Variable capacitance diodes

Variable capacitor diodes

I have heard on the ham bands that variable capacitor diodes cannot be used to tune a front end radio circuit. In fact they can only be used to tune an LC tuned oscillator. Well, that is not exactly so, as I will now attempt to explain.

See below a few vari cap diodes circuits, from the well known source, but if wiki wishes to add my article, please feel free to do so.

The internals of the vari cap diode look as follows:

This diagram I think is from a text book, however greater the “d” layer, the lower the vari cap diode capacitance. Now why is it the case that a rectifier diode can be used a electronic voltage tuned capacitance. The diodes depletion layer thickness is where the magic happens. However one may be assuming or thinking that the diodes bridge voltage ( threshold voltage ) needs to to overcome for the vari cap diode to function in any way, well this not so. While the threshold voltage is not breached, the radio signal will pass through the diode, as the diode acting as a capacitance connection component. Provided the RF signal voltage is not greater than the threshold voltage, the vari cap diode would provide an capacitance type component in an RF circuit. The RF circuits from no:1 to no:3, illustrate this type of circuit arrangement principle. However, a band pass filter can be designed to be use as an front end tuneable band pass filter, below diagrams.

The above bandpass filter design is constructed around a single parallel tuned circuit, L1 and the vari-cap diode. The principle that the tuning of the voltage tuned circuit would peek select the intended radio signal, by bootstrapping the primary side with the signal return from the tuned secondary resonance, thus the more present signal upon the RF output coupling. In any extent, the "Q" factor of the tuned circuit would thus that be of the dynamic resistance, and hence the circuit bandwidth on the 3dB points. The R.F. coupling with this circuit design, is the two primary windings, one for the RF input, the other for the RF output, while the secondary winding is the bandpass filter tuned section for selective signal filtering.

An additional point relating to the above bandpass filter circuit, the two primary circuits can be centre connected, creating a common ground wire connection, thus a balance primary transformer input connection wiring. However, there is no reason why one half of the balanced primary could be the R.F. input, while the second half of the balanced primary, the output tuned responsive filtering onto the follow on R.F. circuits of the radio.

Alternatively, the input primary side of the bandpass filter, may perhaps an offset wring, that is the R.F. input of say only 20% of the R.F. output side of the common grounded centre wire of the primary circuit, circuit diagram. This would provide an auto transformer impedance step up, while the bottom end side of the wiring grounded, the input first half of the primary circuit to the antenna, on the common connected centre wiring, while the second half of the primary side, the top end side of the overall winding, connected to the R.F. amplifier circuit. Similarly, the same circuit could be used for a tuned bandpass filter auto transformer impedance step down.

In the second bandpass filter design, the vari-cap diodes are tuning both “L1” and “L2”, as one complete single inductor, made from two parallel inductances, connected as the secondary windings. The centre tuned RF circuit would thus resonant at the desired frequency, while also providing an isolated tuned circuit coupling with only the magnetic coupling between the input and the output circuit, of the tuned transformer circuit connections. The dynamic resistance of the tuned circuit, namely the RF circuit “Q” factor, and hence the 3dB point bandwidth of the tuned circuit, would hopefully not be disturbed. Provided the RF signal voltage is less than the diodes threshold voltage, then only the capacitance provided by the diodes depletion layer would be present to the RF tuned circuit.

All other R.F. connections are magnetically coupled via the primary windings, of "L1" and "L2", which are thus then R.F. coupling to the secondary windings of "L1" and "L2" of the R.F. coupling transformers, the secondary windings thus voltage tuned circuit for selective signal filtering transformer.

However, to prevent a higher RF signal voltage from breeching the threshold voltage, where-by the diode would then act as an rectifier diode circuit, a higher vari cap tuning voltage would have to be used. The principle being that the maximum RF signal voltage would be at least 0·7 volts less or more say 1·5 volts less than the maximum tuning voltage for the vari-cap diode.

In both cases of the vari-cap tuned bandpass filter circuits, there not any other components surrounding the actual tuned circuit wiring, that would sub-due the "Q" factor of the the dynamic resistance and hence the RF bandwidth 3dB point reference. The "d.c." only connections to the vari-cap tuned circuit, is the vari-cap tuning voltage, all other R.F. couplings are made via a magnetically coupled tuned circuit for bandpass selective signal filtering.

How would one implement this into a 40m radio.

If say one would to use an “Arduino Uno” to control the digital oscillator, as an AD9850 or an si5351 vfo circuit, this would act as the local oscillator for the radio. To tune the front end bandpass filter, the Uno would then program a DAC, a digital to analogue convertor, this to provide the variable capacitance diode tuning voltage for the 40m radio RX front end antenna fed circuit.

The vari-cap tuning voltage and thus the bandpass filter centre of frequency, need only be within the 3dB point of the bandpass filter. However with an 8 bit DAC, there would be 255 steps of vari-cap tuning voltage to tune the front end bandpass filter to the desired signal, in accordance to the local oscillator DDS vfo.

I found a pdf reference for denco coils and circuits, be thought-full though, please do not over load this hams website. http://www.g4dmp.co.uk/dencocoil.pdf