AF Frequency for Max. Sensitivity - elmers Forum at eHam.net

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AF Frequency for Max. Sensitivity	Reply
by KG4YTL on August 18, 2005	Mail this to a friend!
I am trying to figure out what AF frequency is the most sensitive. This would be useful for PSK31. I have heard that it is about 1000hz since this is the average human voice and companies design radios with this in mind. When I use the IF shift on my IC-718, it seems the filter is centered on 1400hz. I do this by looking at the water fall while I switch the IF shift full right and full left every few seconds. The pattern indicates the center of about 1400hz. Although, this seems to change on different bands i.e. 10 meters has the center closer to 1700hz. Any comments on what AF is the most sensitive and why the filter center appears to change?

RE: AF Frequency for Max. Sensitivity	Reply
by W6VPS on August 18, 2005	Mail this to a friend!
Is this really the question you want to ask? Don't you really intend to ask what audio frequency is most easily heard by people or detected by a receiver's audio chain? The audio range of 2k to 5k is where most speech characteristics lie. There are many frequencies higher and lower of course but you understand as we age our hearing begins to be less accute and the range of frquencies detected starts to limit considerably. 1k hertz is a common tone frequency to use...but there are some folks who do not tolerate it well and prefer tone frequencies a little lower. For what it's worth a 1k hertz tone at 0 Db was how we always set the audio levels in brodcasting. Paul

RE: AF Frequency for Max. Sensitivity	Reply
by N6AJR on August 18, 2005	Mail this to a friend!
I have lots of damage from jet noise in the Air force and Wind noise from driving with the window down for 28 years in emergency service for the state of California.., so there are lots of areas I just can't hear at all, but I prefer a tone about 700 to 800 cycles as easiest to hear for me. This may be common to a lot of folks my age ( late 50's) old viet nam vets and ex military. The Military was not too concerned about hearing protection, and the Mission was always first priority, and I guess we were a bit expendable. So a bit of hearing loss is ok as long as as we made it home.. YMMV

RE: AF Frequency for Max. Sensitivity	Reply
by AA4PB on August 18, 2005	Mail this to a friend!
If your SSB IF filter is as it should be then the audio output level should be pretty constant over a range of about 300Hz to 3KHz (depending on the particular filter characteristics). For PSK31 it shouldn't matter much where you put your frequency within that range. If you want to view your filter characteristics on the waterfall, tune to an empty frequency on the 10M band so you hear nothing but white noise. The brilliance of your waterfall should be pretty consistant across the 300Hz to 3KHz range. The center frequency of narrower IF filters will depend on the design of the radio, the alignment of the injection oscillators, and the accuracy of the filter.

RE: AF Frequency for Max. Sensitivity	Reply
by KG4YTL on August 18, 2005	Mail this to a friend!
I don't care what the human ear is sensitive to. I guess I am asking what AF corresponds to the IF that is most sensitive. (What AF is best detected by the receiver's audio chain.) When looking at a waterfall of random static, the frequencies which appear stronger are around 1500hz on my radio. This 1500hz relates to the IF filter center frequency. Frequencies that are higher or lower ar attenuated as part of the IF filter shape. Thru experimenting, I have found the most sensitive part of the radio is around 1500 AF. Does this sound right? Is there a way I can find this spec officially/mathmatically? Thanks

RE: AF Frequency for Max. Sensitivity	Reply
by AA4PB on August 18, 2005	Mail this to a friend!
Well, the filter is supposed to be flat over most of its bandwidth (that's the design goal). It will drop by 3dB at the far ends of the bandwidth because the specified bandwidth is at the -3dB points.

RE: AF Frequency for Max. Sensitivity	Reply
by K0RFD on August 18, 2005	Mail this to a friend!
I understand what he is asking. Depending on the rig, and whether you have any Rx equalization programmed, the rig's frequency response probably won't be a limiting factor. If you are using a sound card interface, many of them use inexpensive isolation transformers that are nowhere near linear. My homebrew interface uses Radio Shack transformers that roll off at about 400 Hz on the low end and about 2500 Hz on the high end. I have found that staying in the middle of the passband, the supressed carrier frequency plus about 1500 Hz, gives me the best results. This is actually less of a problem on receive than on transmit. It's pretty easy to compensate for the receive audio being rolled off. However, if you adjust your Tx audio to set your power at the low or high ends of the passband, your power will be much higher, and you might overdrive, at the center of the passband. Tune your rig to a place on the band that is quiet, and observe the "snow" on your waterfall. That will give you a good visual clue as to the audio frequency response of your system -- receiver, interface, and sound card taken together.

RE: AF Frequency for Max. Sensitivity	Reply
by AA4PB on August 19, 2005	Mail this to a friend!
Even more than the interface coupling transformer response, the values of any coupling capacitors can have an effect on the overall response. The frequency response of the radio however should be relativly flat over the filter bandwidth. You certainly should not have to be adjusting your receiver volume control or transmit audio level over the range of say 600Hz to 2400Hz. My receiver gain and power output stays pretty constant over that range.

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