[amsat-bb] Re: SINAD Vs. Noise Figure Testing
kl7uw at acsalaska.net
Wed Dec 10 00:14:26 PST 2008
In this modern day, soundcard programs display very weak signals much
below what can be heard. The method described below can be modified
using a soundcard spectrum program whereby you can lower the signal
on the display to equal the noise floor and thus achieve very close
to S+N/N = 0. Measuring the signal in dBm that produces this level
will give one a good measure of sensitivity.
For example, my SDR-IQ is calibrated in dBm so I can see this
directly. Typically with 2.1 KHz wide SSB the SDR-IQ has a noise
floor of -130 dBm at 28-MHz into a 50-ohm load. On an antenna you
see a much higher noise floor that shows the amount of noise the RF
When I use a 144/28 MHz convertor before the SDR-IQ, the absolute
reading is likely shifted by the gain and NF of the convertor. Use
of a calibrated signal generator will give a good benchmark for
sensitivity. One must insert a test signal using a directional
coupler to know what actual sensitivity is when connected to an antenna.
73, Ed - KL7UW
At 01:53 PM 12/9/2008, Glen Zook wrote:
>The original SINAD was a 10 dB figure which was developed at least a
>couple of decades after the 20 dB quieting figure. The "proper" way
>to measure the 20 dB quieting point was to use an AC voltmeter
>utilizing a cupric oxide rectifier (which was what most of the "olde
>tyme" VOM units used - i.e. Simpson 260). With the squelch of the
>FM receiver "wide open" the voltmeter was placed across the speaker
>and the volume control adjusted for a convenient reading (1 volt was
>very common). Then an on-frequency unmodulated signal was applied
>from a signal generator having an accurately calibrated attenuator
>and the signal level was adjusted to the point where 1/10th the
>voltage was recorded on the VOM. Since a reduction in voltage by a
>factor of 10 represents a 20 dB reduction this point was referred to
>as the "20 dB quieting" point.
>Most experienced two-way radio technicians could actually make a 20
>dB quieting measurement "by ear" and did not require the use of an
>AC voltmeter. When verified by someone watching a voltmeter while
>the technician did the measurement by ear virtually always resulted
>in a measurement that was well within 1 dB which was "close enough
>for government work" for field measurements.
>Using a well calibrated dB meter (i.e. one made by Hewlett Packard)
>across the speaker will usually not result in the same reading as
>that done with a "plain old VOM" using the simple cupric oxide
>rectifier. The 20 dB quieting measurement was done by field
>technicians who often did not have such fancy test equipment as a
>calibrated dB meter.
>An easy to make and a very meaningful sensitivity measurement is the
>LDS (least discernible signal) which involves turning on the BFO,
>tuning the receiver for maximum signal strength, and then reducing
>the signal level (using a well calibrated attenuator on the signal
>generator) to the point at which the signal can just be detected by
>ear. The reading in microvolts from the signal generator is the
>LDS. Of course reducing the bandwidth does affect the LDS. By
>doing an LDS measurement you are measuring the weakest signal that
>the receiver is capable of receiving through the noise generated
>within the receiving system.
>--- On Tue, 12/9/08, Michael Heim <kd0ar at sbcglobal.net> wrote:
>From: Michael Heim <kd0ar at sbcglobal.net>
>Stan makes a good point here, especially for FM sensitivity. I
>worked in the 2-way business for a long time, and typically a sinad
>measurement would be something like the value of microvolts at the
>antenna input for 20 dB of quieting of the receiver. It is
>basically a measurement of FM receiver sensitivity. It will also
>show mis-aligned IF strips and detector, but for the sake of
>arguement in this case, lets say the receiver is functioning
>normally. The goal should be the minimum amount of signal input at
>the antenna connector quiets the receiver 20 dB.
>I am not certain that a sinad reading would be of much use on an SSB
>receiver. Typically a similar measurement for an SSB receiver would
>be MDS, or "Minimum detectable Signal", which would be the amount of
>signal that is injected into the antenna connector that produces a
>faint but detectable signal in the receiver.
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