How to Check Signals
This page still under construction revised 1/8/2004!!
I noticed W2VJN has proposed using absolute signal level at a fixed test spacing for click measurements. While I agree with George's suggestion we establish a test method, I strongly disagree that a 15Hz BW signal level measurement of the peaks accumulated in dozens or hundreds of scans is any more useful that listening off-frequency on a known good receiver.
Some radios, like the FT-1000 series and other Yaesu radios, click on both make and break. Radios that do that are much more disruptive than radios that click only on make or break. The slope of the roll-off is also very important.
What we really need for click measurements is a peak and average power measurement on the adjacent frequency.
My local wintertime 350Hz BW noise (after preamplifier) compared to a sample of signals on one night was:
The dynamic range between noise and W4ZV on one occasion was over 85dB! It also illustrates how important antennas, location, and propagation are rather than power. The dB difference between signals from the same area is profound. Many signals run at or near noise floor. This may not be typical of every night, but it shows how large the signal level variations between weak DX and strong signals are.
W8LRL and I have heard each other while running about 30 micowatts!There are three main reasons we hear clicks and splatter off-frequency:
Dynamic Range (IM3)
The most useful specification for receivers is called dynamic range. Dynamic range (DR) tells us the ratio in dB (anything described in decibels is a ratio) of the weakest signal that can be heard to the level where problems start.
IM3 DR would be the ratio of two equal level signals (creating a third-order product by unwanted mixing) to the noise floor of the receiver.
Imagine we have two strong CW signals spaced 1kHz apart, one at 1840kHz and another at 1839 kHz. As the level is increased, the receiving system has a increasingly non-linear response. The 2nd harmonic of 1840 can mix with the fundamental of 1839, and the result would be a new signal at 2*1840-1839=1841kHz. Another signal, if the two original signals are equal strength, appears at 2*1839-1840=1838kHz.
(There are also sums, but they fall outside the filters and tuned circuits. We normally can't hear them, so we generally just ignore them.)
The response of this product is non-linear. The level of the mixing product increases faster than the level of either individual "real" signal.
When we can hear that "phantom signal" above the noise floor of the receiver, it adds interference to other weak signals we might be trying to hear. We reference the main signal level this occurs at to the noise floor, because that is the level where it would start to be noticeable.
Always remember actual overload is the result of the vector sum of signals in the passband of the system at that point. One way to look at it is that overload is an accumulated power problem, not an absolute level problem with one signal. This means a great number of weaker signals can cause the same problem as a few strong ones.
Transmitters are a special case partially covered in a checking transmitters page.
Higher Order IM
We can have higher order IM products, but they are always odd sums or differences. For example the 2nd harmonic of one signal can mix with the third harmonic of another, and we have 5th order products (2*f1 minus 3*F2 called fifth because 2+3=5).
Although they can affect CW receivers, higher order products create most of our SSB problems. This occurs because higher-order products fall well outside the filter passband of a typical SSB receiver or transmitter.
Blocking is the point where we can detect a change in sensitivity or gain from a single strong unwanted signal. Blocking "pumps" the receiver gain and can actually make false clicks. We always have to sure any click reports are given with signal levels well below the blocking point!
Blocking again is a ratio referenced to noise floor, since loss of sensitivity (either through increased noise or decreased gain) can affect readability of weak signals.