VFO SLOW TUNING

now a days peoples  are using the dds vfo and all sort of digital techniques in the vfo. The days of slow vernier drive and band-spread coils etc are gone. Also there is no more production of this mechanical devices. We still love those metal gangs and slow drive !  But unfortunately we could not find them easily.... here is the todays solutions.

Now peoples use the varactor diodes. This varator diodes need to be given some DC voltage. One solution is to use the multiturn pot.

You can use the fig.1 ckt. I used it but it did not give a very clear and broad range.
Then somehow, I got hold of this ckt in PY2OHH, the Brazilian ham. In one of the VFO he used  I liked it. I am going to implement into all my vfo.

It gives a very clear and very slow drive. It can work with the 33v zener used as varactor. Earlier is used the 1n4007 as varactor. The voltage is not linearized. The two 100k pot has to be linear ( not the logarithmic) for further very slow tuning you can use a multiturn pot. How much shift it gives just enough to resolve the 5khz shift. Ok when you are using the homebrew rx/tx.

You would like to use the MV2109 etc etc varactors. Here nothing is hard and fast rule. Use whatever is on your hand.

The values of the slow tuning is given in the ckt itself.  Wire as shown in the ckt.  Don't forget where the ground is taken from ! (Will upload the fotos laters on ... ok see below )

That's it folks wheel have been re-invented successfully.
 

BFO for measurig inductance

I use this ckt very extensively for knowing the resonance of my tuned IF stages (4mhz, 10 mhz), band pass filters , oscillator tank ckts.

As such it does not meaure inductance directly. For measuring you have to do the calculations. Connect Inductor-L and put 100 pf capacitor across it to make a tuned ckt. Now, read the freq in the counter. with the help of formula calculate the inductance.
Please note you will be not able to work out the RFC values by this as it likes only High-Q inductance !

I use this to circuit to tune the tank (L and C) for a given freq. when either L or C is variable or both variable , change them to get the desired freq in the counter. remove your tank (L and C) from the osc and put it in your IF stage or oscillator stage, bandpass filter and you are on the spot.

This little ckt has proved itself over the time. Use of styroflex and polyster capacitor in this oscillator is a good thing.  I use 12 volt regulated dc.

Years ago i used when i did not have frequency counter, with help of analogue communications receiver ( no digital freq readout) and other xtal marker used to put the tanks of 4 mhz IF into resonance...