18-04-2017, 09:38 AM
(18-04-2017, 06:00 AM)Crackle Wrote: What frequency range would I be expected to pickup on a SW radio if the FM oscillator is working.
As I think you've already discovered, you won't!
The local oscillator of any receiver is always going to be Fs ± Fif where Fs is the signal frequency and Fif is the IF frequency.
There is an Airband immediately above Band II, so thousands of badly screened local oscillators interfering with aircraft communications would not please the authorities so, as you've found, the local oscillator operates 10.7MHz below the signal frequency.
Note that the local oscillator for any frequency above 87.5 + 10.7 = 98.2MHz will actually be in Band II - your signal generator test only just misses the bottom end of Band II.
You might find it useful to experiment a bit - you may find it useful in the future. If you've got any far eastern sets with entirely unscreened front ends, one of those should make an excellent signal source - unmodulated, of course!
Many years ago I was involved in experiments to see if it was viable to use the Labgear Televerter to upconvert the entire VHF spectrum - 22 adjacent channels - to UHF and found two strong sources of interference.
One turned out to be a production fault in the ITT VC200 chassis: a ferrite bead on the base of the first IF amplifier to suppress spurious oscillation turned out to be a white ceramic bead such as used to insulate the leadout wires of heating elements! *
After much head scratching I found that the second one was local oscillator radiation from the UHF tuner in a 12" Loewe Opta which was running on the opposite side of our large workshop - in fact, as the two receivers were both more or less in corners, the distance between them - about 20 feet - couldn't have been greater!
* When I reported my findings to ITT (it wasn't confined to one set) I was told that the sets were built in Portugal and as Portugal, at that time, didn't use UHF for broadcasting, they would probably never have found out!
