Calculating input impedance?

rwgast_logicdesign · 2012-07-01 16:18

Ok this isnt necessarily a prop question, just stuck it here so all the guys that know there stuff would see it.

Ok so ive been working on a frequency counter for quite a while. Acually the one Im building is here http://www.ikalogic.com/build-your-40-mhz-frequency-meter/ its based on an AVR design just using a prop to calibrate it right now. Just chose this cuz I needed a decent counter and didnt wanna waste an 8 dollar prop chip

on something this simple. Anyways ive been working on turning this thing in to a hi quality counter at least as hi quality as I know how to do. So ive been reading alot about probes and interconnects all that jazz! So ive come up with the same problem I had when doing amp designs and tying to conform to higher standards. How do you calculate input impedance? For example when doing an amp design you want your input impedance to be 0 if possible and for this project I need to know my input impedance so I can design some probes or troubleshoot problems with it when measuring higher frequencys (ive already used 100mhz prescalar gonna upgrade that soon).

Like I said sorry if anyone is offended by the avr... I just know theres alot of guys on the board like jmg who are very good with this kind of stuff.

HShanko · 2012-07-01 17:21

Tell us again! You want the input impedance to be '0'? That will short ouf your probe, or input signal if is X1.

Does anyone understand what he meant?

Publison · 2012-07-01 18:17

You never said what frequency range you want to measure.

My probes are usually 22 ga wire.

Or a scope probe.

You probably need to move this to "General" "Chatter". Has nothing to do with a Propeller question.

(The AVR people couldn't answer this, they must have some info ) ?

rwgast_logicdesign · 2012-07-01 19:12

like i said i know this isnt prop related only stuck it.here for now so a few people who mostly only visit this forum would see it. and it is in no way avr specific.

anways im not looking for 0 impedance i have an amp project that needs an input impeadence of less than 2 ohms. for this meter that is set up to mesure 0 to 95mhz i would like 1 mega ohm of impedance. im making custom prob
es out of rca cables.

basically i need to know how to measure input impedance so i can later swap bnc adapters on and use scope probes. from what i understand a scope usually has 1 mega ohm of input impedance. so with a 10 to 1 cable they throw a 9meg resistor in series to achieve a 10 mega ohm imput impedance. right now im not to worried i will be building passive probes tthat are just 1 to 1 or whatever. but as i upgrade to read higher frequencys id like the footwork to be in place. im just trying to build a
quality piece of test equitment that has little interferance compared to my crappy dmmrr

LoopyByteloose · 2012-07-02 01:07

Could it be that what you really want is 'nearly infinite' impedance? There are a lot of op amps of the extremely low noise variety that would happily do this. Then the output from that could be enhanced as required for the microcontroller of your choice. You just need a clean square wave for the microcontroller at the appropriate voltage, right?

It may merely take one op amp stage or you might have to add something.

In other words, impedance enhancement is an analog issue, not a digital one.

Impedance is rarely matched outside of stages in an RF amplification at a known frequency or in long transmission wires or an antenna.

Over broader bandwidths,
The impedance of an input is preferred to be higher (as it requires less power).
The impedance of an output is preferred to be lower (as it provides more power).

So while you want to collect your Rf at near zero impedance, you want your input to be near infinity (to get a more faithful sample with minimal power).

I hope I got that all correctly or a lot of guys are going to stomp on me. You need to seek out an ARRL site if you want somebody that is knowledgeable. After all, they live and eat RF concepts.

Toby Seckshund · 2012-07-02 02:05

On my cheapish counter the two input options are 0 - 100MHz 1M // 35pF which usually just gets a 'scope probe and is the one that gets most of the bench use. The other is 100MHz - 1GHz 50 Ohms which obviously goes to a pre-scaler. For RF work 50 Ohms is a good standard but will not allow for non-intrusive probing (but then all probing will be intusive to some extent) and you will have to be mindful of the power that gets to that one.

I haven't done much to my reversed clocking of a Prop thoughts. On that I was putting a 28MHz signal into a counter object and then from that VCO'ing the 5MHz that drives the Prop a few KHz so that I could keep the freq generator object to an exact multiple of 8, so that the output would be jitter free. That would have a VDU and serial interface for it and the I2C controlled TV tuner. Already thats 3/4 of a Prop used.

