Now I am a little more familiar with the rig and have updated to v1.06 firmware I have at last started to make measurements on this much-anticipated transceiver.
The first measurements made are the receiver noise figure. Unlike preamps and transverters, transceivers don't usually have an RF out from their receiver, so that noise figure/gain analysers like the HP8970 can’t be used to measure sensitivity. The analyser receiver section doesn’t usually go down to audio.
The more usual way to measure receiver sensitivity is to measure MDS (minimum discernible signal or minimum detectable signal). This can be a very misleading name as when measured, as MDS is defined, the signal is usually most definitely still discernible!
The accepted definition for MDS is when an applied signal power equals the receiver noise floor in whatever bandwidth is selected (usually 2.7kHz for SSB and 500Hz for CW). There are two problems here.
First, the signal level must be measured for its true noise power. This requires a true RMS voltmeter. This should preferably be an analogue (moving coil) meter rather than a digital meter, since the nature of noise is such that when measured in these relatively small bandwidths the digital display will be flickering all over the place, unless well damped, and this can lead to other errors. You are measuring a sine wave signal against band limited noise. Never an easy thing to do when the signal is close in level to the noise.
The second problem is that the signal generator must be well screened and preferably a very low minimum output level. It is usual to operate the signal generator a little above its minimum level to improve screening, however, in order to minimise poor screening effects. As the required signal levels will most likely be in the range -130 to -140dBm, the generator is often set at about -90dBm and then an additional 40dB of in-line fixed attenuators plus an 11dB step attenuator used to reach the low level required. The step attenuator is used to exactly set the signal generator signal into the receiver so that the meter shows a rise in level, over no signal, of exactly 3dB, indicating that the signal power is at the same level as the receiver noise floor. The definition of MDS.
This method can be quite accurate and repeatable, but often is not so due to less than perfect screening in the signal generator, uncertainty in the fixed and step attenuator calibration and coax cable leakage. Double screened coaxial leads are most definitely required.
For these reasons I am a little sceptical about some published MDS results and decided I wanted a better way to measure the sensitivity of the IC9700 receivers on each of its three bands.
Since I have an HP346A (5dB ENR) noise head I thought it would be interesting to use the Y factor method of measurement.
The well specified, low level, noise output from an HP346A noise head can be used instead of a signal generator to measure the complete receiver noise figure from RF input to audio output.
Just one problem. How to measure the audio power and overcome the noise flicker?
My solution is to use my SDR-IQ, as it can be used down to 500Hz, to effectively replace the receiver section of the HP8970 noise figure meter, and then measure both the noise power from the receiver with no input and with the noise head switched on and producing the excess noise. The ratio of these two noise powers is the Y factor and it is easily converted to indicate the noise figure of the receiver. The switching on and off of the noise head is done manually as described below.
It is important to ensure that the SDR-IQ range is set a bit bigher than the bandwidth selected in the receiver. I used 4kHz with a centre frequency of 2kHz
The SDR-IQ, in Continuum mode, can be set to give a great deal of smoothing and increase the FFT. Since the noise source is manually switched on and off, the amount of time in each state is effectively unlimited, leading to improved resolution of the Y factor.
The SDR-IQ, when running Spectravue in Continuum mode, shows the noise power as a dB difference (Y factor when converted) between noise on and noise off. The HP8970 driving the noise head can be switched between these two states with the 14.1 and 14.2 Special Function and the actual ENR used in the measuring band can also be shown using the 5.x Special Function.
To be continued..
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