KB6NU's Ham Radio Blog

///////////////////////////////////////////
2016 Extra Class study guide: E4C - Receiver performance

Posted: 13 Mar 2016 07:15 AM PDT
http://feedproxy.google.com/~r/kb6nu...m_medium=email

E4C Receiver performance characteristics, phase noise, noise floor, image
rejection, MDS, signal-to-noise-ratio; selectivity; effects of SDR receiver
non-linearity

In the past, sensitivity was one of the most important receiver performance
specifications. Today, instead of sensitivity, we speak of a receiver’s
minimum discernible signal, or MDS. The MDS of a receiver is the minimum
discernible signal. (E4C07) This is the weakest signal that a receiver will
detect. One parameter that affects a receivers MDS is the noise figure. The
noise figure of a receiver is the ratio in dB of the noise generated by the
receiver compared to the theoretical minimum noise. (E4C04)

A related specification is the noise floor. When we say that the noise
floor of a receiver has a value of -174 dBm/Hz, it is referring to the
theoretical noise at the input of a perfect receiver at room temperature.
(E4C05) If a CW receiver with the AGC off has an equivalent input noise
power density of -174 dBm/Hz, the level of an unmodulated carrier input to
this receiver would have to be -148 dBm to yield an audio output SNR of 0
dB in a 400 Hz noise bandwidth. (E4C06)

Another important receiver specification is selectivity. A receiver’s
selectivity is the result of a lot of things, including the filters a
receiver has. 300 Hz is a desirable amount of selectivity for an amateur
RTTY HF receiver. (E4C10) 2.4 kHz is a desirable amount of selectivity for
an amateur SSB phone receiver.(E4C11)

In addition to a 300 Hz filter and a 2.4 kHz filter, high-end receivers
also have filters called roofing filters. A narrow-band roofing filter
affects receiver performance because it improves dynamic range by
attenuating strong signals near the receive frequency. (E4C13)

Back in the day, when superheterodyne receivers had intermediate
frequencies, or IFs, in the 400 500 kHz range, image rejection was a
problem. If there was a strong signal present on a frequency about two
times the IF away from the frequency your receiver was tuned to, you might
hear that signal. Accordingly, 15.210 MHz is a frequency on which a station
might be transmitting if is generating a spurious image signal in a
receiver tuned to 14.300 MHz and which uses a 455 kHz IF frequency. (E4C14)

One solution to this problem is to select an IF higher in frequency. One
good reason for selecting a high frequency for the design of the IF in a
conventional HF or VHF communications receiver is that it is easier for
front-end circuitry to eliminate image responses. (E4C09) A front-end
filter or pre-selector of a receiver can also be effective in eliminating
image signal interference. (E4C02)

Another way to get rid of image signals is to use a narrow IF filter. An
undesirable effect of using too wide a filter bandwidth in the IF section
of a receiver is that undesired signals may be heard. (E4C12)

Because most modern transceivers use digital techniques to generate a local
oscillator signal to tune a receiver, synthesizer phase noise might be a
problem. An effect of excessive phase noise in the local oscillator section
of a receiver is that it can cause strong signals on nearby frequencies to
interfere with reception of weak signals. (E4C01)

Software-defined radio (SDR) is becoming more popular in amateur radio. It
is, therefore, necessary to know something about SDR receiver
characteristics. The SDR receivers analog-to-digital converter sample width
in bits has the largest effect on an SDR receivers linearity. (E4C17) An
SDR receiver is overloaded when input signals exceeds the maximum count
value of the analog-to-digital converter. (E4C08) Distortionis caused by
missing codes in an SDR receiverÃ*s analog-to-digital converter. (E4C16)

Finally, here are two miscellaneous questions on receiver performance
characteristics. Atmospheric noise is the primary source of noise that can
be heard from an HF receiver with an antenna connected. (E4C15) Capture
effect is the term for the blocking of one FM phone signal by another,
stronger FM phone signal. (E4C03)

The post 2016 Extra Class study guide: E4C Receiver performance appeared
first on KB6NUs Ham Radio Blog.


///////////////////////////////////////////
2016 Extra Class study guide: E4B - Measurement techniques

Posted: 12 Mar 2016 11:01 AM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email


E4B Measurement techniques: Instrument accuracy and performance
limitations; probes; techniques to minimize errors; measurement of Q;
instrument calibration

One thing about test instruments is that you need to take the readings with
a grain of salt. By that, I mean that chances are that the instrument
reading is not exactly the value of the parameter you’re measuring. The
reason for this is that no instrument is 100% accurate.

