Saturday, March 26, 2011


My old buddy K3RR and I were corresponding and he mentioned the difference between his cascaded 250hz filters and my 12hz DSP filters. The theoretical difference is 10log 250/12 =13.2 dBm. I was sitting on 80M seeing if I could hear anything out of A52J:

(after all hope springs eternal) and since the band was so quiet I decided to measure the difference and make a little document.

Here is the radio with 12hz filters:

The RX2 receiver's vertical antenna has a little less output than the full size vertical on 80M connected to RX1. Best case today is in the -118dB range. For the 250hz case:

-107 dBm was the best I could do which is about 11 dBm difference. So the filter difference extends me another 11 dB into the noise floor. I decided to look at 500hz filters as well:

Predicted is 16.2 dBm and I got 14dBm difference, very consistent with the data point above. Some of the difference between predicted and measured could be due to AGC action since certainly by the time I get 16dB above the noise there is some AGC starting to flatten out the signal. I had "best" AGC-T (agc threshold, or the point at which the AGC just starts to reduce gain) set on the 12hz filters:

When I readjusted on the wider filters it didn't make any difference. The AGC-T adjustment is one of the best features of this radio. The AGC is designed to effectively turn down the sensitivity of the radio. If the AGC is not active the radio is most sensitive. By adjusting the AGC-T you can set the AGC to turn on at some point considerably higher than the band noise. This is a big deal when working DX since in most radios the AGC is already well engaged and turning down the sensitivity due to the band noise itself, so you are never really able to realize the full sensitivity of the radio or the full dynamic range. Since you don't need the radio to be more sensitive than band conditions allow by adjusting the AGC-T your chances of hearing someone right at the noise are improved.

Here is a shot at 500hz with the noise blankers turned off


and now at 500hz with the blankers turned on

Without the noise blankers, the AGC-T and the 12hz filters (and diversity) I basically would only be working G's, OK's and DL's on the bands.

Here is a shot of the radio on 10M with a resonant vertical attached

and here is a shot with no antenna connected to RX1

Still no Bhutan but VU4PB just showed up on 40 to tease me:

A nice European pile up and a K1 who is hearing him. It's the K1's gray line time so maybe tonight is the night: