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 environment adds.
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.
Glen, K9STH
Website: http://k9sth.com
--- On Tue, 12/9/08, Michael Heim kd0ar@sbcglobal.net wrote:
From: Michael Heim kd0ar@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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