WEBVTT 00:00.120 --> 00:02.760 This is three video series on single diode mixers. 00:02.760 --> 00:06.000 Design and simulation using Microwave Office. 00:06.120 --> 00:12.680 In today's video, we are going to talk about what is RF mixers and where do we use them in RF and microwave 00:12.680 --> 00:13.320 engineering. 00:13.400 --> 00:18.440 And in the next section of this video we will talk about what is single diode mixer, its application 00:18.440 --> 00:23.040 and how to create a single diode mixer circuit using microwave office. 00:23.120 --> 00:24.160 So let's get started. 00:24.280 --> 00:27.040 Now let's start with what are RF mixers? 00:27.080 --> 00:30.640 RF mixers are three ports active or passive devices. 00:30.760 --> 00:36.360 They are designed to yield both a sum and a difference of frequencies at a single output ports. 00:36.400 --> 00:40.080 When we insert two distinct frequencies on input ports. 00:40.120 --> 00:45.880 Now as you can see in the block diagram, as we insert frequency one on input one port frequency two 00:45.880 --> 00:50.880 on input two ports, then the output will be sum and difference of frequency one and frequency two. 00:50.920 --> 00:55.480 Now let's understand about these ports and naming we use in RF mixers. 00:55.560 --> 01:00.200 So if we see from the picture below this is the symbol of RF mixers. 01:00.240 --> 01:01.600 So here is one declaration. 01:01.600 --> 01:02.850 Before moving forward. 01:02.850 --> 01:08.210 I want to make it clear that in this entire course of mixer discussion, I'll be talking with respect 01:08.210 --> 01:11.530 to radio frequency receivers, not the transmitters. 01:11.570 --> 01:13.530 Now let's go back to our port discussion. 01:13.650 --> 01:18.650 So as you can see, the first port which is RF input signal is coming from antenna. 01:18.690 --> 01:21.370 I just want to highlight one more point between mixers. 01:21.370 --> 01:23.770 And antenna will have other circuit blocks as well. 01:23.770 --> 01:26.730 So it's not just you can follow this block directly. 01:26.730 --> 01:28.730 So there will be other circuit blocks. 01:28.850 --> 01:35.130 For example there will be some amplifiers, filters, etc. for now we will focus on mixers only. 01:35.170 --> 01:38.130 Now in the second port there is another input. 01:38.130 --> 01:40.650 We denote it with local oscillator input. 01:40.690 --> 01:41.050 All right. 01:41.090 --> 01:47.250 So that local oscillator circuit we just introduce some frequency based on our output requirement. 01:47.250 --> 01:52.210 So we design our local oscillator circuit based on the desired frequency of this mixer. 01:52.250 --> 01:57.890 We'll talk about it in detail when we'll be working on or when will be designing the circuit of mixer. 01:57.930 --> 02:00.130 Now the third port is an output port. 02:00.130 --> 02:03.290 As you can see we denote it with intermediate frequency output. 02:03.330 --> 02:09.130 Now if Now, if we'll talk about from the definitions or from the poor discussion of, uh, RF mixers, 02:09.210 --> 02:11.730 the output of the mixer will be. 02:11.890 --> 02:18.730 So we can just finalize the formula based on our previous discussion will be the sum and subtraction 02:18.730 --> 02:20.010 of input frequencies. 02:20.090 --> 02:26.890 So if I just write it down f I f which is the frequency of I of port will be is equal to flow, which 02:26.890 --> 02:30.890 is the frequency of local oscillator port plus minus f rf. 02:30.930 --> 02:34.130 Next we will understand the concept of up and down conversion. 02:34.170 --> 02:39.330 So these are just the terms that we will use in this entire series of videos. 02:39.330 --> 02:44.330 And yeah so in general application of mixer we don't keep both frequencies. 02:44.330 --> 02:48.050 So as I've told you using mixers we can produce two frequencies. 02:48.170 --> 02:51.450 The sum and subtraction of input port frequencies. 02:51.490 --> 02:51.770 Right. 02:51.810 --> 02:55.050 So in general we don't keep those both of those frequencies. 