# Tag Info

20

System bandwidth is a combination of probe bandwidth and oscilloscope input bandwidth. Each can be approximated by an RC lowpass circuit, which means delays add geometrically: t_system^2 = (t_probe^2 + t_scope^2) f_system = 1/sqrt((1/f_probe)^2 + (1/f_scope)^2) This means that a 10MHz 'scope with 60MHz probes can measure sinusoids of frequency 9.86MHz ...

15

Summarised solution: A single transistor and 3 resistors will take a 0V \ "5V or more" signal and produce a 5V/0V output. With example resistor value, load on signal is about 80 uA at 5V and 250 uA at 15V. This can be reduce to say 8 uA/25 uA if desired and even lower if necessary. (Larger version of diagram below). A 390 ohm resistor and a 4V7 zener ...

7

Yes, the TSOP1738 will do at this short distance. The 0.65 relative responsitivity means that at 36 kHz your IR LED needs to be $\sqrt{0.65}$ = 0.8 times closer to see the same signal strength, due to the inverse-square law. So if your TSOP1738 sees a certain level for 38 kHz at 1 m, you'll have to hold the transmitter at 80 cm to get the same signal ...

6

You might want to consider having two (or more) ranges. The issues with capturing very low frequencies are somewhat different from the issues with the higher ones. As you've already noted, at the high end of your range you have counter overflow problems. But consider at the low end of your range, your accuracy will suffer from not having enough counts in ...

6

Mixing outgoing and returned signals and taking the difference as an indication of relative velocity is by far the easiest way of calculating velocity from Doppler shift. As often, a useful place to look for the general principles is Wikipedias article on Doppler shift and I don't need to duplicate much of this. The most important aspect is how to ...

5

Just use an inverter made out of a single transistor and a couple of resistors. Since you are measuring the frequency, it does not matter if the signal is inverted or not - the frequency is the same. You can use a "digital transistor" that has the resistors inside or you can use almost any regular transistor and add the (10K or so) base resistor outside (the ...

5

The easiest way is to clamp the input signal to Vcc (+5V): The resistor value is not critical, but it shouldn't be too small; maybe in the range 10-100 kOhms. If you are really picky about the Vcc+0.3V requirement, you should use a Schottky diode; but I think your µC wont be harmed if you use an ordinary 1N4148. EDIT: To support my opinion that it is ...

5

In the astable configuration, the low time is given by 0.693×R2×C and the high time is given by 0.693×(R1+R2)×C. The factor of 0.693 is the scale factor (relative to the RC time constant) for the time that the capacitor takes to charge or discharge exponentially between 1/3 Vcc and 2/3 Vcc. The precise value is -ln(1/2) = 0.693147... ...

4

If possible I'd suggest selecting a microcontroller that supports a counter operation using the timer inputs; rather than manually incrementing a counter inside an ISR (which at high frequencies quickly ends up saturating the microcontroller activity) you allow the hardware to handle the counting. At this point your code simply becomes a matter of waiting ...

4

the oscillator is designed to drive a high-impedance IC clock input In this case, you will want to add a buffer chip to be able to drive the 50 Ohm coax and the spectrum analyzer input. The appropriate chip depends on what type of load the oscillator is designed to drive (TTL, CMOS, ...?), the frequency, the available power supply voltages, etc. ...

3

Your output filter response should basically be the 20 dB/decade response of the capacitor. The ESR zero will depend on the number and type of capacitors you're using. It's the natural corner frequency formed by the effective ESR and total capacitance: $F_z = \dfrac{1}{2 \pi \times R_{esr} \times C_{out}}$ The gain slope will swing more positive at ...

3

Use a CD4046 PLL at 1:1 frequency ratio. The VCO is reasonably linear (1%), so its control voltage will be proportional to the input frequency. Looks like CD4046 sells for $0.15 -$0.20 in quantity: http://octopart.com/partsearch#search/requestData&q=cd4046 Digikey sells them for \$0.354 ea in 100 quantity (DIP, other packages cheaper): ...

3

You didn't give the all-important information about what frequencies you are using for the communication. You can't know if a filter is right if you don't know what frequencies you want to separate. However, the main point is to attenuate the power line frequency by a large factor. Here is one simple passive way to do that: This is a simplistic filter ...

3

10 MHz analog bandwidth means that a 10 MHz signal at 10 V will look like a 5 V, in other words your amplitude will be halved at 10 MHz. The 10 MHz bandwidth means that your 50 MHz signal will be dampened down quite a bit, but how much is hard to speculate on. The 50 MS/s means that you cannot realistically work with signals much over 5 MHz if you hope to ...

