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Bandwidth rules Part 97.307
Note: Bandwidth measurement
dynamic range requirements are based on typical signal-to-noise ratios I have
observed over the past several years. They are not the extremes of what I have
seen, but rather are typical values. Some information on my receiving system and
noise floor is available in (NOISE) and (RECEIVING).
A receiver can be used to check BW if we understand what we are
doing! The common mistakes are:
- Using a band-scope or spectrum analyzer having wide filter
bandwidth
- Leaving a noise blanker on (some receivers have a bit of this problem
even with the NB off)
- Using excessive (or SSB) bandwidth while tuning to check signal
bandwidth
- Using an inferior receiver or a receiver with poor close-spaced
strong-signal performance (many DSP-only radios are pretty poor)
- Using excessive gain
- Having excessive background noise or signals
- Relying on an S-meter (most are not linear or accurate).
1.) Bandscopes and Spectrum Analyzers
Bandscopes or spectrum analyzers using wide filters are often
unreliable signal bandwidth indicators. While such devices are always good for monitoring band activity,
they often have too much bandwidth and/or are subject to overload. When
bandwidth is filled by many strong signals, the accumulated signals demand
headroom from amplifier and mixer stages. Many spectrum analyzers and band scopes
(panadaptors) do not have narrow filters and
low internal distortion. The old HP-141 series of analyzers, for example,
require extreme care in use to prevent overload.
You can test bandwidth by viewing a pure unmodulated carrier on the display.
When bandwidth of a carrier is nearly zero, the display should ideally show a perfect single spike of negligible
bandwidth.
Bandscopes (and spectrum analyzers) with poor dynamic range or wide filter bandwidths are all but
useless for determining bandwidth or signal defects.
2.) Noise Blankers
Noise blankers must be OFF when checking bandwidth or working close to strong
signals.
In order to remove noise, noise blankers add a form of intentional distortion to
signals, They do this by turning a switch or gate in the IF amplifiers off and
on. An abrupt rise in peak input signal
level over average signal level triggers the switch, and shuts the receiver off.
The bandwidth of the noise detector is very wide, and this means a strong signal even
5-10 kHz away will
activate the NB gate and distort signals.
Some receivers (like Yaesu’s) do not fully remove the effects of the NB
system, even when the NB is off! In some receivers you have to turn the NB off plus
turn the NB gain down, the FT1000MP and FT1000MP MK V
are examples of this. In others, like the FT1000D,
you actually have to modify internal wiring to correct NB problems. The
mechanism is explained in the links to the receiver mods.
3.) Bandwidth
You must select the narrowest filter possible to measure TX BW,
certainly less than a 500 Hz filter with good shape factor for SSB measurements.
Receiver
bandwidth and shape factor directly adds to the transmitter’s bandwidth. This
means a perfect brick wall 2kHz bandwidth receiver
tuning across a perfect 2kHz wide transmitter makes it sound like the actual
signal bandwidth is 4kHz. Theoretically it is possible to deduct the receiver
bandwidth from apparent measured bandwidth to obtain real bandwidth, but this
generally means you have also decreased the dynamic range of the receiver (or
spectrum analyzer). In practice, deducting bandwidths often produces unreliable
results.
The slope of the receiver (or analyzer) filter is also important. If the receiver response
is -6dB at 4kHz and -60dB at 8kHz, you will hear stuff out 8kHz (plus
transmitter bandwidth) on very strong signals if you are in a quiet location.
4.) Inferior Receivers
Some radios, in particular DSP only radios, have very poor strong signal
performance. They can’t be trusted to
give accurate BW reports.
Look at tests here, Sherwood
Engineering’s tests, or ARRL
tests of close spaced receiver performance. Many receiver are not all that
good. Most Yaesu receivers have a built-in design problem in the noise blanker
amplifier that seriously deteriorates close spaced IM performance even when the
noise blanker is OFF.
Even the Sherwood engineering test is too wide for some receivers. The
Sherwood test, for example, inflates performance of R4C’s with the CF-600/6
filter. This happens because the 2 kHz test measures outside the
filter passband (600Hz). The second mixer in the R4C is a horrible design,
especially the early FET mixer. Close-spaced tests should always be done inside
the bandwidth of the roofing filter, the roofing filter should be considered
the narrowest reliably useable selectivity.
5.) Noise
If the band is noisy you really can’t check a signal for low-level IM,
clicks, or splatter. The noise will cover up any weak signal defects.
There must be at least 50dB headroom between the peak
signal level being tested and your noise floor to check bandwidth on SSB, or
80dB of signal to noise headroom to check CW bandwidth if you operate near weak
signal areas of the band. For general ragchewing away from weak signal areas
50-60dB headroom is generally enough.
Some bands are a special case because SSB operates near weak signal CW
stations. 160 meters is one example. Bandwidth of higher power SSB transmitters
operated near weak CW stations can be problematic. I often hear spits from Icom
756 and TS 2000 transmitters on SSB as far as 10kHz away on 160
meters.
Receiving noise floor is probably the single most common source of false
“clean signal” reports to what actually are problem transmitters. If
the noise or QRM floor is high, you won’t hear spurious signals.
Wide-audio operators are particular
victims to giving each other false assurances of how narrow and clean they are.
They often use “opened-up” receivers that absorb more noise power
from wide bandwidth (remember noise power is directly proportional to receiver
bandwidth) and they often live in noisy environments. It takes a good weak
signal narrow receiver in a quiet location to properly check
bandwidth.
Results
The result of the factors above is that some people will report a nasty
signal “clean” when it isn’t, and some will report a signal
“wide” when it isn’t.
Understanding how to test will correct problems, and help us use our
own equipment better. Receivers make very good measurement devices if used properly.
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