02:55.050 --> 02:56.410 We reject one of those. 02:56.410 --> 03:03.570 Either we reject flow plus plus FRF or we just reject flow minus FRF. 03:03.570 --> 03:07.500 And that is again based on the requirement of our mixer design. 03:07.500 --> 03:13.420 So if we want to produce an output frequency that is lower than the input RF frequencies, then it is 03:13.420 --> 03:14.540 called the down conversion. 03:14.540 --> 03:19.460 So if we have input frequencies coming from antenna is higher than the desired frequency at the output 03:19.460 --> 03:20.700 of mixer, then it will be. 03:20.700 --> 03:23.220 Then we have to down convert the frequency right. 03:23.260 --> 03:24.300 As its name suggests. 03:24.300 --> 03:29.860 And similarly, if we want to produce an output signal whose frequency is higher than the input frequency 03:30.100 --> 03:32.900 at the RF port, then we call it up conversion of the signal. 03:32.940 --> 03:36.900 Let's try to understand this concept better with the help of one example. 03:36.900 --> 03:41.820 So let's say we want to up convert from 2GHz to 10GHz signal. 03:41.820 --> 03:47.620 So we'll simply see if we'll add eight gigahertz into this input frequency we can achieve the target 03:47.620 --> 03:48.980 frequency of ten gigahertz. 03:49.220 --> 03:49.460 Right. 03:49.500 --> 03:52.740 So let's apply that into the formula that we just discussed. 03:52.740 --> 03:56.460 So here RF input will be is equal to two gigahertz. 03:56.460 --> 03:58.380 Low input will be eight gigahertz. 03:58.380 --> 04:02.380 And the formula is f I f is equal to f l plus minus FRF. 04:02.380 --> 04:06.180 So then we'll produce two frequencies ten gigahertz and six gigahertz. 04:06.180 --> 04:06.230 Regards. 04:06.270 --> 04:06.710 All right. 04:06.750 --> 04:08.590 We want to keep the ten gigahertz. 04:08.630 --> 04:09.030 Right. 04:09.070 --> 04:14.470 Then what we'll do, we'll just add a high pass filter and reject the six gigahertz of frequency. 04:14.550 --> 04:19.790 So this is an example of upconversion because we have converted frequency from 2GHz to 10GHz. 04:19.830 --> 04:20.990 Now there is another example. 04:20.990 --> 04:25.030 Let's say we want to produce 0.5GHz from two gigahertz input. 04:25.030 --> 04:28.110 So let me know in the comment section what will be the low frequency. 04:28.230 --> 04:34.670 Now before I start discussing the single diode mixer, let's talk about what is the difference between 04:34.710 --> 04:39.150 mixers and frequency multipliers divider or RF synthesizer. 04:39.150 --> 04:45.510 Because we can also achieve the desired signal with the help of frequency multiplier, divider and synthesizers. 04:45.550 --> 04:45.790 Right? 04:45.830 --> 04:51.310 So in contrast to frequency multiplier and dividers which also change the signal frequencies. 04:51.430 --> 04:58.070 So the advantage with mixers is mixers theoretically preserve the amplitude and phase without affecting 04:58.070 --> 05:04.270 the modulation properties of signal and its port, which is not the case in case of multiplier and dividers. 05:04.270 --> 05:10.270 So that's why whenever we are using multiplier and dividers, we need some sort of feedback or control 05:10.270 --> 05:10.550 loop. 05:10.590 --> 05:14.710 Let's say PLL, by which we can able to sync the phase of the signals. 05:14.790 --> 05:18.990 And we also use amplifiers to maintain the amplitude of the signal. 05:19.030 --> 05:19.350 Right. 05:19.390 --> 05:26.030 So that makes this circuit a bit difficult to place when you are dealing with RF section where we need 05:26.070 --> 05:28.990 really neat and clean layout and small circuits. 05:29.030 --> 05:29.350 Right. 05:29.390 --> 05:36.750 So that's why we always prefer mixers for RF whenever we need up and down conversion in the RF application. 05:36.790 --> 05:41.830 Next we will understand what are single diode mixers or diode mixers in general. 05:41.950 --> 05:46.070 And pros and cons of diode mixers over other types of mixers. 