3

A time domain transient sweep is what you would use in OrCAD PSpice - LTSpice should have something similar. A DC bias point analysis is of no help with a dynamic signal (pulsed source). An AC analysis is for things like gain and phase analysis of filters. If the results look a little funky, see if there's an option to skip the initial bias point ...

3

I suspect a big part of the drift you're seeing is related to this equation in the LM2907 datasheet: This shows that the LM2907 output voltage will drift linearly if your Vcc drifts. If you are not using an extremely good regulator to power the LM2907, this could easily account for 0.2% of drift. If you are using a precision reference circuit for your ...

2

Using prescalers even GHz measurement can be achieved. This is simple 40MHz frequency meter with ATMEL AVR AT90S2313: http://www.myplace.nu/avr/countermeasures/index.htm Here are some other similar projects: http://www.ikalogic.com/freq_meter_2.php http://www.saturn.dti.ne.jp/~khr3887/lfcd_e.html ...

2

Posting this code as an alternative per @timrorr's suggestion to my previous post. This compiles for the ATTiny24A using c99 language standard, but I haven't actually tested it in any way beyond that. #include <stdint.h> #include <avr/io.h> #include <avr/interrupt.h> #include <util/atomic.h> #define TIMER0_PRELOAD 0x83 // for 8MHz ...

2

Can't you just use a 16-bit timer's input capture and overflow interrupts (plus a variable) to do the measurement? Here's how I would do it with the ATTiny24A with AVR-GCC (untested and potentially buggy of course): #include <stdint.h> #include <avr/io.h> #include <avr/interrupt.h> #define TIMER1_BITS 16 // 16 bit timer ...

2

Instead of doing a 1 second counter, make it a 0.1 second counter and multiply the count by 10? If it's just a matter of storing the counter number, can't you use additional code to keep track of when the counter is about to overflow and write to another memory location to keep the tally?

2

Maxim has an app note showing how to use their high-side current sensor parts. They also show a basic op-amp circuit as the "traditional" way to do this measurement, and this is probably good enough for your application: The output from this circuit could be run to a comparator to generate a digital '1' whenever the pulse is "on" and a '0' when the pulse ...

2

You're saying little (nothing!) about the protocol. Is this Ethernet Over Powerline? The protocol matters because of the frequencies used. Powerline communication both has a much higher frequency and much lower level than the 50/60Hz mains power. The higher frequency has its limitations: mains wiring isn't really fit for it, and the signal radiation will ...

2

Your formula is written such that it assumes that Vdd and Vss are symmetric with respect to ground. If this is not true — e.g., you are using only a single power supply with Vss tied to ground — this formula does not apply. If you want to use this circuit with a single supply (or asymmetric supplies), you'll need to connect the grounded end of ...

2

You have to consider all possible sources of drift; any term in the equation from Photon's answer is a possibility, plus (potentially) the temperature of the chip itself. the Photon pointed at probably the chief culprit : the 9V supply. However, the values of R1 and C1 may change with temperature, and so may the "constant" K as the LM2907 temperature ...

2

I would use a zero-crossing detector circuit (google - tons of them, very easy) and then (depending on the frequency you're trying to measure) either use an input-capture timer/counter to automate the counting, simply reading & resetting the count at given measurement intervals (much lower CPU load), or use the input pulses to gate a fast counter (to ...

2

As @angelatlarge said your first task is to view the disassembly of your interrupt vector to determine how many cycles your code actually takes. Second, you have to realize that there is additional overhead associated with a call to an interrupt vector. Here is an extract from the ATMega328P datasheet, section 7.7.1 - this should be representative of all ...

1

_delay_ms() is a very unprecise function, besides it takes precessor time to call it and return back. There are many ways you could use atmega's peripherals to measure frequency, if you want to go by feeding input frequency to T1, I would suggest using another timer to measure time. You could set up and start both timers simultaneously, then catch overflow ...

1

Using Daniele's convention from comments: $k = \dfrac{R1}{R1+R2}$ $Vs1$ and $Vs2$ are the maximum and minimum output voltages, resp., and $Vth$ and $Vtl$ the higher and lower threshold, resp., I get the following equations: $\begin{cases} (Vth - Vs2) e^{-t1/(R C)} = Vtl - Vs2 \\ (Vs1 - Vtl) e^{-t2/(R C)} = Vs1 - Vth \end{cases}$ ...

1

Remember that spectrum analyser can work without cable, just with antenna ? You can make an air core ("styrofoam core") transformer: draw a 10-15 turns spiral on a board and terminate it with high-Z load. Glue an insulating pad on top of it and slap a 3-5 turns of insulated solid wire soldered to BNC connector. Cover everything with pile of glue. Connect ...

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