Monday, March 2, 2009

Static-- A new way to think about tuning the band

I spoke about the AGC and the ability to tune the intelligibility using the AGC-T control and the AGC decay.. You can see the various controls in the above pic.

Here are 2 audio files one without anything but 75M static and one with static plus a QSO

The band is running s-7 constant noise (-88dBm) with static crashes to +10 (-63 dBm). The signals in the QSO are running S-9 (-74dBm) with some mild QSB. Typical 75M thunderstorm season conditions. There are storm conditions all up and down the Atlantic coast, and out into the ocean tonight. The stations are out in the far mid-west and mountain states and I am using my 80M flat top oriented broadside NW-SE.

The controls are AGC-T set to 90 which is what the F5K comes out of the box set to and I might set it to this on a very quiet winter night. AGC is medium, and the Audio is set to 16. After recording at these parameters I reduce the AGC-T to 51, set the AGC decay to long, and I add some audio boost by switching in some flat boost I have set up in the in the RX equalizer. The settings are the same for both audio clips


(the little distortion spike in the middle occurs because I kick in the RX EQ boost too soon, aka operator error)

Static plus QSO

(in this sequence you can hear me kick in the RX EQ near the end)

This level of static is typical of summer time conditions here in Florida

Here is a pic of the menu item that has more complete control of the AGC. The greyed out portions (like decay et al) become available when you choose "custom" on the front panel, and you can set the AGC up anyway you want. I haven't really played much with these controls since the SDR-1000 days but it may be able to further tune the AGC for even better static reduction.

The values in the greyed out parts (Attack, Hang, and Decay) are what is chosen when you make a choice in the AGC drop down on the front of the radio console. The choices are fixed, long, slow, med, fast, and custom. For static reduction on SSB, long works best for me.

As I understand it the AGC in this radio is a dual loop system, kind of how a phase locked loop works, with major and minor loops controling. This allows the gain to be controlled basically under parallel control, so you get very rapid major gain reduction and minor tweaks, and it makes the AGC VERY smooth with virtually no over shoot or pumping. It is this characteristic that allows for the AGC to handle wide values in static crashes without wide variation audio output. If you ran the AGC at 51 on a quiet band , you would miss half of what was there to hear, but as I said before, all signals are always present, its just a matter how far under the noise you are capable of trolling to find them. I usually run AGC-T at 75-80 on the quietest bands during the winter, sometimes as high as 90 on bands like 30M where my noise floor approaches -130dBm. 75M summertime conditions do NOT allow you to dig very far down into the noise, so you may just as well set up your radio for that given set of noise conditions and enjoy the relative silence. The effect is less pronounced on CW with narrow filters but still useful.

Note the AGC is completely different than the "noise reduction" (NR button on the front panel) feature. That function along with auto notch (ANF) and the noise blankers are controlled from an entirely different setup menu. I'll discuss them at a later date.