Little transceiver TX/RX offset question

[neazoi]

Hi,
I made this small multi-band transceiver. The TX/RX switch is shown in the TX position.
I notice that there is a TX/RX offset in frequency, which is desired.
However This offset depends on the frequency band the transceiver operates, because the fixed capacitance value has a different effect on each band.

Now I would like to make an adjustable offset for that reason and I first want to investigate where this offset comes from?
Capacitance ot the base to the ground in combination with the 100nf coupling capacitor?

 

[betwixt]

The only practical way to shift the frequency is by varying the voltage across the varicap diode. To make an adjustable offset, you need a potentiometer to set the amount of offset and connect it such that it increases or decreases the varicap voltage when needed. I'm not sure which way you want the offset to go but it should increase or reduce the voltage from the wiper of the tuning control, either directly or through another 100K resistor.

Brian.

 

[neazoi]

The only practical way to shift the frequency is by varying the voltage across the varicap diode. To make an adjustable offset, you need a potentiometer to set the amount of offset and connect it such that it increases or decreases the varicap voltage when needed. I'm not sure which way you want the offset to go but it should increase or reduce the voltage from the wiper of the tuning control, either directly or through another 100K resistor.

Brian.

That is why one needs to ask. I thought a second varicap but a single could do it. that way you descripe. How can it increase freq. and how it can decrease it? (circuit connections)

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Hm... a variable resistor from the varicap to the SW1a point that connects to the 100nF would create a voltage divider when the TX is swithed on?

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Correction, I meant a potentiometer not a variable resistor (one of it's ends connected to the gnd)

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Another correction (I cannot edit the post)

a variable resistor from the SW1a point that connects to the 100nF to the ground, would create a voltage divider when the TX is swithed on, with the top end of it being the 100k and the frequency pot?

 

[betwixt]

I don't think you can do it that way, all the points have signal on them. What you really need is another switch contact and to wire it this way:

Add the 'offset' potentiometer in parallel with the tuning control.
Wire the switch in series with potentiometer wiper pin.
Then connect the other side of the switch through a 1M resistor to the top of the varicap.

It means you have two tuning controls when the switch is closed, one as before and the second being a fine tune with less range. Because it can go below or above the tuning voltage (assuming the tuning pot isn't at the end of its track) you can fine tune in both directions.

Note that you sink about 1mA through the tuning control for no reason, if you make the tuning and offset controls 100K instead of 10K it will save almost 2mA current consumption with no side effects.

Brian.

 

[neazoi]

Add the 'offset' potentiometer in parallel with the tuning control.
Wire the switch in series with potentiometer wiper pin.
Then connect the other side of the switch through a 1M resistor to the top of the varicap.

It's been a bit hard to visualize how the schematic should be . Can you draw one for me? You can use this for a quick way. click and place **broken link removed**

Another simple way I am thinking: just add a 100k resistor from the VCC to the SW1a, at the 100nF connection contact.
This will have some effects
1. On TX, TX frequency will be shifted at maximum but at only one direction.
2. Now you set the RX offset instead (on RX mode), using the usual tuning process.
These two previous points will lead to a fixed TX and variable offset RX, but even with the solution you mentioned, by adjusting the RX frequency you also change the offset, so you have to reset it again. These two controls affect one another.
The simple resistor solution, whereas it does not allow for setting large frequency variations on RX (only close to the offset desired), and it shifts only to one direction, it has an advantage though. at any frequency and without external equipment or receiver you do not know what the offset is, but with this method you can reset the RX frequency by adjusting its pot all the way up to the vcc. Then the TX anr RX offset will be zero (apart from difference in capacitance of the RF amplifier that follows the oscillator). Then you can adjust the RX offset easier than before. At least now you have a starting point where you know when the offset is zero.

In practice, it may be easier to call (TX) on a fixed frequency and have the RX offset variable. Because others with expensive equipment will be able to reply on this fixed frequency. Because you know the starting offset in this cheap transceiver with the single offset resistor, you can just set the RX offset at maximum and you will always hear a tone. then bring this tone to the desired frequency you want by adjusting the RX offset.
I mean it is better this way. Because if you adjusted the TX offset, it could be messy if someone was trying to copy your signal.
Of course you cannot set the TX frequency and RX offset at the same time using this simple solution.
How does it sound?