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 Does the local oscillator level affect the sensitivity?
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 Return to top of page · Post #: 16 · Written at 7:52:15 PM on 27 April 2019.
Wirelessfan's avatar
 Wirelessfan
 Location: Sydney, NSW
 Member since 26 April 2019
 Member #: 2349
 Postcount: 18

I recall a letter to RTV&H where a reader had tweaked/reduced the level of an external LO to get a cleaner sinewave into the mixer of their published design.
This improved the s/n of that particular hi-fi tuner...
 
 Return to top of page · Post #: 17 · Written at 8:36:40 PM on 27 April 2019.
Marcc's avatar
 Marcc
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5257

One thing notable is that as the frequency increases many of the mixer converters loose amplitude. That loss of amplitude is reflected in loss of modulation, that is rather why we saw the rise of Pentagrids with separate, mainly triode, exciters. Some early pre-Pentagrid sets used separate valves as exciters. & we saw valves like 6K8 & 6J8 go into SW sets.

The Pentagrids love voltage & it is normal for them to die from lack of volts & drop off at high frequency as they fail. Only one pentagrid is not destabilised by AGC on SW. That's why its not a applied on SW.

If signal for AM is normally applied to the signal grid of the mixer via a series cap. If applied to the antenna, theoretically, on the BC band it should not get through from the antenna. Any connection to the LO will throw it off frequency
 
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