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Selecting the right component values for comb generator.

Sonnyx1

New Member
Helo,

I am working on making a comb generator with around 2MHz base frequency. I have seen some diagrams online for higher base frequency comb generators examples that are very similar to each other so I want to try using them as a base.

circuit.jpg
10MHz base.jpg


So my question is, how do I go about selection the right values of the components if I want to use a oscilator close to 2MHz? Is there also a way to simulate them to see how the output signal looks like before going and making it?

I am grateful for any help.

Sources of the two pictures:
First: https://g0mrf.com/source2.htm
Second: https://www.edaboard.com/threads/ha...-with-fundamental-frequency-of-10-mhz.200668/
 

JimB

Super Moderator
Most Helpful Member
Looking at the second circuit:

1uH and 220pF is resonant at just over 10MHz
So what we have is basically a TPTG Oscillator (Old thermionic valve reference! :)).

If you want a 2MHz comb:
Change the crystal to a 2MHz type
Change 1uH to 5uH, and 220pF to 1100pF to make the collector circuit resonant in the 2MHz region.

The amplifier/shaper circuits with the MAR6 and the diodes should be OK as they are.

As for simulating it, I have no idea.
Just build it and smell the solder fumes!

JimB

PS
Dont try building it on a solderless breadboard, you may be disappointed when it does not work.

Try building it using "ugly construction", on a piece if copper clad board, using the copper as a continuous ground plane.
 

JimB

Super Moderator
Most Helpful Member
Another thought came to me over lunch*,
referring to the second circuit again, the 220pF capacitor and the 1.8nF (1800pF) capacitor form a voltage divider.
This voltage divider performs two functions:
1 It reduces the drive voltage from the oscillator to the MAR6.
2 It prevents the low impedance input (50 Ohm) of the MAR6 from excessively loading the oscillator collector circuit and stopping the oscillations.

With this in mind, it may also be a good idea to increase the 1.8nF to around 9nF or whatever the nearest available preferred value.

* Lunch was lambs kidneys, cooked with onion, bacon and mushroom, and served on a bed of rice.
Very tasty.

JimB
 

Sonnyx1

New Member
JimB
Hi,
first of all, thank you for your help. I have another question. If I wanted for the crystal to be 500kHz. How would I go about changing the circuit and calculationg the needed values for the components? Do only the 1uH and 220pF elements matter if I change the frequency, or wouold a change this big need all the components values to change.
 

JimB

Super Moderator
Most Helpful Member
If you want to go down to 500kHz, my first thoughts are that:

1uH becomes 20uH
220pF becomes 4700pF
1.8nF becomes 33nF
Also the 330pF capacitor in the emitter circuit of the Oscillator should be changed to maybe 0.1uF.

Sometimes, low frequency crystals can be a bit odd and don't like to work in some circuits, it all depends on the characteristics of the crystal.
If the 500kHz crystal does not work in this circuit, try a different circuit.

JimB
 

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