05:46.190 --> 05:49.270 So diode mixers are the most modern mixer design. 05:49.270 --> 05:50.990 Uses Schottky diodes. 05:51.190 --> 05:56.310 The main reason for this is the Schottky diode has major carrier device. 05:56.310 --> 06:01.870 That means it has higher switching speed than other p-n junction diodes, which is suitable for designing 06:01.870 --> 06:04.510 mixers up to 13GHz or more. 06:04.630 --> 06:11.280 The other advantage of using the diode mixer is by adjusting the barrier voltage, we can adjust the 06:11.400 --> 06:18.680 yellow drive, which will be required to obtain low loss mixing, and that leads to have higher linearity 06:18.720 --> 06:20.600 compared to other types of mixers. 06:20.720 --> 06:23.600 Now let's understand that with the help of block diagram. 06:23.600 --> 06:31.360 So there are four building blocks of a diode mixer, RF isolation circuit, RF matching network diode, 06:31.360 --> 06:34.040 or the DC biased diode circuit. 06:34.040 --> 06:36.320 And the last block is filter block. 06:36.360 --> 06:38.920 Let's start with RF isolation circuit. 06:38.920 --> 06:45.040 So as their name suggests, RF isolation circuit is responsible for isolating the input ports which 06:45.040 --> 06:49.680 are RF input or RF frequency input and low frequency input signals. 06:49.720 --> 06:56.440 Next building block is the matching network, which is responsible for matching the input impedance 06:56.440 --> 07:02.680 of DC biased diode circuit and the output impedance of RF isolation circuit. 07:02.800 --> 07:09.320 So whatever is coming out of RF isolation circuit, it should be impedance matched or both ports should 07:09.320 --> 07:12.890 be impedance matched with the DC bias circuit. 07:12.890 --> 07:15.090 And there we use a matching network. 07:15.170 --> 07:17.090 It could be a different type of matching. 07:17.170 --> 07:18.130 We'll understand that. 07:18.170 --> 07:21.450 How to design that matching network in the next step of this video. 07:21.570 --> 07:23.690 And at last we'll have filters. 07:23.810 --> 07:27.890 So as we all know that we have some desired frequency of this mixer. 07:27.890 --> 07:34.610 So to keep that desired frequency and reject all other types of frequency and spurs, we use these filters. 07:34.690 --> 07:37.130 That will be the last stage of this mixer design. 07:37.210 --> 07:43.490 Next we will understand these blocks in detail while designing and simulating the single diode mixer 07:43.490 --> 07:44.850 and different blocks of it. 07:44.890 --> 07:51.490 So step one is let's start with our simulations of diode, so that you can directly download using the 07:51.490 --> 07:56.090 link given in the description, because I have already designed that in previous video. 07:56.090 --> 07:58.530 And we are going to use the same diode model. 07:58.530 --> 08:02.930 Once that project has been downloaded, you can go to our microwave office. 08:02.930 --> 08:07.130 From your start menu, run the tool, go to File Open Project. 08:07.130 --> 08:09.090 You can locate wherever you have saved it. 08:09.090 --> 08:15.450 In my case it is on desktop and just select the single diode mixer amp file and click on open. 08:15.530 --> 08:16.010 Here we go. 08:16.130 --> 08:18.290 So this is the project which we have designed. 08:18.290 --> 08:23.810 And this is nothing just the diode model and the IB curve simulation of the diode model. 08:23.930 --> 08:29.850 So just to recall that this is the package parasitics of the diode which is here. 08:29.970 --> 08:35.130 And we have downloaded the Spice model of this diode from the manufacturer's website. 08:35.130 --> 08:39.050 And then we have created this test Testbench to plot the I-V curve. 08:39.090 --> 08:45.610 And at last we have just matched the I-V curve or the output of this I-V curve with the datasheet to 08:45.650 --> 08:48.370 verify that we have simulated it correctly. 08:48.490 --> 08:53.370 So in the next step we are just going to create the diode DC biasing circuit. 