rather get a GR to BNC direct one piece, but everyone is wanting upwards of $30 for a single unit..

With shipping this used unit is going to cost you $15.00  It looks serviceable.

[TinselKoala]

@Brian:

This should be a suitable replacement for your panel mount 874 connector:
http://www.ebay.com/itm/GENERAL-RADIO-GR-874-PB58-CONNECTOR-NEW-/121589661784?pt=LH_DefaultDomain_0&hash=item1c4f4f0058

This place has a bunch of 874 fittings, etc but I didn't see the precise panel mount connector listed, you may want to call them to see if they have the exact replacement:
http://www.mgs4u.com/RF-Microwave/General-Radio-GR-874.htm

If I were you I'd stick to 50 ohm co-ax and the BNC connector system for all Ts, terminators, attenuators, patch cords and other fittings, and just get the one necessary 874-to-BNC adaptor for the GenRad oscillator itself.

[Brian516]

@Brian:

This should be a suitable replacement for your panel mount 874 connector:
http://www.ebay.com/itm/GENERAL-RADIO-GR-874-PB58-CONNECTOR-NEW-/121589661784?pt=LH_DefaultDomain_0&hash=item1c4f4f0058

Yep, that's the exact one that the manual specifies as it's "Panel Mount Connector".  I did not see that listing when I first searched.

If I were you I'd stick to 50 ohm co-ax and the BNC connector system for all Ts, terminators, attenuators, patch cords and other fittings, and just get the one necessary 874-to-BNC adaptor for the GenRad oscillator itself.

That is the plan.  When I said "Replace the BNC connector" I meant replacing the crimp-on BNC with a Solder-type or Compression-type BNC fitting.  From my experience with cable and satellite systems, crimping, crushing, or even sometimes squeezing a coax cable can cause distortion to the signal. This occurs mostly with digital signals, but just the fact that it can distort a signal at all makes me not want crimp fittings for anything. Not only that, but they aren't very secure. One good yank and the connector could come right off. 
As for the 'making my own patch cables' - the only reason I am planning on doing this for the time being is that I have a surplus of compression BNC fittings, the coax strippers, and compression tool to do it all. I have thousands of feet of RG6 that I can't use, but RG58 is pretty cheap. I could make 12 2' cables for the cost of one actual patch cable, plus I can make them custom length so I don't have a bunch of excess rolled up.  If I had to buy everything, I would just buy a couple pairs of cables instead.

I was playing with my Arduino a little earlier in preparation to build that Inducto-meter.  I built the I2C LCD interface and found out that the coding in the sketch is apparently incorrect... and I have no idea how to fix it.  So I guess until I learn how to write code for it, or someone else fixes it, I will be using the multiple wire LCD setup. I did notice that with that specific IC I can run up to 8 LCD's off of the same two wires by just assigning different addresses.  I'm assuming I could easily add a piezo instead of one of the other screens.  That would bring me up to the Parallax's capabilities for much cheaper than $45. I'll figure out the coding issue in the next few days hopefully. 

[Brian516]

Well, well, well.....  It appears that Mr Tek decided to start acting up again....
Everything has been fine and dandy up until today, when Channel 2 decided it didn't want to trigger above the .2V/div (on 1x, 2V/div 10x) setting.
I have been using it a bit probing around on various circuits, and never had any problems at all, until I got this FG today and started testing it out.

This is where things went wrong:
At first, I was comparing the MAIN output (on CH1 scope) to the TTL output (CH2 scope) for a while, and playing with the sweep settings on the Tek CFG253.  At this point everything was fine.
Next I disconnected from the Tek and connected up to the Wavetek 166.  I tested it out a bit on only channel one for a while, and then hooked up TTL to channel 2 of my scope, and that is when I noticed something was wrong with Channel 2.  I then disconnected everything, went back to the Tek 253 and hooked up the MAIN to CH1 and got it displaying a waveform.  I set CH2 of the scope to the exact same settings (above .2V/div) and switched my BNC from the MAIN of FG to CH2, and could not get it to trigger and display the waveform.  When I put it on AUTO I get a nearly flatlined trace, and cannot get a stable trigger at all.   

I could just leave it like this, and only be able to use CH2 for up to 16V max with a 10X probe, but who wants to have a piece of equipment that they can't use the full range of features on??  Not me!   If I can't solve this, it's going to drive me nuts!    I'm going to look thru the troubleshooting and schematics of the manual and see what I can figure out here after I make a video to show what's happening.  For now I really just wanted to get this info out here to see if anyone has any ideas that can help me get a bead on what's wrong here.

A video demonstrating the problem using a small device and also a function generator:
https://www.youtube.com/watch?v=YGbAhnIFfeU

Thank you for all of your help everyone!   You've all gotten me this far in such a short amount of time, which is incredible.  Hopefully this problem can be resolved fairly easily and the full functionality of this scope restored.