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The amplifier
Many things in an amplifier datasheet describe the performance of the amplifier. Two of these figures are the intercept point, IP3, and the 1dB-compression point. These values are a way of describing how linear the amplifier is.
These two figures are not always found in a pre-amplifier specification but it is only more and more important with good intercept and 1dB-compression values. Ask the manufacturer next time you are buying a new pre-amplifier!
The reasons why we need better values are the increasing amount of radio amateurs but also different commercial radio systems. These commercial radio systems often use digital modes with very high peak-power and sometimes they are very close to our HAM-frequencies.
1dB compression point
The 1dB-compression point is defined as the level where the output power does not follow an increase in input level. We assume that an increase of 1dB input power will increase the output power with 1dB, ie the amplifier is linear.
When this is plotted in a diagram one will see a certain point where the amplifier output no longer follow an increase of input power. When the difference is 1dB from the ideal curve then this is called the 1dB-compression point. In other words, the amplifier is linear up to this point. Normally this is also defined as the end of the “spurious free dynamic range”.
Intercept point
We normally hear about the third order intercept point but there exist of course many other intercept points, as the second, the fifth and so on.
There are at least two ways of measuring the third order intercept point, ie the most used intercept point, and these are briefly showed here. If the amplifier is;
- very broadband, we measure the increase of the third overtone
- more narrowband we use two-tone testing.
The second method is the most usual way of doing this since most amplifiers are narrowband. Even if the amplifier would be very broadband it would be hard to use the first method since amplifiers always are frequency dependent in some way. It would therefore be hard to say at what frequency the measured third order intercept point really is. The important thing about this first method is that it gives you an idea about how this work and it also show that the amplifier is actually a unlinear device.
The products, ie the signals “created”, are usually called intermodulation or intermodulation products. These products are always “created” when several frequencies are present together with an unlinear device.
First method
When measuring the third order intercept point, IP3, of a broadband amplifier we use an adjustable oscillator and a spectrum analyser.
By increasing the output power from the oscillator the increase of the products on the amplifier output is observed on the spectrum analyser and noted.
When the gain curve is dotted together with the third overtone, these lines are enlarged until they cross each other. This is the third order intercept point, IP3.
If you are interested in the IP5 you do the same but use the fifth overtone.
Second method
The second method is the one normally used. Two oscillators and a spectrum analyser are used. The idea is to measure the mixing products from the amplifier caused by the active device.
The two oscillators are added and feed the amplifier. The amplifier output is measured with the spectrum analyser. It is very important that the oscillators are “clean”, ie low phase noise.
The oscillators also must be isolated from each other in some way. Attenuators are often used together with a good quality combiner.
Both oscillators should have the same power output but on different frequencies, let say f1 and f2, (f2 is in this example the higher of the two). In amateur radio applications the difference is often 10 or 20 kHz but one often see frequency differences of up to 1MHz.
By varying the combined signal with an adjustable attenuator one adjust the amplifier input level to a suitable level. (It is not recommended to change the output from the two oscillators, the result will be better with an attenuator and the attenuator will also help to keep the return loss on a low level.)
The amplifier will “create” mixing products at;
f3 = 2*f1 - f2
and
f4 = 2* f2 - f1
By measuring the difference in power between the original frequencies, f1 and f2, and the “created” frequencies, f3 and f4, the third order intercept point can be calculated.
The formula for calculating the third order intercept point is;
OIP3 = Pout + Pdiff / 2
Example
If the output power from each of the oscillators are –10dBm and the power difference between f1 and f3, or between f2 and f4 is 40dB the IP3 is then 10dBm,
(–10+40/2=+10dBm).
The power difference between f1 and f3, or between f2 and f4, should normally be the same. If not, one usually use to large frequency distance between f1 and f2.
Calculation of cascaded amplifiers
As with noise-factors one can calculate the resulting intercept point when cascading amplifiers. The formula;
1/IPtot = 1/IP31 + Gain1/IP32 + Gain1*Gain2/IP3+…
(HTML-code is simply not made for mathematical expressions…)
Intermodulation in connectors
We normally doesn’t talk about intercept points or intermodulation in coaxial-connectors, but sometimes these components show results of intermodulation. It’s especially important if you transmit with high power levels and have nearby DX-stations with good receivers!
The phenomena were “first” observed by mobile phone-operators using the same antenna for both transmitting and receiving – and also at the same time. The mobile-operators only use 10-100W but since the signal isn’t attenuated “in the air”, ie not even transmitted before it’s on it’s way down to the receiver, it will be very strong.
This could be a problem since some DX-stations use very high powerlevels and there could be nearby DX-stations with very sensitive receivers.
Important things to look out for are;
- Corrosion
- Low contact pressure
- Less than 360-degree contact
- Poor surface finish
- Dust within the connector
This also shows that one must never use crimp-connectors for high power levels! They will not give 360-degree contact. Clean all coaxial-connectors when mounted and use only waterproof types, ie not PL-connectors. 7/16 or N-connectors will be a much better choice if available.
Corrosion is probably an important factor when this intermodulation is created. When the metal corrode small crystals are created. Compare this with old crystal-radios! (During the WW2 war-prisoners made crystal-radios by pressing a needle against a razorblade, in those days the text was not printed it was etched! Etching is a sort of corrosion and small crystals are created. Together with some other components the prisoners were able to receive distant broadcasting transmissions.)
Conclusion
What we find out from the formula is that the first stage must be able to handle much power, ie having a large IP3-value. It is also important to balance the gain in the system since higher gain will decrease the total intercept point. It is very much as with noise factors, the first stage is very important!
When comparing intercept point of amplifiers, it is important to ensure that the measurements have been made with the same difference between f1 and f2. It is much easier to get a high value with a larger difference! This is unfortunately used by some manufacturers!
It is also very important to point out that this does not only concern amplifiers. All components used in a radio system have an intercept point. When calculating the resulting total IP3 one must include mixers and other devices as well.
When it finally comes to the coaxial-connectors, talk with your DX-neighbours! You will not be able to measure any intermodulation if you are using a coupler on the feeder – you must measure this intermodulation with a separate antenna! (The intermodulation could be created at the last connector, far from the coupler.)
Try to only use high-quality connectors, at least in the transmit path. If you think they are too expensive try to look after used connectors at a flea market. Do not use or buy connectors if they are in bad condition!
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