SM 5 BSZ - Inter Modulation in Receivers

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SM 5 BSZ - Intermodulation in receivers.Dec 30 2004)

The third order intercept point. (IP3)

Background

This is the definition: The third order intercept point (IP3) is the point at which the the extrapolated third order intermodulatiolevel (IM3) is equal to the signal levels in the output of a two-tone test when the extrapolation is made from a point below whic

the third order intermodulation follows the third order law. IP3 may be given as the input level or as the output level for that poand which one has to be specified. One uses the terms input intercept point IIP3 and output intercept point OIP3.

The third order law says that the intermodulation product grows with the input amplitude raised to a power of three. Theintermodulation will grow by 3 dB for a single dB increase of the input.

IP3 is a figure of merit that characterizes a receiver's tolerance to several signals that are present simultaneously outside the desirpassband. IP3 is a power level, typically given in dBm, and it is closely related to the 1 dB compression point. Both IP3 and the 1compression point are meaningful only when given in relation to the noise floor. Adding a 10 dB attenuator to a receiver will

improve both IP3 and the 1 dB compression point by 10 dB, but it will also degrade the system noise figure by 10 dB so the 10 dattenuator is not improving the dynamic range, it just shifts the power range upwards.

The third order intercept point, IP3, although a number that is obtained from an extrapolation, has a real physical meaning and it well defined. The fact that different standardized procedures give different results may be because not all of these proceduresmeasure IP3 correctly. They may be based on incorrect assumptions or some theoretical mistake. There may also be overtones oftest signals present. This article Receiver Dynamic Range which has appeared in DUBUS and CQ VHF discusses some practicalaspects of IM3 measurements.

Another thing is that IP3 and the theory behind it is not applicable on all receivers. In such cases the number that comes out of astandardized procedure will not be IP3, it will just be the outcome of a specific procedure. The most obvious case is a DSP radiocontaining an A/D converter in the signal path. If both test signals reach the A/D converter, IM3 will not have any tendency at allfollow a third order law at low signal levels. More here: IMD in Digital Receivers QEX Nov/Dec 2006, pp 18 - 22. See also Dynarange observations for the SDR-14

Analog receivers may have side circuits such as noise blankers that contain amplifiers, detectors or other circuits that produceintermodulation at signal levels where the main signal path is very linear. Such intermodulation may leak into the main signal pathdue to inadequate screening or buffering causing irregular behaviour at low signal levels, thus making IP3 and the associated theoinvalid.

A perfectly linear amplifier will have no intermodulation at all, but at some point it must saturate. The third order intermodulationassociated with abrupt saturation does not follow the third order law, something that can be seen on the plot of the Delta44 below

IP3 is a figure of merit that is useful to characterize the linearity of a receiver for signals well below saturation. IP3 is typically afunction of the frequency separation. Stages that are preceeded by filters that remove one (or both) of the test signals will notcontribute to IM3 in a two tone test, but as the frequency separation is made narrower, both test tones will pass through more of filters and cause IM3 further into the receiver, usually with a much lower IP3 than the one associated with the front-end.

In some cases IM3 at very low signal levels has been reported to be orders of magnitude stronger than expected. For example thereview of TS-450S and TS-690S in QST April 1992 shows IM3 at the noise floor for an input signal of -64 dBm while the third orlaw gives the expected IM3 level about 60 dB lower. If the phenomenon is real, the reason for the very strong intermodulation at signal levels is most probably some side-circuit. Something non-linear that receives an amplified signal and produces intermodulathat leaks into the main signal path due to inadequate isolation. I have measured the IM3 produced in a TS-690S at the RS-03meeting and did not see any trace of low level IM3. The serial number was much later compared to the 1992 ARRL Lab test,Kenwood may have corrected whatever problem that was present in early TS-690S units - or maybe there was a problem with theARRL measurements.

Excessive IM3 at low levels has been reported implicitly in several tests from the ARRL lab as shown in table 1.