08:53.410 --> 08:58.090 Now before doing that I'm just going to add some of the requirement of our diode mixers. 08:58.250 --> 09:02.650 So in case of this diode mixer the RF frequency. 09:02.770 --> 09:08.690 So we'll just denote it with f RF will be 4.25GHz. 09:08.690 --> 09:12.010 And the output frequency we need is 0.5GHz. 09:12.250 --> 09:15.940 So F I f will be 0.5GHz. 09:16.100 --> 09:16.380 All right. 09:16.420 --> 09:17.740 So this is the requirement. 09:17.740 --> 09:22.420 And to achieve that we need the flow of 3.75GHz. 09:22.460 --> 09:22.620 Right. 09:22.660 --> 09:24.740 So this is our sort of requirements. 09:24.740 --> 09:30.700 You can add as much as information you want in this design notes, which is very helpful I find throughout 09:30.700 --> 09:37.420 the designing and simulating different circuits because simulation can be as huge as possible. 09:37.540 --> 09:40.140 So it's good to keep the design notes. 09:40.140 --> 09:44.340 And yeah, just make make sure you're just noting down and mentioning all the information. 09:44.340 --> 09:46.540 It could be impedance of the tracks. 09:46.580 --> 09:48.380 Let's say you have designed a microstrip line. 09:48.380 --> 09:50.260 I want to use it throughout the simulation. 09:50.260 --> 09:54.980 That number or the thickness of or the width of that microstrip line. 09:54.980 --> 09:56.540 So you can just note it down here. 09:56.540 --> 09:58.260 And then it would be helpful to recall. 09:58.300 --> 09:58.860 All right. 09:58.860 --> 10:02.260 So these are our requirements as you can see on your screen. 10:02.260 --> 10:06.940 Now in the next step we are just going to create the diode DC biasing circuit. 10:06.980 --> 10:11.300 To do that we'll just simply go to our circuit schematic. 10:11.460 --> 10:16.020 Right click Create New schematic I'm going to name it Diode Biasing. 10:21.060 --> 10:22.780 And click over create button. 10:22.820 --> 10:26.100 Now in the next step we need to create the Subcircuit. 10:26.100 --> 10:31.300 As you already know from the previous tutorials of our design environment. 10:31.340 --> 10:37.860 Now, instead of going through with the process again, as we already know that we have the subcircuit 10:37.900 --> 10:38.980 of diode here. 10:39.140 --> 10:44.220 So if I just select that and go to inner layer, it will lead me to this diode circuit. 10:44.260 --> 10:44.580 Right. 10:44.620 --> 10:47.340 Which are which is the package parasitics of the diode. 10:47.420 --> 10:53.020 So instead of creating it again from scratch I'll just copy that from the diode I-V curve circuit. 10:53.060 --> 10:56.300 So again to go or it's not open I guess. 10:56.300 --> 10:57.060 So we'll just. 10:57.100 --> 10:57.780 Here we go. 10:57.820 --> 11:01.940 So we'll just copy it and go back to diode Biasing and paste it here. 11:02.060 --> 11:03.540 So I want to rotate it. 11:03.540 --> 11:06.580 So to rotate that we just press Ctrl R. 11:06.580 --> 11:08.340 And I want to rotate it like that. 11:08.500 --> 11:08.900 There we go. 11:08.940 --> 11:13.180 And we'll add the ground port at the cathode of the diode. 11:13.220 --> 11:14.060 Let's place it here. 11:14.100 --> 11:20.390 Now we can't just directly connect the anode of the diode to DC biasing circuit, because we know that 11:20.470 --> 11:23.990 the the input of the anode will be a RF signal. 11:24.270 --> 11:30.270 And if we just connect that directly to a DC biasing circuit, then there will be a RF current flow 11:30.390 --> 11:33.670 from this to the DC circuit, which is not good. 11:33.750 --> 11:37.910 So to avoid that we have to add an inductor or we call it choke. 11:38.070 --> 11:44.430 And that choke will act as a high impedance whenever a AC current will try to flow into the DC section. 11:44.430 --> 11:47.270 To do that we'll just go to elements from elements. 11:47.270 --> 11:51.710 We'll go to lumped model, go to inductor and select a closed form of inductor. 