Home |
Search |
Today's Posts |
|
#1
![]() |
|||
|
|||
![]()
On Thu, 31 Mar 2005 12:28:46 GMT, Jaggy Taggy
wrote: Richard, what do you mean by "If you lack sufficient shielding for the detector, this will degrade the nulls considerably." The source is the built in noise generator in the bridge. My detector is my Icom communications receiver, connected via coax to the bridge. What additional shielding would be needed?? Hi Uwe, This is entirely revealed by practice. Simply answer how wide is the null? If you don't know how wide it should be, then chances are you don't know how well your system is connected. As I pointed out elsewhere, I can obtain nulls that are 100dB deep. This is one simple measure of width; although not in terms of width, it is very distinctive and correlates with the same consideration. Far more frequently my nulls are 120+dB deep (down into the receiver noise from a signal level of 1mW). The problem with a noise bridge is power. To observe that same depth of null requires a noise source of several Watts (not sure of the actual figure now, but retail sources don't come close). If you balance your bridge and you run out of signal before you run out of balance, then your settings are at the wrong Z. What appeared to be a balanced null was actually the lack of source power to indicate you were off-set. For a noise bridge, you have to keep adjusting even when the noise seems to have been nulled. You keep going until you find the noise signal climbing again, and then split the difference between the two edges of the silent span of readings. If that is too wide, you could have quite a time of this. Now, consider if your shielding is not tight and you could hear your source along a path that was not through the bridge = you never hear a null, or you get a very very broad null that still contains noise/signal. A receiver's shielding that is good to -100 to -120dB is exceptional. In our hobby it is not exactly commonplace, but it is found in good equipment. However, how good your equipment is can only be determined through actual testing as shielding is also a function of grounding, and there everyone is an expert but few are well grounded. 73's Richard Clark, KB7QHC |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
FA: Palomar RX Noise Bridge | Swap | |||
Icom 746pro Testimonial | Shortwave | |||
FS: Palomar Engineers R-X Noise Bridge | Swap | |||
FS Tennatest RF Noise Bridge 1-150 MHz | Swap | |||
Automatic RF noise cancellation and audio noise measurement | Homebrew |