mixer downconversion noise output level dB/Hz - Page 1
Forum section: electronics / projects

author: john23
author_rank: Frequent Contributor
timestamp: February 10, 2025, 10:24:26 am
content: Hello, I have a mixer shown in the link below.The way I calculate the noise coming out of the mixer is as following:-174+NF-conversion loss=-174+0.5-6=-179.5dB/Hz so far I am correct?also theMixer Noise Figure typically measures within +0.5 dB of conversion loss for IF frequencies greater than 5 MHz.However I was told that we add another 3dB because we convert noise from RF twice.If its true, Is there some visualisation I could use to see why we add another 3dB?Thanks.https://markimicrowave.com/products/microstrip/mixers/m1-0818ne-1/datasheet/

author: mawyatt
author_rank: Super Contributor
timestamp: February 10, 2025, 02:15:36 pm
content: The 6dB conversion loss should be accounted for as part of the mixer NF, so in your calculations it should be 6dB not -6dB as it can't improve the noise floor as you've shown as -179.5dB/Hz, which BTW should be dBm (DB Milli-Watts).Don't forget to include the noise degradation due to the LO Phase Noise.Best

author: john23
author_rank: Frequent Contributor
timestamp: February 10, 2025, 05:51:52 pm
content: Hello, I was told that conversion loss decreases the signal and the noise thats why i did -6 where my logic is wrong?How can I know the LO phase noise degradation?Thanks.

author: RFDx
author_rank: Regular Contributor
timestamp: February 12, 2025, 09:34:47 am
content: Quote from: john23 on February 10, 2025, 10:24:26 amThermal noise PSD is limited to -174dBm/Hz @300K. Mixer total NF=6.5dB. The main 6dB because the mixer is passive and has typ. 6dB loss and 0.5dB on top of that because the diodes inside the mixer add some noise of their own.

author: john23
author_rank: Frequent Contributor
timestamp: February 12, 2025, 12:33:25 pm
content: Hello , I am confused regarding this 6dB .this 6dB is not Noise figure and its not conversion loss.There is some thoery I know that when our LO frequency is very close to the RF frequency then we add 3dB.but when they are far away then we need to look at some plot.If it sounds femiliar I would be glad to see some datasheet with visual example I could see this noise being converted into IF logic?Thanks.

author: mawyatt
author_rank: Super Contributor
timestamp: February 12, 2025, 02:09:22 pm
content: The noise floor is kTB, where k is Boltsmann Constant 1.38 X 10^-23 J/K, T is temperature in Kelvin, and B is Bandwidth in Hz, thus noise floor is -203.859dBw (watts) or -173.859dBm (milli-watts) @ 298K in 1 Hz Bandwidth. The only way to make the Noise Floor lower is to cool things down.The mixer loss is just lumped into its Noise Figure as mentioned. BTW theTheoretical NF Limitfor an ideal perfect lossless passive Bi-Phase mixer (diode mixers qualify) is 3.92dB due to signal lost in the mixer conversion process (Fourier Analysis shows this). ThisTheoretical Limitwas taught in all texts and until the discovery of the PolyPhase Mixer (PPM), or N-Path Mixer was thought to be the ultimate limit in Mixer NF.https://www.eevblog.com/forum/rf-microwave/polyphase-or-n-path-mixer/The reason for the added NF when the RF is close to the LO (low IF) is due to the LO Phase Noise in the 1/f region and also the 1/f noise of the core mixer switching elements (diodes, fets). Check out plots of oscillator phase noise and note how it gets worse as you get close to the carrier.Best

author: RFDx
author_rank: Regular Contributor
timestamp: February 12, 2025, 05:32:14 pm
content: Quote from: john23 on February 12, 2025, 12:33:25 pmWhich 6dB are confusing? The mixer has 6dB conversion loss and a noise figure of 6.5dB.

author: john23
author_rank: Frequent Contributor
timestamp: February 16, 2025, 12:08:30 pm
content: HelloI am using the following mixer to downconvert a -60dBm 10Ghz signal .The mixer needs 17dBm signal into the LO so I provide it using the amplifier shown below.At 10Ghz the Rf conversion loss is -7dB ,NF =0.5(above 5MHz)So what is the output noise?Thanks.mixer:https://markimicrowave.com/products/surface-mount/mixers/mm1-0320hsm-2/datasheet/amplifier:https://www.analog.com/media/en/technical-documentation/data-sheets/hmc634.pdf

author: john23
author_rank: Frequent Contributor
timestamp: February 17, 2025, 09:19:56 am
content: Yes, So in a mixer we are downconverting twice the noise.two questions:1.what if the LO frequency will not be so close to the RF frequency ,what do I need to do in such case?could you give an example?2.I am driving the LO input of a mixer with the follwoing amplifier.I know that LO power influences the conversion loss, is there something else?Is the LO SNR signal makes a contribution to the output noise?do i need to look somewehere in the datasheet of this amplifier to see its contribution to the output noise of a mixer?https://www.analog.com/media/en/technical-documentation/data-sheets/hmc634.pdf

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 18, 2025, 02:29:33 pm
content: Quote from: john23 on February 10, 2025, 10:24:26 amIt's incorrect to suggest the NF of the mixer is actually 3dB higher due to noise from image frequencies (though from a system-level perspective the result can sometimes be equivalent).Frequency mixers make calculations of system-level noise figure much more complicated. Things get especially confusing once you encounter differing definitions of noise figure (for example, Friis' original definition vs the IEEE's definition. Some also use terms like "single/double sideband noise factor").I don't have a straightforward explanation, but I suggest reading these two articles as a starting point:https://www.microwaves101.com/encyclopedias/noise-figure-one-and-two-friis-and-ieeehttps://www.analog.com/en/resources/technical-articles/system-noisefigure-analysis-for-modern-radio-receivers.html

author: RFDx
author_rank: Regular Contributor
timestamp: February 19, 2025, 05:04:55 am
content: Quote from: john23 on February 16, 2025, 12:08:30 pmMixer: IL=7dB, NF=7.5dB.Noise floor at the output of the mixer is limited to -174dBm/Hz. Noise floor at the input of the mixer is therefore -174dbm/Hz + NF = -166.5 dBm/Hz. The (noiseless) -60 dBm input signal and the -166.5 dBm/Hz noise floor will be 7 dB (= mixer IL) less at the output of the mixer.

author: john23
author_rank: Frequent Contributor
timestamp: February 19, 2025, 07:13:52 pm
content: Hello, Two questions:1.regarding the mixer why dont you add the insertion loss into the calculation?-174dbm/Hz + NF = -166.5 dBm/Hz.2.The mixer is converting the signal and its image why dont you add 3dB to the result ?why converting twice?3.I am driving the LO using this amplifier below.Is there some math i need to do regarding the PA contribution to the mixer output moise (except insertion loss)https://www.custommmic.com/resources/cmd241p4-08-18-1.pdf