11:51.750 --> 11:54.790 Now to rotate it we'll just right click and place it here. 11:54.830 --> 11:58.070 Now to limit the current we need to add a resistor. 11:58.150 --> 12:01.270 As we all know that resistor has the property to limit the current. 12:01.310 --> 12:05.910 We just select a closed form of resistor right click and place it in series with this inductor. 12:05.950 --> 12:10.230 Now in the next step we are just going to add a DC supply. 12:10.270 --> 12:12.750 To add that we'll just go to source. 12:13.030 --> 12:16.990 And from the sources we'll find the DC voltage source. 12:17.030 --> 12:19.680 We'll just drag it, rotate and place it here. 12:19.720 --> 12:26.520 All right, so as we all know that we need to add a couple of capacitors to this DC supply to avoid 12:26.520 --> 12:29.760 all the transient noise due to this DC voltage. 12:29.880 --> 12:30.240 All right. 12:30.280 --> 12:35.640 Now to add that we'll again go back to our lumped model capacitor close form of capacitor. 12:35.640 --> 12:38.080 And we'll add three parallel capacitors to it. 12:38.120 --> 12:41.280 So for now I'm just making some space here. 12:41.320 --> 12:42.680 Let's keep it here. 12:42.840 --> 12:43.440 Control C. 12:43.480 --> 12:44.280 Control v. 12:46.320 --> 12:46.880 Here we go. 12:46.920 --> 12:49.600 And next we are just going to connect all these. 12:49.760 --> 12:51.320 So we'll just connect it. 12:51.600 --> 12:54.760 This this and these all will be connected to ground. 12:54.760 --> 12:58.840 So we'll just place one ground port here which is here. 12:59.000 --> 13:00.600 Let's place it here and that's it. 13:00.680 --> 13:03.840 So this is a sort of DC biasing of a diode. 13:03.880 --> 13:05.960 Next we are just going to enter the value. 13:05.960 --> 13:09.160 So as you remember from the I-V curve that yeah. 13:09.160 --> 13:12.600 So we have looked for couple of voltages across the diode. 13:12.720 --> 13:15.320 And what will the amount of current will be flowing through that. 13:15.320 --> 13:17.760 So we'll target the ten milliamps of current. 13:17.760 --> 13:18.960 So as you can see can see here. 13:19.080 --> 13:22.760 So this is the point where we'll have approx ten milliamp of current. 13:22.760 --> 13:27.000 And then the voltage across the diode will be 0.369V. 13:27.000 --> 13:34.920 And so if I'll be precise then at ten milliamps the voltage across the diode will be 0.3685V. 13:34.920 --> 13:37.640 And you can also tally that from the datasheet of the diode. 13:37.640 --> 13:39.520 So let's go back to diode biasing. 13:39.560 --> 13:43.040 Now if you simply apply the Ohm's law V is equal to IR. 13:43.280 --> 13:49.560 Here V will be 3.3 minus the voltage across the diode or the target voltage across the diode will be 13:49.720 --> 13:56.120 0.3685 is equal to I will be ten milliamps and r will be unknown. 13:56.120 --> 14:00.560 So from that we'll get the value of resistance will be two 90 ohm. 14:00.560 --> 14:06.200 And if we just look for that resistance let's say on different distributors website we'll find the nearest 14:06.200 --> 14:08.280 resistance of two 70 ohm. 14:08.320 --> 14:08.640 Right. 14:08.680 --> 14:11.120 So we'll just make it 270 here. 14:11.120 --> 14:14.640 And similarly if you want to calculate the value of inductance. 14:14.640 --> 14:21.050 So the purpose of inductance as I told you it is going to act like a high impedance path at the input 14:21.050 --> 14:21.650 frequency. 14:21.650 --> 14:25.570 So our input frequency is to 4.25GHz. 14:25.650 --> 14:30.290 If you recall that from the design notes then we can simply calculate the impedance is equal to two 14:30.330 --> 14:32.490 pi f into L or omega L. 14:32.530 --> 14:32.970 Right. 14:33.010 --> 14:37.490 So it is coming across 1.8 kilo ohm or something around that. 14:37.570 --> 14:40.650 If the inductance will be 68 nano Henry. 14:40.690 --> 14:41.170 Right. 14:41.210 --> 14:43.530 So I'm going to use 68 nano Henry here. 14:43.610 --> 14:49.450 It could be any value which is sort of making it high impedance path for the input frequencies right. 