BSZ - Intermodulation in receivers. http://www.sm5bsz.com/dynrange/inte

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Unit MDS IM3DR IP3 IP3 IP3

publ. calc. diff. QST

(dBm) (dB) (dBm) (dBm) (dB)

========= ==== ===== ===== ===== ===== =======

FT-857 -132 87 +4.1 -1.5 +5.6 Aug 03

FT-900 -129 98 +22 +18 +4.0 Feb 95

FT-1000/5 -127 101 +25.7 +24.5 +1.2 Nov 00

HF-1000 -133 97 +30 +12.5 +17.5 Dec 94

IC-736 -139 95 +9.7 +3.5 +6.2 April 95

IC-738 -139 94 +8.1 +2 +6.1 April 95

IC-756PII -131 97 +20.25 +14.5 +5.76 Feb 02

Pegasus -132 77 +7.2 -16.5 +23.7 Feb 00

R8 -135 90 +6 0 +6.0 March 92

SG-2020 -130 88 +15.5 +2 +13.5 Oct 98

TS-2000 -129 94 +19 +12 +7 July 01

Table 1 Deviations from the third order law as calculated from ARRL equipment reviews by the formula IP3=1.5*IM3DR+MD

It has to be understood that ARRL Lab has its own definition of IP3: "IP3 is the result of a third order extrapolation of the IM3response at the S5 level"This procedure surely gives an accurate value for IP3 for all receivers that really follow a third order lawbut back in 1992 when the TS-690S was measured, ARRL Lab used another definition of IP3. The TS-690S IM3 value at the noi

floor was extrapolated with a slope of 3 leading to a published IP3 value of -33 dBm. With today's procedure, extrapolating fromS5 level, one gets a line that crosses the signal at about +4 dBm from the very same measurement. The difference between the oldand the new procedure is 37 dB!! None of the methods is acceptable. A receiver that has a third order response like TS-690S in thARRL Lab test does not have any IP3 at all. IP3 must not be redefined to be the outcome of a procedure.

IP3 is a well defined quantity that has a good theoretical model behind it. Some receivers may contain circuits that make the modincorrect and in such cases one can not use a concept from the model.

The TS-450S/TS-690S units measured at the ARRL Lab probably have an IP3 of +5 dBm for the front-end as determined from FA in QST April 1992 page 70. A measurement at 100 kHz separation would probably have shown a normal third order responsesince the two tones would probably not have reached the circuits responsible for the irregular behaviour in case the observation wreal and not an artifact.

There is an article in QEX Jul/Aug 2002 p. 50, in which Ed Hare, the ARRL Lab Supervisor argues that there is not a true intercepoint for a receiver because the IM3 curves do not follow the third order law. This statement is fine, but the three different responcurves in this very article are however not good examples. The curves giving "IP3 from MDS" are not drawn correctly. The respocurves show the reading of an RMS voltmeter and they are curves showing S+N. The position of the noise floor is known and thecorrect point to draw the lines from is a point 3 dB below the point where the S+N response is 3 dB above the noise floor. At thispoint S and N are equal, both S and N are 3 dB below the sum of both of them. The line drawn from the correct place at the MDSlevel coincides exactly with the line "IP3 from Best Fit in Linear Output Range" in Fig C and the answer to the question at the sidof fig. C: "What is the 'real' intercept point of this icom IC-765?"isIP3 = +9.5 dBm. The third line in fig C is not compatible withe definition of IP3. This non-linearity may be caused by loss of gain in which case it would disappear if a third signal were usedkeep track of receiver gain variations as shown in fig 11 below.

Table 1 and the published result for the TS-450S indicate that IP3 is not a useful figure of merit for some receivers because there no point below which IM3 follows the third order law. For receivers having large discrepancies, IP3 should not be given at all, onhas to specify some signal levels and the corresponding IM3 levels. Best, of course with a diagram as was done for the TS-690S, for the receivers in table 1, giving IM3 below the signal level for a test signal pair X dB above the noise floor would be a better wto present the results.

It is my personal belief that any analog radio that gives much more IM3 at low levels than one would expect from the third or

law suffers from an easily correctable design error. Table 1 may be an example of such receivers.

I have tried to find a receiver that has high IM3 levels when the measurement is made at the noise floor because I would like toinvestigate the mechanism. I have not yet found a single one. Measurements at the noise floor are difficult, the phenomenon coulbe an artifact. I will continue searching for it by bringing instruments to ham meetings. Details of performance measurements aregiven here.


