Podcasting since 2005! Listen to Latest SolderSmoke

Tuesday, July 20, 2021

Mythbuster Videos 8 and 9 -- The Old Military Radio Net plus "Zero Beat and The Vertical Skirts"


I like to listen to the Old Military Radio Net on Saturday mornings. This week I was listening with the Mythbuster receiver. The AM carriers provided a good opportunity to observe the effects of the steep skirts of the 10 pole crystal filter. We start at zero beat, with the BFO exactly on the carrier frequency. If I turn the VFO dial in one direction, I in effect move the passband in a way that puts the carrier in the passband. And it is no longer zero beat with the BFO, so we hear the heterodyne (the beat!). But if I turn the VFO dial in the other direction, the carrier is now outside the passband. Even though the BFO would produce a tone, we don't hear a tone, because those steep filter skirts are keeping the carrier out. We do continue to hear some of the sideband frequencies, because they remain in the passband. The very sharp drop-off of the carrier tone is a good indication that the steep skirts of the crystal filter are doing the job.



"Zero Beat and the Vertical Skirts" Sounds like the name of a Punk Rock band, doesn't it? Anyway in this video I explain what happened in Mythbuster Video #8 (above). I explain why we can hear the Old Military Radio Net carriers when I tune the VFO in one direction, but not in the other.

2 comments:

  1. Significantly better than any of my homebrew SSB crystal filters. What's your secret Bill?

    ReplyDelete
  2. Careful measurement of the motional parameters. And using 10 poles. Cohn topology. Looking at what both AADE and Dishal software recommended. Changing the caps when the first result was too narrow. Using NanoVNA and Antuino to check passband. Paying attention to terminaton impedances (to get rid of ripple). Termination Insensitive amps on either side of the filter. 73 Bill

    ReplyDelete

Designer: Douglas Bowman | Dimodifikasi oleh Abdul Munir Original Posting Rounders 3 Column