The IBO is the power of the input signal normalized to the maximum input power. So 0 dB IBO means you are at maximum input power of the amplifier. Let’s say that your amplifier can handle a maximum of 1 watt input power, and can produce 10 watt output power. The IBO at this point is 0 dB, and so is the OBO, which is normalized to 10 watts. Since these are normalized to their maximum values, you can not determine the absolute gain of the amplifier from the IBO, OBO values. Now as you reduce the power, the IBO will decrease. 3 dB IBO means you have input half a watt. The OBO will also go down, depending on your IBO/OBO relationship, which is different for every amplifier. Before any kind of analysis of an amplifier, you need to know this relationship, which is sometimes given in IBO/OBO and sometimes in actual power out such as dBms. To get IBO/OBO from dBms. read my tutorial. IBI/OBO are usually referred to as positive numbers because they are backoffs from the maximum number. There is no such thing as a Saleh amplifier. This is a way of characterizing the IBO/OBO relationship by way of curve fitting. So to measure your IBO, just measure the power out of the modulator and divide by maximum power. For an OFDM signal this is just the amplitude squared. 1111111111111111111111111111111111111111111111 Introduction To define the performance of a receiver or transmitter, various specifications are recorded which are obtained from measurements carried out. Perhaps the least understood of these in amateur radio circles is intermodulation performance and how it is measured. The aim of this article is first to discuss intermodulation
products and how they are produced and then look at how they are defined and measured. What are Intermodulation Products? When a single frequency (f1) is fed through a device whose output is not a linear function of its input, harmonics of f1 are generated, i.e. 2f1, 3f1, 4f1, 5f1, etc (no device is perfect and so harmonics are always generated even at low levels). Now, if two separate frequencies exist together in a nonlinear device, sum and difference frequencies are also produced in addition to the harmonics. This can be shown mathematically to be the result of a multiplication process between the two original frequencies and hence the two new frequencies are called products. If the two original frequencies are f1, f2 and the highest frequency is f2, then we can expect two other components (or products) of (f1+f2) and (f2-f1). However, it doesn't stop there. Since there are harmonics of f1 and f2, then there will be sum and difference products between all of the harmonics and the fundamentals and between each other. These are the intermodulation products which are frequency components distinct from the harmonic components discussed in the previous paragraph. Of course, if there are more than two fundamental frequencies, then the multitude of products is compounded further. It can be shown, using a mathematical series, that when harmonics are generated, the harmonies extend upward in frequency to approach infinity, progressively decreasing in amplitude as the frequency increases. Likewise, the intermodulation products could also be considered to be infinite in number. However, we are only really interested in those of practical significance, that is of such a level that they might deteriorate the quality of our signal beyond an acceptable level. To examine intermodulation products we will consider two frequencies f1 and f2 and some of the orders of intermodulation products. To define the order, we add the harmonic multiplying constants of the two frequencies producing the intermodulation product. For example,
(f1+f2) is second order, (2f1-f2) is third order, (3f1-2f) is fifth order, & etc. Let's consider f1 and f2 to be two frequencies of 100 kHz and 101 kHz respectively, that is 1 kHz apart. We now prepare Table 1 showing some of the intermodulation products.
Looking carefully at the table, we see that only the odd order intermodulation products are close to the two fundamental frequencies f1 and f2. One third order product (2f1-f2) is 1 kHz lower in frequency than f1 and another (2f2-f1) I) is 1 kHz above f2. One fifth order product (3f1-2f2) is 2 kHz below f1 and another (3f2-2f1) is 2 kHz above f2. In fact it is the odd order products which are closest to the fundamental frequencies f1 and f2. Let's expand further the odd order products as shown in Table 2. The series of odd order products can be seen to descend and ascend progressively in increments of 1 kHz from the two fundamental frequencies f1 and f2 respectively. A typical spectrum produced could be depicted as shown in the chart of Figure 1.