14:49.490 --> 14:51.370 So here we have all these values. 14:51.410 --> 14:53.890 Next I'm just going to put the value of capacitors. 14:54.090 --> 14:55.410 So let's make it. 14:55.450 --> 14:56.610 It is in Picofarad. 14:56.690 --> 15:00.050 So we are going to add let's say one microfarad capacitor. 15:00.210 --> 15:02.170 So it would be one E6. 15:02.410 --> 15:04.970 And then we'll have 1200 picofarad. 15:05.010 --> 15:07.330 And let's add one more smaller capacitor. 15:07.450 --> 15:08.970 Let's say 100 picofarad. 15:09.010 --> 15:11.170 So these values are not going to impact anything. 15:11.250 --> 15:15.250 These are just to remove the transient noise from the DC source. 15:15.450 --> 15:18.090 So our circuit is pretty much ready. 15:18.210 --> 15:24.340 And next we are going to just quickly run the simulation so, as you already know, to run the simulation 15:24.340 --> 15:27.340 to get the voltage and current, we have to annotate the circuit, right. 15:27.380 --> 15:30.540 To do that, we'll just go to project and go to DC biasing. 15:31.020 --> 15:33.020 From here we'll click on Add Annotation. 15:33.020 --> 15:39.420 So here as you can see we are going to annotate it for DC which is the DC input current for all the 15:39.420 --> 15:40.060 elements. 15:40.100 --> 15:40.540 All right. 15:40.700 --> 15:41.500 Click on apply. 15:41.700 --> 15:44.820 And then all the DC voltage for all the nodes apply. 15:44.820 --> 15:45.180 And okay. 15:45.220 --> 15:47.420 And at last we'll just run the analysis. 15:47.420 --> 15:52.220 And if I zoom in here as you can see, the amount of current which will be flowing through this diode 15:52.220 --> 15:53.940 will be 10.8 milliamps. 15:53.940 --> 15:57.380 And the voltage across the diode will be 0.381V. 15:57.420 --> 16:00.380 Make sure the supply voltage is 3.3V. 16:00.380 --> 16:01.100 So yeah. 16:01.100 --> 16:03.660 So from that our circuit is sort of verified. 16:03.660 --> 16:07.620 And next we are going to add the DC block capacitors. 16:07.620 --> 16:12.340 Because we also do not want any DC current will be flowing in the direction of RF path. 16:12.340 --> 16:17.460 So we have avoided the current flow from this direction to this direction by using this inductor. 16:17.500 --> 16:24.500 Now, to avoid the current flow from this DC path to RF path by adding the DC blocking capacitors so 16:24.500 --> 16:27.260 we can simply place two closed form capacitors. 16:27.420 --> 16:30.140 One will be here and just copy it. 16:30.140 --> 16:31.580 Paste one will be here. 16:31.620 --> 16:36.740 Let's make the circuit and the value of these capacitors will be 100 picofarad. 16:36.740 --> 16:41.060 And that is again based on the frequency we are going to pass from these capacitors. 16:41.260 --> 16:44.700 So it should be low impedance for that frequency. 16:44.820 --> 16:46.540 And for DC it should be high impedance. 16:46.540 --> 16:48.220 And next we are going to add ports. 16:48.260 --> 16:52.300 So let's place one port here and we just copy it rotate. 16:52.340 --> 16:53.380 Place another port here. 16:53.380 --> 16:55.180 So this is our DC biasing circuit. 16:55.340 --> 16:57.100 I'm just going to name these ports. 16:57.100 --> 16:58.340 So this is port one. 16:58.340 --> 16:59.340 Double click on this. 16:59.420 --> 17:00.500 This window will open. 17:00.500 --> 17:03.220 So this will be the input of the DC bias circuit. 17:03.220 --> 17:06.060 And this port will be the output of the DC bias circuit. 17:06.060 --> 17:08.940 So I'll just double click here and this will be out. 17:09.500 --> 17:11.340 Next we are just going to save the project. 17:11.340 --> 17:17.580 So this will be the biasing circuit of the diode which we are going to use it in another schematic as 17:17.580 --> 17:18.940 in sub circuit block. 17:18.940 --> 17:20.220 So that's it for this video. 17:20.300 --> 17:25.140 So next we are going to design the RF isolation circuit and their matching network.