Hi, everyone
Now I am doing a Gm-C RF tracking band pass filter whose working freq is from 48M to 300M by using Tow-Thomas architecture; the tuning circuit is confused me a lot. i try to use PLL as tuning circuit, i use the same Gm-C cell to building a vco, but the oscillate freq is not the same as the BPF, why? anybody can help me, thank you:)  or can other proper method be used as tuning circuit?
Spring

rfcooltools.com wrote on Jul 10^th, 2010, 11:00pm:

thank you for you reponse:)
i know for active RC the method you mentioned is effective3, but for Gm-C circuit, the Gm is not easy to measure.

Good grief no!

Tune the -6dB point of the filter.

Create an input frequency signal at the -6dB point of the filter.
Put input signal into filter.
Put input signal resistor divider
Compare amplitudes of the 2 signals (build an amplituide detector)

Adjust filter until amplitudes match.
Done.

loose-electron wrote on Jul 11^th, 2010, 12:19pm:

Thank you for you response:)

Why choose -6dB point of the filter, i see same papers choose 0 dB point of the filter, but the accuracy of the amplitude detecor  limit the tuning accuracy of the filter.

Spring

Hi Loose_electron,


you mentioned tuning to the -6 dB frequency. I think you may have misread the post since this is a bandpass filter. Otherwise could you please explain a bit further. Thanks.

Hi Spring,

IMO if you're going with tuning using the method mentioned by loose_electron, then you could do a phase comparsion at the input and output of the filter since the phase shift should be zero at the center frequency.


cheers,
Aaron

Your architecture will depend on your system.  Can this be a one time tune (with external reference signal), an occassional on chip tune between operations, or a continuous tuning to track temperature/VDD, etc. (ie, master/slave operation).

The question of how to make 1/R from a Gm is pretty fundamental.
A Gm can be a single xter or diff pair, with sometimes complex biasing (ie, CMFB, degeneration tuning).  
The outputs are connected to the inputs of an appropriate polarity.

The ascii stick figure someone drew looked more like a delta-VGS bias than a Gm to me.  Not like a trans-conductance cell.

I recall the Johns & Martin text has some tuning examples that might be useful to you .

Good luck,
Wave

;)

aaron_do wrote on Jul 13^th, 2010, 2:03am:


Ok, my bad, I was thinking a LPF.

For a BPF its a little different.

If the BPF is narrow in BW you tune for an amplitude peak using a band centered reference signal.

If the BPF is a bit wider, you generate two reference signals that sit on band edges (one above band center, one below), and you adjust  your tune for amplitude balance between the two signals.

Another option is using a band centered test signal and adjusting the tuning up-down and find the -6dB points of the
tuning system, and then splitting the difference to center the BPF. Tuning system has to be linear.

Many possible options there.

The reason I suggested -6dB is because it's easily generated, V/2 is pretty easy to do right?

gmC filters will require updating due to temps and process corners. Will need to update settings periodically.

Oh, and I am a big proponent of digital tuning systems. Switched systems are just a bit more predictable than an analog FB system.

Hi Rfcooltools,

Thanks for the detailed derivation. I had assumed proportionality and not equality. For Gm =1/R, agreed that it should be 4:1