Figure 1 - Spectrum of Inter modulation Components Of all the harmonics and inter modulation components produced, are often only interested in those which fall in. the pass band of our equipment and, in the case of the inter modulation components, those which happen to be closest to our fundamental frequencies. The third order components are the closest and also usually the highest in amplitude. Because of this, they are usually the products of most concern and are those which are commonly measured and defined in transmitter and receiver performance specifications. Effects of lnter modulation Components The existence of inter modulation components affects the performance of equipment in various ways. First let's look at audio amplifiers. The presence of any component at the output of an amplifier, but not fed into it, degrades the quality of the signal being amplified. We call this distortion, which can be the result of non-linearity in the amplifier causing the generation of harmonics of the signal frequencies and, in turn, inter modulation components. So we have harmonic distortion and inter modulation distortion which can individually be defined. We often see inter modulation distortion abbreviated to IMD. Some different effects can be experienced when nonlinearity exists at RF in a transmitter and, in particular, in the final linear amplifier of the transmitter. Consider the amplifier delivering sideband components to the antenna at radio frequencies and because of nonlinearity, harmonics of the various sideband components are
generated plus various inter modulation, components. The harmonic components and the even order inter modulation components will be well spaced away from the operating frequency and hopefully attenuated by the tuned amplifier tank circuit and the antenna tuning system. Not so for the odd-order inter modulation components which are closely spaced around the fundamental components from which they were generated. First of all they will show up as audio distortion after being received and detected by the radio receiver. However, that's not all! We have seen from the previous paragraphs that the odd order components spread out either side of the fundamental components in progression gradually decreasing in amplitude. The effect is to broaden the radiated signal and in receiving the signal, we experience the familiar sideband splatter. As most of us well know, this causes interference to others trying to use another channel near in frequency. Another application where those odd order, inter modulation components are of considerable concern is in the first Mixer stage of a super heterodyne receiver. The special function of the mixer stage is to produce some form of non-linearity so that an intermediate lower frequency is formed from the sum or difference between the incoming RF signal frequency and a local oscillator frequency. The mixer stage is, therefore, a prime spot for other inter modulation products which we might not want. Let's look at an example. Our receiver is tuned to a signal on 1000 kHz but there are also two strong signals, f1 on 1020 kHz and f2 on 1040 kHz. The nearest of these (f1) is 20 kHz away and our sharp intermediate frequency (IF) stage filter of 2.5 kHz bandwidth is quite capable of rejecting this signal. However, the RF stages before the mixer are not so selective and the two signals f1 and f2 are seen at the mixer input, free to produce inter modulation components at will. Now work out the third order inter modulation component (2f1-f2) and we get (2x 1020-1040) = 1000 kHz, right on our signal frequency. This is just one example of how inter modulation
components or out-of-band signals can cause interference within the working band. Another form of interference in receivers which results from the mixing of inter modulation components is Cross Modulation. This becomes more apparent when dealing with AM signals and the modulation on a strong out-of band signal transfers itself across to modulate the signal being received. The process is probably complex but, due to non linearity in the receiver, one can well imagine the carrier and sideband frequencies of the out-of-band modulated signal mixing to produce difference secondorder components at the audio frequencies of modulation. Due to the same non-linearity, the unwanted audio components inter modulate the signal being received. If the receiver is designed for good inter modulation immunity, it will also have good cross modulation immunity. 1111111111111111111111111111111111111111111111 The SFD given usually refers to the Flux Density needed for full transponder saturation. With an IBO 4 dB, you will have a flux density that is 4 dB lower so -88.5 dBW/m² arriving at the spacecraft. You are not fully saturating the transponder since it obviously operates in multi-carrier mode (hence also the values for IBO/OBO). You are using 10*log(9/36) = - 6 dB of the bandwidth (hence the power) so the operational flux density of your carrier will be 6 dB lower of -94.5 dBW/m² it's calculation is comming arround 20*log ( 9/36 ). Please Confirm. Wheathere it is 10*log ( 9/36 ) or 20*log ( 9/36). There is an error in the code of that site. Where it says ofd=1.0*form.satpfd.value1.0*form.satibo.value+10.0*Math.log(form.upbw.value/for m.tbw.value); there should be /Math.log(10.0) added. If this is not done the log in base e (ln) is calculated rather
than the log in based 10. I did already write an email to the author in order to have this corrected. The required OBO is usually determined by the satellite operator. It depends on the operating mode of the transponder. As a rule-of-thumb, you can take 6 dB as OBO in case of multi-carrier operation. But to be sure, always check with the satellite operator. I will reuse part of a previous answer (discussing saturation fluz density and downlink EIRP), as i believe covers your question the saturation flux density is a measure of the power density required at the input of the satellite (right before the receiving antenna) in order to deliver the maximum EIRP from the amplifier in the output, considering the backoffs set by the satellite operator. it can be specified independent of the position where you are in the coverage area, as it is transponder specific. in some cases it is given as a range as in here: http://www.eutelsat.com/satellites/pdf/Data_sheets/ab2_d atasheet.pdf where it is made to depend on the G/T of the satellite in the direction of your station. Some other operators actually provide contours of the SFD. SFD is used as a reference point to determine the input backoff, as the difference between the saturation density and the operational flux density (that achieved by the carrier you are activating and which depends on the EIRP of the earth station) is the input backoff. If you know the input backoff, there are two ways to determine the output backoff : either the operator tells you
