SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
This is especially true regarding the Input 3rd Order Intercept Point or IIP3
, which is given for every product, for each frequency and for all sensitivity levels. The meaning of IIP3 is generally known and in any case descriptions and explanations are easily found on Internet. For our analysis it is enough to understand that IIP3 is a Figure of Merit
which defines the input overload power level (in dBm) at which our receivers will start producing spurious signals within their passband, or in other words, their own interference.
By using the wealth of data provided, we can easily have a quantitative idea of our receiver’s intermodulation performance across its entire frequency range. Try to obtain this info for other apparently similar products!
As an example let us consider a practical case in the 40 m band. The 8 MHz IIP3 specification for the RSP-1A is: IIP3 = -4.24 dBm at the maximum sensitivity of 0.3 uV, which corresponds to a received power of -117 dBm / 50 Ohm.
The 40 m band extends from 7.0 to 7.3 MHz. Through the day, but at night especially, at F1 =7.3 MHz operates a very strong broadcast station, sometimes coming in at – 40 dBm. At F2 = 7.25 MHz another broadcaster comes in at an average power of -70 dBm. These two stations can potentially produce two in-band intermodulation products as follows:
IIP3(1) = (2*F1) – F2 = 14.6 – 7.25 = 7.35 MHz
IIP3(2) = (2*F2) – F1 = 14.5 – 7.30 = 7.20 MHz
This second product then could appear smack into the 40 m. band as a self generated spurious signal and interfere with an on air signal at that frequency. But what would the level of this spur be? A practical rule of thumb states that the power level Ps
of the spur will be three times the difference Pd
between the strength of the strongest of the two incoming and the rated IIP3 value of the receiver. In our case (see SDRplay specs) IIP3 = - 4.24 dBm
Pd = 40 – 4.24 = 35.76 dB
Ps = 35.76 * 3 = -107.26 dBm
But the rated sensitivity of the RSP-1A is -117 dBm, so that this spur would be received as a real signal with a level 10 dB higher than a true weak signal at the same frequency!
In reality we must consider the noise floor and look at the real sensitivity of our receiving system, receiver + antenna
. Let us look at Table 3 of this thread:
https://www.sdrplay.com/community/viewt ... f=5&t=3685
which I have again uploaded below. If we were in a QUIET RURAL area, our most likely noise floor would be -112 dBm, so our receiver’s sensitivity would exceed the noise by: 117 – 112 = 5 dB. If we now used a 5 db attenuator between antenna and receiver, our useful sensitivity would still be the best possible, but both F1 and F2 signal power would decrease by 5 dB with a very large effect, considering the times 3 factor:
Pd = 40 + 5 – 4.24 = 40.76 dB
Ps = 40.76 * 3 = -122.28 dBm
Thus the spur could not give us any problem because it had dropped below the level of the noise floor!
It can be argued that the same result could be obtained reducing the RSP-1A gain by 7 dB by placing the gain slider position to 1: our IIP3 would increase to +1.94 dBm and we would have:
Pd = 40 + 1.94 = 41.94 dB
Ps = 41.94 * 3 = -125.8 dBm
Again placing the spur below the noise floor.
But we must remember that reducing the RF gain can non-proportionally degrade the noise figure, so the most efficient solution is to use an external attenuator. For example, looking again at the specification, going from position zero to position 4 for the RF gain, NF goes from 18 to 50.5 dB (32.5 dB worse) while IIP3 goes from – 4.24 to +13.68, or 18 dB better.