Inserting a fixed capacitor in series to the variable tuning capacitor;
and inserting another capacitor in series to the variable local oscillator capacitor,
the tuning span can be reduced. Is that right ?

I inserted 22pF to each variable capacitor on a FM broadcast tuner and works as intended. The range changed from 88-108 to 96-106 MHz for the full tuning knob.
Am planning to reduce even more the span with 10pF in series instead.

Any related considerations beyond this modification, to avoid unexpected side effects ?

 

Hello,

Has ths tuning capacitor more than one section?
I know that tuning capacitors for an AM / FM radio has 4 sections.
Like this one:



The 0-20 pF are the FM and the 0-266 pF the AM capacitors.

Bertus

 

I think it is no more complicated than your description. This is a common method of reducing the capacitance range.

 

Inserting a fixed capacitor in series to the variable tuning capacitor;
and inserting another capacitor in series to the variable local oscillator capacitor,
the tuning span can be reduced. Is that right ?

I inserted 22pF to each variable capacitor on a FM broadcast tuner and works as intended. The range changed from 88-108 to 96-106 MHz for the full tuning knob.
Am planning to reduce even more the span with 10pF in series instead.

Any related considerations beyond this modification, to avoid unexpected side effects ?

The resonant tank circuit can have a higher Q
https://www.allaboutcircuits.com/te...rent/chpt-6/q-and-bandwidth-resonant-circuit/

About the circuit; a narrow band works but the ability to tune full band is reduced.
When weak reception must be improved upon then having less range might be more acceptable.
When the Q is higher at one part of the band the I/Q normal change is less even across the band.
https://www.allaboutcircuits.com/te...anding-i-q-signals-and-quadrature-modulation/
If weak reception is an issue it might be addressed at the antenna base as a frequency selective pre-amp.

 

Has ths tuning capacitor more than one section?

Thanks.
The tuning capacitor has two sections, A variable+trimmer paralleled for RF and a variable+trimmer paralleled for local oscillator.
Type is "224" which I understand as "two-twentyfour pico farads" and there is only FM broadcast tuning.

----> http://ntfs.diytrade.com/sdp/77422/...9693/variable_capacitor_CBM-224F_E-4B4_5.html

Am concerned if sensitivity will degrade by shrinking the tuning span, and if any other side effects may show up. I expect an improvement in selectivity. Goal is to tune only along a 10MHz or less span instead of 20MHz original 88-108.

 

Hello,

When the tuning capacitor has small correction caps, you can optimize it for the selected range.
When the oscillator is tuned, the input section can be tuned for maximum signal.

Bertus

 

Inserting a fixed capacitor in series to the variable tuning capacitor;
and inserting another capacitor in series to the variable local oscillator capacitor,
the tuning span can be reduced. Is that right ?

I inserted 22pF to each variable capacitor on a FM broadcast tuner and works as intended. The range changed from 88-108 to 96-106 MHz for the full tuning knob.
Am planning to reduce even more the span with 10pF in series instead.

Any related considerations beyond this modification, to avoid unexpected side effects ?

In principle, you will get what you want.
Just remember that the Midpoint of your Reduced Tuning Range will keep shifting to the Higher Frequency side. 88-108 goes to 96-108, then maybe 104-108. (Not sure how yo got the upper end came down to 106.)
But why do you need this setup in a FM Broadcast band?