Helping to understand conversion gain of a mixer - Page 1
Forum section: electronics / rf-microwave

author: profdc9
author_rank: Frequent Contributor
timestamp: June 10, 2020, 03:40:06 am
content: I have never designed an active mixer before and I am having trouble getting conversion gain, or at least understanding what it is.I found this BJT transistor called the BFP740 that has a transition frequency of 44 GHz. It looks like this would be a good candidate to make a mixer that could perhaps work up to 6 GHz. I was thinking of making a single-balanced mixer that could be used to demodulate a RF to IF with a low-pass filter at the end.The schematic is below. What it is modelling is a LO which is at 4000 MHz and a RF which is at 4080 MHz. The mixer correctly mixes the IF frequency at 80 MHz.The input goes to an emitter-follower stage to provide a low-impedance RF input to the mixer. The LO would be from a chip like the ADF4351 which can provide balanced output and would be AC coupled. The IF stage is coupled out with another emitter-follower which again is used to boost the impedance of the output signal so that it the output minimally loads the transistor in the single-balanced mixer. The goal is to use all parts that can be constructed from 0402 capacitors and make the layout as compact as possible so that the discrete design might be realized.The input RF frequency is 0.1 V amplitude. However, the output voltage at the IF is only 0.01 V amplitude, indicating about -20 dB conversion loss. The transient output graph is showing the IF signal.I am not really sure how Gilbert cell mixers and single-balanced mixers are able to get conversion gain. I started making a mixer in simulation with 2N3904's and got a similar result. Perhaps conversion gain doesn't mean what I think it means?

author: bob91343
author_rank: Super Contributor
timestamp: June 10, 2020, 03:47:50 am
content: I think you have it right. Conversion gain seems always to be negative.

author: profdc9
author_rank: Frequent Contributor
timestamp: June 10, 2020, 04:42:10 am
content: The reason I asked is that mixers like the SA612 have a positive conversion gain, usually 17 dB:https://www.nxp.com/docs/en/data-sheet/SA612A.pdfI was wondering how they got that.I worked on it a little more, and with a harder square wave drive I can get the "conversion gain" to 0 dB, so that the signal at the IF is the same amplitude as the RF:

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 10, 2020, 06:26:02 am
content: Conversion gain can be bigger than 1 (or 0 dB).Usually you want to drive the gates/bases of transistors in your mixers hard, because then you are really switching, and that gives you less loss in the transistors. In your design you are also kinda throwing away half of the signal by looking at only one output instead of the differential output.And ofcourse, conversion gain is usually expressed in reference to power with 50 ohm ports. In order to really see what the performance is you should look at what it does given a match at the output and look at power.

author: paul002
author_rank: Regular Contributor
timestamp: June 10, 2020, 07:06:30 am
content: Yes it is the ratio between RF in signal and IF out signal. But most of the time you are only interested in one sideband product so that is a loss already of -3db. There is a lot of literature on this subject like "Communications Receivers DSP, Software Radios, and Design By Ulrich L. Rohde And Jerry C. Whitaker" or "Experimental Methods in RF Design" by w7zoi. You can find it on internet. I tested some active JFET mixers but they had more attenuation than simple diode mixers. But the unwanted products were stronger ..

author: vk4ffab
author_rank: Frequent Contributor
timestamp: June 10, 2020, 10:08:17 pm
content: Quote from: profdc9 on June 10, 2020, 03:40:06 amI note that your LO is -10dbm 0.1V. What happens when you drive the mixer harder. A Gilbert Cell usually requires 0dbm 0.6V p-p and a Diode Ring 7dbm 1.4v p-p, perhaps your LO drive is a bit low?

author: profdc9
author_rank: Frequent Contributor
timestamp: June 11, 2020, 01:28:44 am
content: I tried to drive it harder but the IF did not increase with more LO drive. I think the transistors are pretty much turning fully on and off with the applied LO.Dan

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 11, 2020, 05:45:58 am
content: Is there any way for your simulator to show you the region your transistors are in? I think your tail current source should be switching on and off, but the differential pair should be in constant-current right?

author: profdc9
author_rank: Frequent Contributor
timestamp: June 13, 2020, 04:49:32 am
content: So I did the simulation and I think it confirms that the tail current is being modulated by the RF signal while the LO signal is being divided between the two transistors of the differential pair. Pr1 is the current in the tail, while Pr2 is the current in the transistor that is not loaded, and Pr3 is the transistor that is loaded. There is asymmetry between the two transistors because only one is being loaded with the emitter follower output.

author: rfbroadband
author_rank: Unknown
timestamp: June 17, 2020, 09:05:02 pm
content: The gilbert mixer consists of 2 stages: a) switching core 2) gm cell at the bottom.The theoretical gain of an ideal switching core is 20 log (2/Pi) = -3.9dB assuming a perfect rectangular waveform for the LO. In reality you never a perfect square wave, thus the loss is more realistically around -6dB.The mixer gets its overall gain by the gm stage below the switching core multiplied by the load resistors on top of the switching core, thus gm * R - switching loss. In general you have to distinguish between single balanced and double balanced mixers, you are showing the single balanced version. Because of the gm stage this mixer is called an "active" mixer since it exhibits gain.Ring mixers for example are purrley "passive" mixers and always have a loss associated with them since there is no inherent amplification build into the passive mixer. Realistic gain values for passive mixers are ~ -6dB.I hope that helps.