I got very distracted from that by http://sdrsharp.com/ and a £15 EZCAP USB dongle.

Mark_T · 2012-07-02 05:44

Scope input circuits get their high input impedance by using a FET stage, usually JFET pair. The impedance is not that high really because there is significant capacitance (10x probes are tunable RC dividers to get a broad-band match to that input impedance with 1/10th of the capacitance).

I suspect most FET input stages can be made to work as a high-input impedance compatible with a scope probe, you just arrange the DC impedance to be 1M. You don't want the input capacitance to be too high though.

For a frequency counter I would personally suggest having two inputs, a high-impedance input driving a broadband gain stage, and a 50ohm input. Some sort of gain select is handy (to be able to look at 5V logic signals as well as low-level RF signals). There are probably some video/broadband op-amps available that fit the bill for the gain stage - perhaps even with a high-impedance input stage.

LoopyByteloose · 2012-07-02 06:21

My thinking is that the RF source is likely to be an antenna and available at milliVolts if not less, maybe microVolts. But if could come through a preamp and a 50 ohm coax.

So if one were to need a 3.3volt high, an op amp or two would be required to have a high enough voltage to trigger the I/O. Trying to make a single op amp produce a gain of 3600x is rather dubious and considered bad engineering for several reasons, but two stages at 60x would do a pretty good job. Bandwidth would be listed in the PDF for the op amp.

Gain can be tweaked with variable resistors in the gain stages to assure that it meets specs of the microcontroller.

Inserting a resistor load on the front end would pretty much dictate input impedance. A 1 meg resistor with one leg to ground and one leg to input would effectively create a 1 meg impedance. A 50 ohm resistor with one leg to ground and one leg to input would effectively create a 50 ohm impedance.

Why so? The 'ideal' op amp is suppose to have near infinity impedance on its inputs. So the resistor dominates the characteristics. If you want another number, just use a different resistor.

If output is peak-to-peak 3.3 volts, a fast diode can clip half the wave and the microcontroller can do the rest.

Capacitance is pretty much determined by construction and length of the wires involved.

I am not sure you need anything near to oscilloscope accuracy as you are NOT going to trace the actual curve of the signal, just count the triggering events.

I am sure someone out there wants to use a single rail voltage power supply rather than a dual rail plus and minus voltage supply (something like a +6/-6 with high noise rejection). Generally, I feel that these create more complexities that often need to be undone later. The really good op amps are intended to be most useful in a dual rail setting.

LoopyByteloose · 2012-07-02 09:44

This really is all about what you want in frequency range and what you want for a front.
It would be easiest to get a good proven design from Ham radio enthusiast via the ARRL as the higher you go in frequency, the difficult the problems get. And the lower you go in to signal power, the more you have to fight signal to noise ratio.

Googling around a bit I found several projects.

1. uses a 10k resistor directly into and AVR
2. uses a BF199 radio frequency transistor to create a gain of about 40 before inputting a PIC
3. uses two stages of J309 jfets to boost the signal before entering a microcontroller

Various schemes seem to have different means to enhance the signal. I was thinking a Jfet op amp, but it may or may not be as good as just a couple of stages of jfets, or it may be more costly.

BTW, a freq counter for digital use is not as complex as one for analog use. After all, the digital signal is always driven to a predictable voltage level - no microvolts or millivolts

Mark_T · 2012-07-02 16:02

Loopy Byteloose wrote: »

My thinking is that the RF source is likely to be an antenna and available at milliVolts if not less, maybe microVolts. But if could come through a preamp and a 50 ohm coax.

Any signal from any kind antenna will be _swamped_ with high strength broadcast / mobile / WiFi signals at all sorts of frequencies - you wouldn't expect to see a microvolt signal in that mess without a RF tuner/amplifier to pick it out. The typical signal you might want to measure with a frequency meter would be something like a local-oscillator, usually in the range of several milliwatts. Anything more sophisticated and you probably need a spectrum analyser anyhow - a frequency meter can only work with signals that are predominantly at one frequency - give it a mix of signals of the same strength and you'll get very misleading readings.

Calculating input impedance?

Comments