72367Re: Softrock Dynamic Range and MDS Redux
- Feb 22, 2013Hi Warren,
The IP3 value is a hypothetical value where the signal you are hearing at 7.221 MHz is the same strength as two separate signals at 7.224MHz and 7.227MHz. The 7.221 signal is the 3rd order mixing product of those two other signals in the mixer under test i.e. a value of say +20dBm.
Of course you are not going to be listening to signals of that strength but what strength would the mixing product be at say a more normal S9, -73dBm? Depending on how poor the mixer is, the third order product may be there at say S2 or S3. So in real life how many of those stations you see on 40m are actually true stations on that frequency and not mixing products from much stronger stations further up the band? How can you actually tell? The stronger the IP3 of the mixer the more likely they are real stations and not mixer products.
Have a look at:
Of course the mixer IP3 is only the start. How good or how poor do you think the IP3 is of the toroids you are using for the input filter? Why does PA3AKE use T94 toroids in his input filters if he could get away with T25's in front of the >+45dBm IP3 of his H-mode mixer? He uses T94's because the IP3 of T25's is way too low such that the ferrite is actually the cause of the IP3 rather than the mixer.
Here in Europe, 40m above 7.2MHz is packed with commercial AM stations with huge huge signal levels. You have to have a very good Rx not to get IM products. My old FT101 was dire. A softrock has a way better mixer than that had. On an FT101 you would think the band had nothing on it but commercial stations. It was simply IM products covering up the real, weaker signals. A +20dB attenuator was a good start!
For the money the softrock Rx has phenomenal performance.
Have a look at Sherwood Engineering's radio comparisons
--- In firstname.lastname@example.org, "warrenallgyer" <allgyer@...> wrote:
> I understand and agree with all you said except below.
> My radios all exhibit severe overload symptoms at -12 dBm or less. +30 dBM would be well beyond the severe overload level on on any radio or sound card.
> This level is easy to measure and predict and therefore a good comparative benchmark. But the practical application to real life operating is hard for me to understand.
> Warren Allgyer - W8TOD
> > It's not an arbitrary designation. The overload level is defined as that level where the 3rd order products are at the same amplitude as the MDS. It follows that the Two Tone Dynamic Range is 2*(Pi-MDS)/3. There is an algebraic explanation in one of the Appendices to "Solid State Design for the Radio Amateur". It can also be derived graphically.
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