Identifying correct coax to connect RF Source to RF Input - Page 1
Forum section: electronics / beginners

author: jstarr
author_rank: Regular Contributor
timestamp: November 29, 2016, 04:26:13 pm
content: How do you determine the correct coax to connect a high impedance RF source to a high impedance input so that the RF sine source characteristics, eg power, Vpp are accurately reproduced. Can you use short runs (2-3') of 50 ohm coax?john

author: David Hess
author_rank: Super Contributor
timestamp: November 29, 2016, 06:43:24 pm
content: Quote from: jstarr on November 29, 2016, 04:26:13 pmIt is simple; you don't. The low impedance of the coaxial cable will load down the high impedance RF source.Passive oscilloscope probes do this by either using a compensated series resistance at the input to make an attenuating probe (9 megohms for a x10 probe) or living with low bandwidth however oscilloscopes probes are objectively specified using a terminated 50 ohm source or 25 ohm source impedance!What is your application?

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: November 29, 2016, 08:32:13 pm
content: Yes - more infoWhat are you testing ?What gear do you have to test with ?What scales / amplitudes are you dealing with ?In order for answers to have valuable meaning , the questions have to have values / conditions - you only gave high impedances .

author: edpalmer42
author_rank: Super Contributor
timestamp: November 29, 2016, 08:53:53 pm
content: In general, you match the impedance of your cable to your source and load. If they are different, you might need a transformer. If you don't do the matching, you can get reflections where the impedances change. Depending on the power and frequency involved, these reflections can damage your source. Power and frequency will also affect which cable you need, regardless of the impedance.We can't give you a more detailed answer without more info from you.

author: jstarr
author_rank: Regular Contributor
timestamp: November 30, 2016, 07:05:23 pm
content: Thanks, I am beginning to dabble with oscillators and have an old (old vernier dial - nothing digital) signal generator that outputs 100 KHz - 10Mhz with a high impedance output. I wanted to first have a fairly accurate measurement of the frequency so I bought a RF freq meter on EBay (eBay item number:181861025799) but this only has a high impedance input. Others (Ham radio operator ZL2PD,http://www.zl2pd.com/8digitCounterBox.html) who have used this say it requires about 0.6Vpp input but I am finding I need more than 4Vpp to get a stable and accurate reading ... and this is with a 2' piece of RG-58 coax. So what is the correct way of doing this? I am obviously missing something very fundamental... do I place a ceramic cap in series to couple? If so I guess I need to calculate the value of the cap based on target frequency of 10Mhz and 50 ohm coax?john

author: David Hess
author_rank: Super Contributor
timestamp: December 01, 2016, 02:25:43 am
content: The schematics are informative; the inputs do not have a controlled impedance and are AC coupled into a back to back pair of diodes so signals are limited to 1.2 volts peak to peak. The impedance mismatch between a 50 ohm cable and either frequency counter input is going to cause all sorts of problems depending on the length of the cable compared to the wavelength; with no termination, the cable capacitance or inductance will be presented to the load.Depending on how high the impedance and level is, a series resistor between the cable and source and a 50 ohm shunt termination at the counter will be sufficient; this duplicates how a low impedance oscilloscope probe works. Alternatively duplicate a high impedance passive probe using a high value of input series resistor shunted by a small amount of capacitance; this could get the load capacitance down to 10 to 20 picofarads or so. If the impedance by this is still to low, then a simple single transistor buffer like an emitter or source follower can be used and is what I have commonly done when tapping IF and demodulator outputs.

author: edpalmer42
author_rank: Super Contributor
timestamp: December 01, 2016, 02:47:32 am
content: John, you still haven't given us any significant info on the generator.Okay, flying blind here. Let's assume that the generator's output impedance is 600 ohms. As David says, the input impedance of the counter is somewhat undefined, but let's assume that it's 40 ohms, just for a number - it may be less. Let's also assume that you measured the 4Vpp with a high impedance scope probe. Now if you do the calculation for a voltage divider between the 600 ohm output impedance of the source and the 40 ohms input impedance of the counter, the 4Vpp ends up at about 250 mVpp into the counter. That's still a long way from the spec of 60mV, but it's at least in the ball park - considering all the assumptions involved.In this case, the cable is irrelevant. I think it's too short compared to the signal wavelength to even look like 50 ohms. It's the impedance difference that's killing your level, not the cable.You can confirm all this by finding a transformer that's at least approximately 600 ohms (or whatever the output impedance is) to 50 ohms. Use a frequency that's appropriate to the transformer and see what happens. The poorly characterized input of the counter will still mess things up, but it's the best you can do.

author: jstarr
author_rank: Regular Contributor
timestamp: December 01, 2016, 03:57:04 am
content: Thanks David for taking a look at the counter schematic. If the counter input (let work with the simpler low channel) doesn't have a controlled impedance, how do you decide what would be a good match? Wouldn't a transistor (bipolar or jfet) buffer take the high Z input from my signal generator and provide a very low impedance output to the counter?many thanks for your patiencejohn

author: David Hess
author_rank: Super Contributor
timestamp: December 01, 2016, 08:28:38 am
content: Quote from: jstarr on December 01, 2016, 03:57:04 amAt low frequencies compared to the cable length, the impedance matching does not matter. But if you are measuring a high impedance point you have two problems:1. The inputs to the frequency counter are clamped at 1.2 volts peak to peak and will happily clamp the source through the cable and 100pF or 1000pF AC coupling capacitor.2. While the cable is short, it still adds a capacitive load to the source which will be a problem if the impedance is high enough. Either a buffer or an attenuator can be added to fix this.