A digital oscilloscope operates by digitizing the signal with an analog-to-digital converter (ADC) before processing and displaying the result. An ADC is limited in the range of frequencies that can accurately be converted by what is known as the Nyquist frequency. This is typically 1/2 of the sampling rate of the ADC.

See http://en.wikipedia.org/wiki/Nyquist_frequency

Hint: There are limits on how many samples you can get.

Last edited by marvsherman419. Register to edit

Tags: none

Envision an oscilloscope with a line going from left to right. The line going from left to right represents increments of frequencies. The line going up and down represents the strength (amplitude) of the sum of the signals present at the frequency. Most frequencies appear as spikes in compressed mode. Expand the line out, and the spikes appear as sharp mountains. This tool is useful in viewing frequencies close to the target frequency, bandwidths and RF shielding assessments.

More info here: https://en.wikipedia.org/wiki/Spectrum_analyzer

Last edited by w0rcf. Register to edit

Tags: arrl chapter 7

A spectrum analyzer displays the strength of a signal, and signals above and below the signal's frequency. Since spurious signals and/or intermodulation distortion products appear above and below a SSB signals frequency, the spectrum analyzer is a useful test instrument for displaying these.

You can adjust the vertical and horizontal scales on a spectrum analyzer - the vertical scale is the signal strength in Decibels, and the horizontal scale is the width of the spectrum being displayed.

Hint - The question asks about "spurious" signals and the answer is "spectrum" analyzer. Both words start with "sp".

Last edited by kwyatt. Register to edit

Tags: arrl chapter 7 arrl module 7b

The probe is adjusted until the horizontal flats of a square wave are as flat as possible.

This adjustment is equivalent to making the probe have uniform attenuation over the range of frequencies to be measured. In passive probes this adjustment is usually a small adjustable capacitor.

Silly trick: Cubes are three dimensional, but squares are as flat as possible!

Last edited by radio1984. Register to edit

Tags: arrl chapter 7 arrl module 7b

A prescaler is an electronic counting circuit used to reduce a high frequency electrical signal to a lower frequency by integer division. The prescaler takes the basic timer clock frequency and divides it by some value before feeding it to the timer, according to how the prescaler register(s) are configured.

Mnemonic: think of -divisions- on a -scale-.

Last edited by salvyhead. Register to edit

Tags: arrl chapter 7 arrl module 7b

Nyquist's sampling theorem states that the highest frequency that can be unambiguously reconstructed is at half the sampling rate, and above this, aliasing occurs.

If one should sample at 20MHz, signals of up to 10MHz can be reconstructed.

For example, if one supplied a 20,000,000 Hz signal to this oscilloscope, the oscilloscope would sample the same value every cycle, and the signal would totally disappear. If one instead supplied a 20,000,001Hz signal, the signal would drift in phase with the sampling at 1Hz, and a false 1Hz sine wave would be displayed.

Generally, an oscilloscope sampling at 20MHz will have filters to reject signals above 10MHz for this reason.

Last edited by bapaige01@yahoo.com. Register to edit

Tags: arrl chapter 7 arrl module 7b

An antenna analyzer has its own built in signal source.

Last edited by ben.pope. Register to edit

Tags: arrl chapter 9 arrl module 9g

The SWR, or Standing Wave Ratio, is the (voltage) ratio of the largest voltage on the transmission line to the smallest voltage, and it depends on how much signal is reflected back from some device, typically your antenna. When no signal is reflected back, then you have a "traveling wave", with the intended signal at full power all the time. When some power is reflected back, the forward and backward waves will interfere with each other, sometimes increasing and sometimes decreasing the voltage. The ratio of those two voltages is the (V)SWR.

All three of the named devices can measure SWR.

• The Vector Network Analyzer is the most general. It can measure many properties of a system, including SWR. You plug it into your feed line and antenna without the radio.
• An antenna analyzer is made specifically to measure SWR, typically over a range of frequencies. You also plug it into your feed line and antenna without the radio.
• A wattmeter goes between your radio and feedline. It just measures the power your putting out, but a directional wattmeter also measures how much power is coming back. That can be used to calculate SWR.

Last edited by mstenner. Register to edit

Tags: none

The probe tip and ground of an oscilloscope acts like a sensitive antenna loop. The bigger the loop the more undesired signals or noise it will pick up. Also, the inductance of the ground wire increases with length, which can distort high-frequency signals. So, keep it short as possible.

Hint: Practice, possible, probe.

Last edited by katzelover. Register to edit

Tags: arrl chapter 7 arrl module 7b

This question does not yet have an explanation! Register to add one

Tags: none

This question does not yet have an explanation! Register to add one

Tags: none