Let’s consider frequency counters. Frequency counters are useful
instruments for measuring the output frequency of amateur radio
transceivers. While a number of different factors can affect the accuracy
of an instrument, time base accuracy is the factor that most affects the
accuracy of a frequency counter. (E4B01) The time base accuracy of most
inexpensive frequency counters is about 1 part per million, or 1 ppm.

Now, let’s see how that affects the accuracy of a frequency measurement. If
a frequency counter with a specified accuracy of +/- 1.0 ppm reads
146,520,000 Hz, 146.52 Hz is the most the actual frequency being measured
could differ from the reading. (E4B03) Practically, what this means is that
while the frequency counter reads 146,520,000 Hz, or 146.52 MHz, the actual
frequency of the signal might be as low as 146.519853 Mhz or as high as
146.520147 MHz.

More accurate—and therefore more expensive—frequency counters might have a
specified accuracy of .1 ppm. If a frequency counter with a specified
accuracy of +/- 0.1 ppm reads 146,520,000 Hz, 14.652 Hz is the most the
actual frequency being measured could differ from the reading. (E4B04) This
is very accurate for amateur radio work.

Very inexpensive frequency counters might have an accuracy of only 10 ppm.
If a frequency counter with a specified accuracy of +/- 10 ppm reads
146,520,000 Hz, 1465.20 Hz is the most the actual frequency being measured
could differ from the reading. (E4B05) This might be adequate for amateur
radio work, but as you can see, the difference between the frequency
counter’s reading and the signal’s actual frequency can be up to ten times
as much as with the frequency counter with a 1 ppm accuracy.

Voltmeters

Probably the most common test instrument in an amateur radio station is a
voltmeter. The voltmeter may be part of a digital multimeter (DMM) or
volt-ohm meter (VOM). DMMs have the advantage of high input impedance, and
high impedance input is a characteristic of a good DC voltmeter. (E4B08)
The higher the input impedance, the less effect the meter will have on the
measurement.

The quality of a VOM is given by the VOM’s sensitivity expressed in ohms
per volt. The full scale reading of the voltmeter multiplied by its ohms
per volt rating will provide the input impedance of the voltmeter. (E4B12)
A higher ohms per volt rating means that it will have a higher input
impedance than a meter with a lower ohms per volt rating.

RF measurements

Directional power meters and RF ammeters are two instruments that you can
use to make antenna measurements. With a directional power meter, you could
measure the forward power and reflected power and then figure out how much
power is being delivered to the load and calculate the SWR of the antenna
system. For example, 75 watts is the power is being absorbed by the load
when a directional power meter connected between a transmitter and a
terminating load reads 100 watts forward power and 25 watts reflected
power. (E4B06)

With an RF ammeter, you measure the RF current flowing in the antenna
system. If the current reading on an RF ammeter placed in series with the
antenna feed line of a transmitter increases as the transmitter is tuned to
resonance it means there is more power going into the antenna. (E4B09)

There are a number of instruments that you can use to measure the impedance
of a circuit. An antenna analyzer is one. Some sort of bridge circuit is
another. An advantage of using a bridge circuit to measure impedance is
that the measurement is based on obtaining a signal null, which can be done
very precisely. (E4B02)

That’s the principle behind the dip meter. You adjust the meter’s controls
so that the reading “dips� to a minimum value. The controls then indicate
the resonant frequency. When using a dip meter, don’t couple it too tightly
to the circuit under test. A less accurate reading results if a dip meter
is too tightly coupled to a tuned circuit being checked. (E4B14)

For some experiments, you’ll want to know not only the resonant frequency
of a circuit but also the quality factor, or Q, of the circuit. The
bandwidth of the circuits frequency response can be used as a relative
measurement of the Q for a series-tuned circuit. (E4B15)

Another type of instrument that you can use to make impedance measurements
is the vector network analyzer. As with any instrument, you need to ensure
that it is calibrated properly. Three test loads used to calibrate a
standard RF vector network analyzer are short circuit, open circuit, and 50
ohms. (E4B17)

Finally, a method to measure intermodulation distortion in an SSB
transmitter is to modulate the transmitter with two non-harmonically
related audio frequencies and observe the RF output with a spectrum
analyzer. (E4B10) The instrument we use to do this is called, oddly enough,
a two-tone generator. Typically, these generators provide tones of 700 Hz
and 1,900 Hz simultaneously.

S parameters

S-parameters, or scattering parameters, are used to describe the behavior
of RF devices under linear conditions. Each parameter is typically
characterized by magnitude, decibel and phase.

The subscripts of S parameters represent the port or ports at which
measurements are made. (E4B07) The S parameter that is equivalent to
forward gain is S21. (E4B13) The S parameter that represents return loss or
SWR is S11. (E4B16)

The post 2016 Extra Class study guide: E4B Measurement techniques appeared
first on KB6NUs Ham Radio Blog.