RF preamp DCF77 practical approach

[salviador]

best inducer for77500Hz

I would like to build a preamplifier for the frequency 77.5Khz (DCF77).
The problem is having a Qload > 80, for have a bandwidth < 1Khz
Q = 77500/1Khz = ~78
the type of circuit is this:



For have high Rload I use dual gate mosfet, and Tapped Capacitor Matching for output 50ohm

Which is the best inducer to have the max selectivity of circuit?
I need an inductor with high Q

 

[BigBoss]

Re: best inducer for77500Hz

BF908 is not a suitable choice for 77.5kHz, it's intended for VHF UHF applications.You'd better use AF MOS transistor..
High Q factor is solely not sufficient, Load Resistance ( Impedance ) will impact also the Bandwidth.This circuit is not a well designed one.
Clarify exactly what you intend to do..

 

[BradtheRad]

Re: best inducer for77500Hz

Generally speaking, select LC ratio based on the amount of current you want flowing.

High A, high C, low L.
Low A, low C, high L.

Calculate each component's impedance (inductive or capacitive) based on the standard formulae. Values probably are approximate due to phase different, but it will give you something to start from.

 

[salviador]

Re: best inducer for77500Hz

Hi, I chage the circuit with opamp:
Is real this circuit ?


with opamp I have high impedence

 

[FvM]

Re: best inducer for77500Hz

The OP doesn't eliminate the need for a high Q inductor. Also in a practical filter circuit, you would need to tune the LC resonator to overcome component tolerances. Q >= 80 is absolute feasible properly designed ferrite inductor (using air gap and litz wire) but difficult to achieve with off the shelf fixed inductors.

Classical DCF77 receivers often used a high Q resonant circuit build with the ferrite antenna. An active RC filter may be a way to achieve the intended selectivity without a high Q inductor.

 

[salviador]

I have design my preamp for dcf77 (77500Hz), The maximum bandwidth I managed to get is 5.91Khz, not more :-(
I use this schematic:

what do you think?
(with this inductor I can not do better)

 

[BigBoss]

You can not obtain a proper frequency response in this way.
First, Q factor of the transformer is around 130 according to data so BW will be 600 Hz.But you forget Output Impedance of the MOS that is parallel to tank circuit.
Second, the Load will also bring some parallel resistance due to transformation.
The best way is to use a Xtal/ceramic filter at the input of the amplifier then amplify the signal otherwise you try it till you cut your teeth..

 

[betwixt]

I've had some success at 60KHz (MSF time standard) using an op-amp and twin-tee filter using only RC components as a pre-amp but it still relied on a crystal filter in the main receiver to achieve final selectivity.

Brian.

 

[salviador]

twin-t-filter is notch filter, removes an unwanted frequency.
How can be useful to select a specific frequency?

 

[betwixt]

Simple - put the notch in the negative feedback path of the op-amp so it reverses it into a peak.

Brian.