Measure 27Mhz at 660 volts - Page 1
Forum section: electronics / general

author: coromonadalix
author_rank: Super Contributor
timestamp: September 11, 2019, 02:04:08 pm
content: Suggestion to measure a 27Mhz signal accurately and repetably, certifiable / calibrable ...We measure it normally with an specialy designed loads, the load as rg174 cabling 95 cm long, a precision resistor and an inductance with adjustable core and a 1.5 k 30w high precision resistor. We use inductance resistance and capacitance effects in the measurement, but basically its an voltage attenuator for a scope input. We have different scope models. Current passing thru the load is around 450 ma, witch is around 300 watts ??I cant divuldge anything else / schematics of the loads we useBut i can say its unreliable, if the inductance move physically, the read value change, if the rg174 wire is pinched, or changed with an almost identical lenght the read values change, the rg174 wire heat....Lots of things affect the read value, and what bothers me the most, the department fiddle with the scope compensation screw to fit the value between scopes, each scope has its own deficated load. And for reference we have a very stable 27Mhz reference signal, but it use the same load technologyWe use an BK Precision 2kv x100 scope probe red color, i cant find the specs or part number at the moment.I need some suggestions to measure this signal amplitude correctly, a power meter ?? I need to measure it maximum half a minute.And can be calibrated ...thks

author: pigrew
author_rank: Frequent Contributor
timestamp: September 11, 2019, 02:14:27 pm
content: Is this at a broadcast station or something like that?Is your goal to measure the cable, or the load, or the power going through some other transmission line?I think you're partially running into impedance mismatch between the RG174 (50 ohm) and your load (1.5kohm). I'm assuming the source is 50 ohms?The other thing is that you shouldn't need/use a scope compensation screw/passive probe. The input to your scope should be with a "feedthru-termination", not a high impedance input. I'd seed a block diagram to say more.Also, oscilloscopes are not calibrated/designed to have high accuracy amplitude measurements, especially not with standard passive probes. You should really be using a power sensor (either diode-based or thermocouple) if you want calibrated power readings. Thing is... all of the power sensors will be either 50 or 75 ohm.If you're trying to measure the power through some other trasmission line, use a directional coupler (-50 dB or something?). To do calibration you'll need to inject a known amount of power through the main transmission line, check the delivered power, and use that for an offset correction constant. The other way to do it would be to measure s-parameters of all of the individual blocks. That'd be more accurate, but trickier to do.-Nathan

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2019, 02:44:41 pm
content: Use a power attenuator, or a termination resistor after a power tap.This way the measurement stays at 50 ohms and there's no fiddling with compensation screws or inductors (what the heck is the inductor for, anyway?).Since your load is 1.5kohms, you might use that as one leg of a matching attenuator.Tim

author: coromonadalix
author_rank: Super Contributor
timestamp: September 11, 2019, 03:01:22 pm
content: they use the inductor for a maximum gainIts a 27mhz sinusoidal wave, im trying to have the output impedance of the amplifier / driver section, the amplifier works at 48 vdc around 1.8 ampIts not a broadcasting application, its a pulse controled signal going from 0.0 secs to max 1-2 minutes in timeBut we argue a have small feud at my job for a curate measurements, i measure mostly the amplitude levels, and sometimes the pulse lenght an width.I tought of power meters ... but finding a suitable load ??Gonna check with an De5000 the load specs in serial parallel modes ...oops the cable is Belden 9221 at 191.5 cm of lenght, the inductance is an home made 25 turns of 22 awg wire, its in parallel of the 1.5 k resistorthe signal pass thru an 0.1 uf 2kv capacitor and go to the inductance resistance load the way i see it, and we read the amplitude levels

author: CJay
author_rank: Super Contributor
timestamp: September 11, 2019, 03:54:33 pm
content: Why is the load so variable, can you 'nail it down' so it's not?Can you calculate the impedance of the load at 27MHz?

author: coromonadalix
author_rank: Super Contributor
timestamp: September 11, 2019, 04:27:12 pm
content: I cant measure the impedance at 27 mhz, not equiped to do so.Im trying to find if a powermeter could do the jobhere's an home made schematic, managed to find the probe model

author: tkamiya
author_rank: Super Contributor
timestamp: September 11, 2019, 10:38:50 pm
content: What exactly are you trying to measure?Power? Current? Voltage?What is the output impedance?I'd imagine company like Bird, Coaxial Dynamics, etc. has something for this purpose.If you don't know the impedance, you can measure SWR with various load and figure it out. Unless it is matched with source, anything you measure will be meaningless.I'm thinking this is some kind of industrial heating equipment. Assuming 50 ohm load, at 660 volt, current is 12A, Power is 7200 watts. Not an outrageous kind of output.... I've heard of CBers doing more.At this kind of power though, I'd be using Andrew Helix hardline with appropriate high power connectors though.You could also indirectly measure output by knowing final amplifier efficiency, input voltage and current consumption. Without knowing much more about what it is, I can't say much more than this.

author: TiN
author_rank: Super Contributor
timestamp: September 12, 2019, 02:02:01 am
content: Since this thread is in Metrology section and not in T&M or Beginners section first OP should correctly useSI prefix. I hear yells "but you know what OP meant, don't be volt-nut nazi".. Well, answer or a solution would be very different, and it is always important to clearly state a question, so other members have no chance for misinterpretation.Otherwise plain answer would be - just use data-logging DMM with rating up to 750VAC such as HP 34401A to digitize 27 millihertz signal and derive amplitude from the captured data.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 12, 2019, 02:31:01 am
content: ooops my bad thks 27Mhz it is

author: TheSteve
author_rank: Unknown
timestamp: September 12, 2019, 03:53:13 am
content: Quote from: coromonadalix on September 12, 2019, 02:31:01 am"MHz" not "Mhz"