Coax recommendation

[brandon7861]

What coax is good for scope probes? I am going to get some RG-400 to replace my jumpers, but I need something of much smaller diameter that is double shielded and flexible for homemade probes. I prefer it be 50Ω and double shielded and low loss through at least 200MHz. Does RFParts have something similar to what comes on scope probes? I am tired of wielding my high z probe with tv coax coming out of it.

 

[sp5it]

Why coax? It gonna cause inconsistent measurements depending on frequency measured.

 

[brandon7861]

What other option is there?

 

[brandon7861]

My probe is the active type with less than a pF of input capacitance and a 50Ω output impedance. Wouldn't coax like rg178 be the way to go? If there is a different way to get the signal from the probe to my scope, id love to know about it

 

[brandon7861]

I know actual scope cable is special and has an extremely fine inner conductor to limit capacitance, but I don't care about cable capacitance with an amp driving it

Edit: I ordered rg178. Anything is better than the coax I am currently using.

 

[sp5it]

Using coax for scope probe cause measurement errors because Vf changing in frequency domain.

 

[brandon7861]

Using coax for scope probe cause measurement errors because Vf changing in frequency domain.

I assume you are suggesting that the fundamental and the harmonics arrive at the other end at different times and that changes the resulting waveform seen on the scope.

With respect to IF stages, those signals go into tuned circuits anyhow and my main concern is the frequency of the oscillators, the passband of the cans being right, and drive levels at mixers set to minimize IMD. I think I can still do that with a somewhat distorted waveform as the error would decrease as the harmonics and distortion products drop with the adjustments approaching their ideal values.

If looking at the output at the antenna jack, once I get the harmonics knocked down 50 or 60dB, I could go back and re-peak, but I don't think that would be worth while, would it?

Exactly how serious of an issue is this, and why?

Edit: is there even that big of a difference in VF between 27MHz and 54MHz?? I dont do microwave or digital stuff, just regular old CB radios.

Thanks!

 

[brandon7861]

Besides, my high-Z probe with the coax in question is used mostly for frequency adjustments and not for looking at harmonics. It spends most of its life connected to the FC, and I only use it on the scope when I want to do a quick check for all the signals. Distortion from harmonic delay I don't think affects the frequency of the wave, just the shape of it a little. Maybe I am wrong. I use a 10x probe to look for IMD and harmonics because those things are found at stages where a 10x probe won't affect it much, unlike a crystal oscillator that has no buffer.

 

[brandon7861]

@sp5it Thank you. Just got back home and had a chance t look into this further.

I found a bunch of stuff on phase dispersion and, although from first glance it appears to be an issue at VHF and up (more reading to do), I now know that I must measure the delay at each frequency that may be present when pushing any significant bandwidth through a cable at higher frequencies. I never knew about this.

My current impression is that this will be of no consequence on 11m and its first few harmonics though, but I am unsure if I will even be able to measure my new coax as the charts I see suggest sub-nanosecond delays in that frequency range, and my scope only has a 220MHz bandwidth.