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1-1 of 1 messages
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Designing the Ultimate HF Front End
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by G3RZP on June 22, 2009
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Personally, I like the old fashioned idea of two or preferably three tuned circuits. Tuned either by a variable capacitor or relay switched cpacitors in in a 1-2-4-8 sequence. Not varicaps or PINs or ferrite cores because of linearity considerations. Precede this with a variable attenuator, going in about 5dB steps. Then an RF amp that can be switched out, the mixer, and crystal filter appropriate to the signal bandwidth. Now we don't have a general coverage rx out of this, but does that matter?
Then use an IF of sround 1.4MHz. This is because the IMD performance of crystal filters tends to be better at lower frequencies. Hooke's law applies. When you stretch a spring, extension is proportional to load - up to a point. The same happens with the crystal: the more voltage, the greater the mechanical distortion of the plate. The thicker the plate, the more volts it takes to get the crystal to not to be linear in terms of mechanical distortion against voltage. Thus the IMD is better with lower frequency crystals. I suspect that the 0.25 inch thick X cut 456kHz crystals in pre war HROs would be very good! there are some advanatges in using two filters, each fed from a port of a 90 degree hybrid: each sees 3dB less signal, so in theory gives 9dB less IMD. The signal powers add to give then original power (less filter loss) while the IMD powers, although they add, are still lower. Incidentally, crystal filters can often give better results on IMD one way round than the other.
My articles in QEX (2002) and NCJ (2008) with the results of spectrum measurements at sunspot peak and sunspot minimum suggest that in Europe, about 100dB of instantaneous dynamic range is all that is needed. As signals on 7MHz get bigger in the evening, so does the noise floor - which is not surprising. Note, incidentally, that measured noise floors at my QTH suggest noise somwheta below the ITU median for 'quiet rural'.
Intermodulation is not the only bogey. Much more insidious in its effects is phase noise. If you attenuate the signals by 10dB at the antenna, IMD products hopefully go down by 30dB. Certainly, they will go by more than 10dB. Phase noise will only drop by 10dB, however.
Now it's certainly easier to get good phase noise at lower frequencies. Some of the older rigs, such as the FT102, are very good in this respect. So I am not convinced that up conversion is the best for a ham bands only receiver. Of course, commercial pressures tend to ask for general coverage, even for transceivers - there are a surprising number of commercial users of ham equipment outside the ham bands.
It's because the old tube receivers had the tuned narrow band front end that they work as well as they do, considering that in many cases, the IMD performance is not very good - intercept points of -20dBm or so. They did tend to be good on phase nosie though. That is not surprising when you consider that some of them had nice high Q oscillator coils, well spaced from the screening can, and a triode oscillator running about 1.5 watts input and no lossy varicap diodes!
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