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Theory

Let us start by looking at the intermodulation generated in a single building block of a receiver. It could be an amplifier, a mixer, ferrite core or something else. A single stage that causes intermodulation can be described by a transfer function that is very closea straight line for voltages well below saturation. If the input is denoted X(t), a voltage that varies with time, the output Y(t+d) cabe described with a power-series expansion in amplitudes only:

Y(t+d) = k1X(t)+ k2{X(t)}2 + k3{X(t)}

3 + ......... (1)

where k1, k2, k3 etc. are constant coefficients and d is the time delay between input and output (not important here). Such adescription is valid if the input signal is small. At larger signal levels, there will also be a phase shift because semiconductors conta capacitance that varies with the voltage in a non-linear fashion. Any reasonable analog circuitry will have k1 > k2 > k3 > so ithe signal levels are really low, the transfer function is well described by the first linear term only. By making X(t)=A*{sin(2*PI*f1) + sin(2*PI*f2) } one can easily find what the output signal Y(t+d) becomes in a two-tone test on a unit that isdescribed by the polynomial expansion (1). X(t) is two sine waves of equal amplitude A and with frequencies f1 and f2. Look herfor the mathematical details The result for the third order intermodulation amplitudes is:

IM3=0.5*k3*A3+ 1.5*k5*A

5 + 3*k7A7 + ......... (2)

Here k3 is in the order of 0.06 if the amplitude A is expressed in units of the voltage at the 1 dB compression point.

When two analog building blocks that both are well described by (1) are cascaded, the transfer function from the input of the firsone to the output of the second can also be written in the form of (1). look here for the details

The wording "well described"here means that the polynomial expansion must have a fast convergency which means that|k1| > |k3| > |k5| > ......... (3)

Almost anything used in the main signal path of a receiver will fullfil this requirement. It is however not difficult to find examplefunctional blocks that do not fullfil this requirement. A class B amplifier in which two complimentary transistors are used to suppcurrent from plus or minus to the output is often used as a power amplifier at audio frequencies. Such an amplifier suffers fromcross-over distortion which means that the transfer function has a small discontinuity near zero. Such a function is not at all welldescribed by a polynomial expansion. Intermodulation caused by cross-over distortion in audio amplifiers does not follow thethird order law at all except for voltages well below the non-linearity which may be very far below the maximum output level. (Asuch low signal levels the transistors run in class A)

Another example is an amplifier that starts oscillating at microwave frequencies for voltages above or below certain levels of outpvoltage. The onset of the oscillations gives an abrupt change of the output voltage, a discontinuity which absolutely not can bedescribed by a polynomial expansion.

A/D converters have very good linearity but a polynomial expansion is not suitable at all to describe the non-linearities.

IM3 in the time domain

At high levels, intermodulation can be seen in the time domain. That is what we see on an ordinary oscilloscope. A typical two-totest signal is shown in figure 1.


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Fig. 1. A pure two-tone signal as seen on an oscilloscope.

Figure 2 shows the output from a simple wideband amplifier that is fed with the two-tone signal of figure 1 at a too large amplituIt is obvious from figure 2 that this amplifier is heavily saturated. The dominating effects of the nonlinearity is the second orderterms, the second harmonic and the sum and the difference frequencies. The difference frequency, 20 kHz, an audio tone, has ab25 % of the peak amplitude of the composite RF signal. The sum frequency and the second harmonics of the two test tones areclosely spaced and they form a waveform that is flattened upwards but sharp downwards during times of large amplitudes as can seen on fig. 3.

Fig. 2. The two-tone signal of fig.1 after passing a wideband amplifier that is not designed to work with strong signals like this


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Fig. 3. Exactly the same signal as in f igure 2, but here the oscilloscope speed is much higher.

The amplifier used to produce these images is a simple wideband amplifier with a BFR91A and a resistor to V+ and a (large)capacitor to the output connector. It is obvious from figure 3 that there is no current through the transistor during a substantialfraction of the time of one period of the input signal at large amplitudes.

When looking at the spectrum analyzer, one can see large amplitudes at overtone frequencies when a wide frequency range isselected. Figure 4 shows the fundamental and some overtones without resolving the individual components.

Fig. 4. A wideband spectrum of the signal of f igure 2. The second harmonics and the sum frequency contain about 6 dB less po

than the two tones together.

