Questions on Mixer
The attached pic is a simplified architecture of a mixer I've once seen. I'm wondering about the function of the capacitor CIF, and the magnitudes of those RCs, ie, RB1, RB2, Cin and CG1.
Has anyone ever seen the same work? Thanks.
Ruri
The Mixer has single end RF input, while tranconductors current is differential; so it's a common Gilbert cell with single end input.
Probably the output capacitor is used to filter RF+LO, or anyway it's a simple low pass, often used in Mixer outputs.
Of course exact values cannot be predicted, but, depending on frequency:
The input components are a DC block serial cap (large value, it's impedance is negligible compared to RF intrinsec impedance), resistors are used to block RF signal to bias (typically some kOhm), and shunt cap on the other input is used to "fix" the base, so it's a large value also.
I hope it can help.
Mazz
Oh, oh
I've seen that IF output is single end, so I'm not so sure anymore...
Probably, if the output is single end, it's large enough to shunt RF+LO and to let it pass RF-LO; by the way which are RF & LO frequencies?
Sorry
Mazz
Functions of the components:
- CG1 to ac ground one of the bases (or gates if CMOS) for single ended input
- RB's for biasing- Should be large for RF signal blocking
- CIF provides simple lowpass filtering. The values depends on the RL's and difference between RF/LO and IF.
Good luck. :)
The CIF is meant to filter out high frequency compenent. If the mixer is a downconverter, CIF filter out RF+LO, RF and LO for a normal Gilbert Cell but, it can't filter out 2*LO becuase that is a common mode signal.
Another pupose on CIF, just like Mazz said. If we do not add such a cap between the output nodes and simply match only one output path to the next stage, the power loss of this output network should be 3dB higer than using a ideal diff. to single-end transformer, because half of the signal power is discard. So we add this cap to utilize a portion of the formally discard signal power to lower down the 3dB loss.
One resemble example is available in Razavi's "RF microelectronics", fig.6.21.
Thank you all for the replies.
Yes, I agree with you that CIF is used to filter out some unwanted signals. But how? and how to derive the transfer function (equation)? How to perform some simulation on it?
Rgs.
Ruri
- How: Lowpass. Imagine a parallel of R and C to AC ground.
- Transfer function: Roughly R_out || 2CIF
- Simulate with and without CIF and observe the RF and LO signal levels at the output
:)
Hi, thanks for Donj.
However, I don't agree with dsjomo, the one of Fig 6.21 in "RF Microelectronics" is totally different from this one. Since the former structure is used as a current combiner, while the structure in this case is used to filter out some HF signals.
Another question:
While using ADS to simulate the NF of this kind of mixer in two ways (see Fig(A) and Fig(B)), the achieved noise figures differ about 20dB. Why?
And, since the output of this mixer is designed as single-ended, which one of the simultion is more reasonable?
Best Rgs.
Ruri