804
« on: December 06, 2019, 2110 UTC »
Monitoring weak (realllllllllllly weak) cw sigs has allowed me to test if an 80Hz filter helps or hurts weak cw signal detection. Some weak sig cw enthusiasts say "find your weak sig, then open the filters up wide".
That may work for some ops but when I tried that on some truly weak cw today they melted into the band noise and the cochannel crap, enabling the 80Hz filter made them pop back into detection, time and again on every sig coming in.
And they were just at the perception level in the 80Hz filter.
I don't recall the narrow filter I normally set in the 756 Pro2 (100Hz) being as effective, mebbe it has more ringing, will have to compare both rigs at the same time same signal.
The diff is the 775 has high quality matched 500Hz xtal filters in the 9mHz and 455kHz IF strips that then feed the dsp demodulator/filter unit.
The 775 has 5 frequency mixes/conversions from antenna jack to audio out. A ADC16071 16bit adc does the a/d conversion in the 775.
The Pro2 has a several kHz wide 455kHz IF ceramic filter it passes all sigs through prior to the mix down to 36kHz and right into the adc and dsp. The Pro2 has 3 frequency mixes/conversions from antenna jack to audio out.
The DSP in the IC-756PRO/756PROII employs a 24-bit A/D
converter. The logical value of the dynamic range of a 24-bit A/D converter is 144dB, however the actual value of the analog performance is smaller than this and performance may differ considerably, depending on the type of A/D converter used.
An interesting effect I noted with the dsp filtering in the 775 is you can open the IF filters wide as in ssb wide and the dsp filter does almost as good a job despite no longer having the protection afforded by the preceding narrow xtal filters, despite being a 16bit adc.
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