It is obvious that the transfer function responsible for the waveforms shown in figures 2 and 3 can not be represented with a

polynomial expansion (1) containing only a small number of terms. When looking at the close range spectrum, figure 5, one canindeed see that fifth, seventh and higher order intermodulation terms are present with substantial amplitudes.


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Fig. 5. The narrowband frequency response of the distorted signal.

The time domain curve shape that is produced by all the components visible in figure 5 is shown in figure 6. Here a 14 MHzbandpass filter is inserted between the amplifier undet test and the oscilloscope. This is what remains from the signal shown in fig2 when a 14 MHz bandpass filter has removed everything except the close range intermodulation products.

Fig. 6. The narrowband distorted signal. This is the time domain view of the spectrum shown in f igure 5.


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Place a ruler on the envelope of figures 2 and 6. You will see that there is a clearly visible difference between the envelopes. Thisdifference can be seen more easily on figure 6. Here the waveforms of figure 2 and of figure 5 are placed on top of each other ana third track on the oscilloscope the difference between the two waveforms is shown. This difference is the sum of theintermodulation products. Adding this waveform to the distorted waveform will restore the original waveform. The intermodulaproducts form the narrowband signal that is required to restore the envelope of the original signal We can understandintermodulation as the effect of amplifier gain varying with the signal amplitude. Figures 2 to 7 illustrate the theory given above. waveforms are exactly what one would expect from a text-book transistor transfer function. Amplifiers designed for high dynamirange use feedback. It is advantageous to make the transfer function more symmetric than in the simple amplifier shown here. Byusing more current one can get saturation in both directions causing the waveform corresponding to figure 3 to go towards asquare-wave at large amplitudes. The power dissipation will grow and one has to choose suitable devices.

Fig. 7. Distorted and undistorted waveforms and the difference between them. Here one can see intermodulation in the time

domain. The amplitude is about 10% (-20dB) as one would conclude from figure 5.

The example and the theory given above illustrates the most important aspects of intermodulation in receivers. It is possible to ma wideband amplifier such as the one used above produce a significantly different level for the third order intermodulation producon each side of the signal pair. The phenomenon can be produced at low levels of intermodulation and the reason is probably thataverage voltage on the collector varies with the difference frequency (20 kHz) and that the amplifier therefore is phase modulatewith this voltage. The output capacitance varies with the voltage. Amplifiers that are designed for good dynamic range do typicalnot show deviations from the simple theory above and receivers typically have very similar levels for the IM3 responses on bothsides.

IM3 in A/D converters

Two signals, spaced by 8 kHz were fed to the RXHF unit. The tones and the third order intermodulation levels were recorded andplotted in fig. 8. With a measurement bandwidth of 5 Hz the noise floor is about 138 dB below saturation which is 132 dB belowdBm where the two-tone test saturates.


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The IM3 levels are accurate down to -147 dBm which is 3dB above the noise floor.

IP3 is something like +20 dBm. The IM3 levels close to saturation give this result when extrapolated. The +20dBm intercept poindue to the analog circuits. If the Delta44 gain is increased and the analog levels reduced to place the input level just belowsaturation, the intermodulation level is reduced by 12 dB which indicates that the IM3 is produced inside the Delta44, in its analofront end. Note that a 2 dB change, from 1 dB below saturation to 1 dB above saturation causes the third order intermodulationproduct to increase by 61 dB.IP3 is not a good figure of merit for a digital receiver.

The intermodulation distortion produced by the non-linearities of the A/D conversion process has a level of about -140 dBm. Itvaries at random with the input signal level. The pattern changes when the frequencies are changed. A third signal -20 dB below test tones is enough to wipe out the third order intermodulation of the A/D converter. The two-tone waveform will no longer repeits pattern exactly along the transfer function. As a result the little jumps in the output waveform will have a time jitter that will sthe third order intermodulation into several much weaker signals that can not be detected.

Fig.8. Third order intermodulation within the passband of a 14 MHz WSE/Linrad system.See text.

Precision measurements of IM3

When measuring IM3, the combined uncertainty of the generator levels, measurement errors and unknown, possibly varyingmismatch losses makes it impossible to detect modest deviations from the third order law. Very large deviations have been reportby the ARRL lab as discussed above but I have measured IM3 of many receivers without seeing any trace at all of irregularbehaviour with a "normal" IM3 measurement setup.

