my varactor extension

[Roff]

DAC resistance affects 2 things in your circuit:

1. The Q of your tank.
The reactance XL of your 100nH inductor at 100MHz is 63 ohms. The resistance of your DAC is about half the resistance of the MSB (lowest value) resistor (0.533 for a 4-bit DAC, to be more precise). Q=R*XL. To avoid killing the Q, you should probably keep the resistance at least 100*XL, or greater than about 6k. Nigel's advice is spot on, as usual.

I would hope that the impedance of the resonating tank would be much higher than just the XL of its inductor. Therefore DAC resistors should be in the hundreds of k ohms. :lol:

True, but the Q is nevertheless R*XL, where R accounts for all resistance in parallel with the tank circuit.

 

[Roff]

I would hope that the impedance of the resonating tank would be much higher than just the XL of its inductor. Therefore DAC resistors should be in the hundreds of k ohms. :lol:

This is all ignoring the series resistor feeding from the D2A to the varactor, which Audioguru has rightly included on various diagrams he's posted.

True. The resistor will help isolate the tank from the parasitic capacitance of the four DAC resistors and their associated pad capacitances.

 

[audioguru]

... the Q is nevertheless R*XL, where R accounts for all resistance in parallel with the tank circuit.

I went to Wikipedia, and they were talking about all the RF stuff I forgot about. They say:

 

[Roff]

... the Q is nevertheless R*XL, where R accounts for all resistance in parallel with the tank circuit.

I went to Wikipedia, and they were talking about all the RF stuff I forgot about. They say:

This applies to series resonant circuits with a series R, or parallel resonant where the resistor is in series with the L or the C. It should be obvious that, for a parallel resonant tank with a parallel resistor, the Q is proportional to resistance, while in the case that you posted, it is inversely proportional to resistance.
Regarding √L/C, if you do the math for Fr=100MHz, L=100nH, you will find that this equals 63 ohms, which is identical to reactance of L and C individually. This is NOT the same as the impedance of the tank, which is, for ideal components, infinite at resonance.