the operational difference between input and output backoff (delta) so that OBO = IBO - DELTA, or they give you a functional form to approximate OBO based on ibo, such as OBO = 0.9IBO + 4 or something like that. Once you know OBO, then the EIRP per carrier of the downlink is EIRP at saturation - OBO. EIRP DL = EIRP sat - OBO You need the EIRP of the earth station to determine the operational flux density and therefore the determine IBO and then OBO and then downkink satellite EIRP. Now, C/No of the uplink is calculated based on the EIRP of the earth station. To estimate C/No you can use the SFD as a reference point as follows (from maral's VSAT networks book)
reference C/No, uplink at saturation C/Nou,sat = SFD - Gref antenna 1m2 - Losses + G/T of the satellite in your direction + 228.6 (boltzmann constant) The gain of a reference 1m2 antenna is the tool used to convert from a flux density to effective power captured per m2, G1m2 = 10Log10(4PI) + 20LOG(F/c) continuing: at this moment we have the C/No that would be achieved if i had saturated the satellite, but this is not what we want. we want the C/N produced by your carrier, at an EIRP much less in the typical case than the required to saturate. OFD, the operational flux density of your carrier, is estimated based on the EIRP as follows
OFD = EIRP + G1m2 - Losses since you now know OFD, you can calculate the input backoff: IBO = ABS ( SFD - OFD ) and then C/No,UL = C/No, UL, saturation - IBO note that i force IBO to be positive. it can be negative or positive depending on how the parameters are defined and specified by the operator. normally you will have an SFD less negative than the OFD so it will be negative. but just remember the concept, IBO is the difference in power from your point and the saturation point. since it represents how much less power you have, i subtract it from the saturation C/N to take me to my operating point. of course there are other ways to calculate C/No uplink, without considering SFD, just change SFD from the first equation to OFD and that would be it. in a simple approach, C/No,u= EIRP + G/T - Losses - Boltzmann's K But in any case you need the SFD and the OFD to estimate IBO and then link the IBO to the OBO to find the downlink EIRP. Without that, you wouldn't be able to have the uplink and the downlink tied up, unless you knew the OBO already, so you can calculate the downlink based on the saturation EIRP from the satellite, and subtract the OBO. 1111111111111111111111111111111111111111111111
This is a question that is too long to answer here. Basically you want to have a look around on the internet and study linkbudget analysis. I'll give you a short summary that you can use for further study: 1) Downlink C/N is the result of the EIRP for that carrier on the satellite. Then the path loss and G/T of the receive antenna is taken into account to calculate the downlink C/N 2) Link margin is the difference between the received C/N and the C/N that is needed minimally to receive and decode the carrier. The link margin is needed to achieve a certain availability of the carrier. This because, when it rains, there will be fading and the overall C/N will drop. The needed linkmargin will be bigger if you want a higher availability and will also increase if higher frequencies are used (for example Ku-band compared to C-band) 3) PDF (power flux density), SFD (saturate flux density) and OFD (operational flux density) are all power densities (expressed in dBW/m²). The SFD is the value that is needed at the input of the satellite in order to fully saturate the transponder. The OFD is the flux density that you carrier achieves at the input of the transponder. When the carrier has a certain input back-off and when it doesn't used the full transponder, it will use an OFD that is lower than the SFD. On this forum, you can find some good explanation about this subject done by Luis. 4) Antenna noise temperature is not calculated, it is a parameter that is given by the antenna manufacturer. 5) C/I is a measure for the interference in the linkbudget. It takes into account the effect of intermodulation, adjacent carrier and satellite interference, cross-polar interference. Basically it is just another C/N value that needs to be taken into account to calculate the overall C/N 6) The composite C/N is the result of the uplink C/N, downlink C/N (which are calculated independently) and the C/I. All the contribute to the final C/N that is achievable in the receiver. You could think of all these C/N values as
resistors that are connected in parallel. The lowest value will dominate the result. IF you have a look around on this forum, there is some practical example around. Bottom-line, you will need to study a bit in order to grasp all the details around link budgets. 1111111111111111111111111111111111111111111111 Cross polarisation can refer to the interference that is the result of the "leakage" of signals from carriers on one polarization into the other. Satellites use frequency re-uses where a carrier can be transmitted onto the same frequency as another but is the opposite polarization. Under ideal conditions the isolation between the two polarizations is more than 30 dB so the effect of one carrier onto the other is negligible. This is why a feed in the antenna that receives/transmits into vertical and horizontal polarisation needs to be properly alligned by turning the feed until the point where the maximum of the wanted polarisation is received and as a result the minimum of the opposite pol. The best way to do this is to look at a beacon signal and turning the feed until the beacon is lowest level in the opposite pol. Also during line-up, the satellite operator will ask you to transmit a pure carrier (CW), this is done to check whether your uplink polarisation is correctly aligned so you don't cause any interference in the opposite pol on the transponder. Even when properly aligned, the satellite transponder and your antenna exhibit a certain amount of cross-polar. You can find this back on the datasheets. Another effect that exists is cross polar depolorisation when it rains, this will cause some of the energy to leak into the other polarisation, effectively increasing the noise in your wanted signal.