To shed some light on the limitations of IP3 as a figure of merit for radio receivers, I have made measurements with the highestpossible precision using standard instruments and a WSE converter chain feeding a Delta44 A/D converter in a computer running


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Linrad.

Generator output levels

Three HP8657A signal generators,HP8657A(1) with SER=3250A05484HP8657A(2) with SER=3346A06286HP8657A(3) with SER=3346A06807

were used for measurements. A first problem is that the output impedance of the generators changes when the output level ischanged. SWR is about 1.5 at high output levels, -3 dBm and above but it is lower although not constant at lower output levels.

A load that is matched to the generator will receive about 4% more power than a 50 ohm load. This corresponds to a measuremenerror of 0.17 dB. This may seem satisfactory, but in case the test object has a high SWR the accuracy problems become severe. Aan SWR of 3, the transferred power may be anywhere between +0.83 and -0.91 dB with respect to the power that would bedelivered from a 50 ohm source.

To calibrate the generators, the levels in 2 dB steps were measured on a HP8591A spectrum analyzer. I do not have any data sheefor it, but presumably the accuracy is specified to be within +/- 1 dB. The input impedance of the spectrum analyzer is very close50 ohms. A HP8712C network analyzer gives SWR as 1.034. The input impedance of the WSE RXHFA converter that convertsfrom 14 MHz to 70 MHz has SWR in the order of 2.0 (it varies with the frequency). A matchbox was added to the RXHFA unit tmake the input impedance of the RXHFA unit exactly 50 ohms (SWR < 1.02) Linrad S-meter readings were recorded for the samlevel settings as those recorded with the HP spectrum analyzer. The Delta44 was run in maximum gain mode which means that Aconverter saturation is at about -25 dBm. Recordings were made with and without a 40 dB attenuator at the output of the signalgenerators. This 40 dB attenuator was verified to have a SWR below 1.02 in both directions. This calibration table shows raw datand the output power of the generators Sept 18 2003. Room temperature was 23 degrees and all the equipment had been runningmore than 8 hours. The file was produced from raw data by use of this little fortran program, gencal.f

The IP3 numbers given below are given with decimals but the absiolute level is unknown within about 1 dB. The decimals reflectprecision by which the third order law is established and the unknown error of the zero point of the calibrated power scale has to added.

IM3 in the WSE converters

The raw data for a two-tone test at 100 kHz separation was used with calim.fand the output from gencal.fto produce calibrated data for the WSE converter chain at 14 MHz For details, see under IC706MKIIG below. The data is plotted in fig. 9 in the usualway, signal and IM3 levels as functions of the input level. Since the levels of the two tones differ slightly, the signal power is takebe the weighted average. The IM3 levels should be proportional to (2*P near+ Pfar)/3 which is listed in the file in decibels. Theweighted average power level is thus that number divided by three.


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Fig.9. Third order intermodulation of a 14 MHz WSE/Linrad system at a frequency separation of 100 kHz.

The difference between the expected third order intermodulation levels for an IP3 of 0 dBm, 2*P near+Pfarin dBm and the observIM3 levels are plotted in fig. 10. This is exactly the same data as in fig. 9 but plotted in a way that makes the deviations from thethird order law more visible.

Fig.10. Third order intermodulation of a 14 MHz WSE/Linrad system at a frequency separation of 100 kHz. This is the data fro


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fig. 9 plotted in a different way.

The near perfect third order behaviour that can be seen in figure 9 is actually not within +/- 0.5 dB at levels above -4 dBm. (Notethat the x-axis of fig. 10 is nominal levels) At - 4 dBm input, the IM3 products are about 60 dB below the two test tones. The firsdata point at a test tone level of -29.74 dBm, has an IM3 level of -139.35 dBm in the table. This is false accuracy, I noted the levewith 1 decimal in the raw data, but the uncertainty is several tenths of a dB because the noise floor is at 157 dBm/Hz so themeasurement is severely affected by noise. The large deviations of the two first data points can be attributed to the uncertainty

caused by the noise.

It is not surprising at all that the deviations from the third order law starts to become significant when the IM3 level is 60 dB belothe test signal or stronger. It is "common knowledge that the 1 dB compression point is something like 10 to 15 dB below IP3. Fothe WSE converter chain, the measured 1 dB compression point is at +15 dBm.

