Driving 20m RG-178 Cable - Page 1
Forum section: electronics / rf-microwave

author: luky315
author_rank: Regular Contributor
timestamp: October 14, 2025, 07:49:28 am
content: I need to transmit an analog signal (BW 500MHz, 1Vpp) over 20m RG-178 cable (50R, ~95pF/m).Which Opamp / Video driver / ... is the "best" solution for this problem? Price is not really a concern and power is also available (+-5V, but I can change that also)

author: David Hess
author_rank: Super Contributor
timestamp: October 14, 2025, 02:08:06 pm
content: What kind of flatness out to 500 MHz are you expecting? Just -3dB?There are plenty of suitable fast parts now which will support a gain of 2 driving a double terminated 50 ohm transmission line to 1 volt peak-to-peak:LMH6559OPA695LMH6703LMH6702What kind of equalization were you expecting? 500 MHz through RG-178 is not going to arrive without significant loss at higher frequencies.

author: luky315
author_rank: Regular Contributor
timestamp: October 14, 2025, 02:31:05 pm
content: My concern is more about stability / ringing.

author: radiolistener
author_rank: Super Contributor
timestamp: October 14, 2025, 04:05:21 pm
content: The main factor that determines ringing in your setup is impedance matching between the cable, the source, and the load. RG-178 has a characteristic impedance of 50 Ω, so the driver and the receiver should both present 50 Ω to the line. If this is done correctly, reflections are minimized and you should see virtually no ringing, regardless of the op-amp or video driver used.the key point is that proper termination is far more important than the exact choice of driver.So focus first on:1) Driving the line with a proper 50 Ω output.2) Terminating the receiving end with 50 Ω (most important part).3) Minimizing stubs or unmatched branches.With these in place, ringing will be negligible.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: October 14, 2025, 07:25:08 pm
content: It might be possible, that BUF602 from TI meets your specs.Datasheet:https://www.ti.com/lit/ds/symlink/buf602.pdf

author: David Hess
author_rank: Super Contributor
timestamp: October 14, 2025, 09:05:46 pm
content: Quote from: Uunoctium on October 14, 2025, 07:25:08 pmIt does, however with a fixed gain of one, it is not suitable for a double terminated application unless additional gain is added elsewhere, or dividing the input signal by 2 is acceptable.Quote from: radiolistener on October 14, 2025, 04:05:21 pmIn a point to point application, termination of the transmission line is only required at one end. So for example the BUF602, or any gain of 1 amplifier, could be directly connected to the 50 ohm transmission line, and a parallel termination of 50 ohms used at the end. This has the virtue of preserving higher bandwidth because additional gain is not required. Alternatively, a transconductance amplifier (current output) could be directly connected to the 50 ohm transmission line. (1) In both cases, the amplifier sees a 50 ohm load at all frequencies.QuoteThe advantage of double termination is that it is more tolerant of mismatches with both ends properly terminated, so it is preferred *if* sufficient gain-bandwidth product is available, which is the case out to 500 MHz these days with available parts. Here the amplifier sees a 100 ohm load at all frequencies.(1) Old oscilloscopes commonly use this configuration because gain-bandwidth product was limited and it was important to get as much as possible out of a minimum number of stages. Later oscilloscopes started using double termination because transistors improved to the point where enough gain-bandwidth product was available to tolerate the loss of gain or frequency response.

author: CaptDon
author_rank: Super Contributor
timestamp: October 14, 2025, 11:55:15 pm
content: Will that analog signal span all the way from D.C. to 500MHz? That is a very broadband 'analog' signal. That is the entire bandwidth of the I.F. on some microwave links. That bandwidth spans many octaves or even decades of band width. Usually when we see a signal with a bandwidth that wide it would be riding on a 1GHz carrier with an occupied bandwidth of 1GHz to 1.5GHz and representing just one half of an octave bandwidth which makes things simple in terms of flatness / twist as they say.