The point where the deviation from the third order law start to become significant, -4 dBm is 19 dB below the 1 dB saturation levwhich means that the peak amplitude formed by the two test signals is only 13 dB below the 1 dB saturation point. This means ththe peak voltage spans 22 % of the transfer function up to the 1 dB compression point and that higher order terms come into playabove -4 dBm.

The third order intermodulation in the WSE converter chain has contributions from many different stages. They are all welldescribed by equation (2) and therefore the entire converter chain is well described by (2).

IM3 in IC706MKIIG

The IM3 measurements were all performed with the setup showm in figure 11.

Fig.11. Setup for IM3 measurements. Generator (1) is set to the notch frequency 14.15981 MHz, Generator (2) is set to 14.139

or 14.05980 MHz while Generator (3) is set to 14.11980 or 13.95980 MHz. The "pilot tone" is 10 Hz above the IM3 product.

The three generators are accurately calibrated only for a load impedance that is very close to 50 ohms. For the WSE converter chand for the IC706MKIIG a matchbox in the form of a PI-filter was used to make the "Out" port of figure 11 see 50 ohms at the tefrequencies. This way the SWR at the generator outputs was made below 1.05.

The first line of the raw data files contains four numbers. The first is the nominal power to which all three generators were set toproduce the power levels at the "Out" port that are written as the following numbers on the first line. These power levels aremeasured with the HP8591A spectrum analyzer that was used to calibrate the generators. The difference between the calibratedoutput levels and these power levels give the attenuation between each generator and the "Out" port well within 0.1 dB.

The remaining lines contain the nominal levels of generators (2) and (3) which are set to the same nominal level followed by thenominal level for generator(1). The last two values are the Linrad S-meter readings for the signal of generator (1) and the IM3


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product. The difference between these two S-meter readings gives the difference in dB between the "pilot tone" from generator (and the IM3 product. The true power of generator (1) is fetched from the calibration data and hence the true level of the IM3product can be calculated with good accuracy.

The raw data at 20 kHz and the raw data at 100 kHz were processed with calim.fand the output from gencal.fto produceic706a20.txt and ic706a100.txt

The output files contain the raw data followed by the observed level of the IM3 product referenced to the "Out" port, the expectIM3 level calculated as 2 * P2 + P3, the true levels from generators (2) and (3) at the "Out" port and finally the difference.

Fig.12. IM3 data for the IC706MKIIG.

The IC706MKIIG is well characterized by the IP3 number which is about -3 dBm both at 20 kHz and at 100 kHz. When looking the difference between the measured IM3 levels and the theoretical computed from 2 * P 2 + P3, one finds an error of -8.5 dB at kHz and an error of -7.5 dB at 20 kHz. The formula would be correct if IP3 were 0 dBm. The full formula is P1 - IP3= 2 * (P2 - I

+ (P3 - IP3). This means that IP3 is half the errors, -4.2 dBm at 20 kHz and -3.8 dBm at 100 kHz. The values obtained graphicallfrom an extrapolation in figure 12 are less accurate, but fairly close. Presumably the first IF filter is narrow enough to not allow bsignals to pass in which case the IP3 number is IP3 for the front-end. The noise figure is 12.1 dB.

The intermodulation level follows the third order law up to about -33 dBm where the deviation is about 0.5 dB. IM3 is then aboutdB below the two tones.

IM3 in FT1000D

The FT1000 has a front-end AGC. At at the point where it starts to reduce the signal, intermodulation generated in the mixer will


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reduced. The minimum front-end intermodulation occurs when the RF volume is set for the S-meter to show S9+60 dB. The inpusignal required to reach this level is +8 dBm. At higher attenuation, the intermodulation caused by the attenuator diodes increaseand dominates over the intermodulation in the mixer.

The front end AGC uses a PIN(?)-diode attenuator and it has some effect on the input impedance of the FT1000 unit. Small chanon the input impedance are visible already at an S-meter reading of S9, corresponding to a signal level of -46 dBm or two equallystrong signals at -52 dBm. With an IP3 in the order of +21 dBm, the FT1000 will have an IM3 level of -52 dBm for an input signthe order of -3 dBm. With the "pilot tone" from generator (1) at -52 dBm present simultaneously, based on the observation that thimpedance starts changing when the meter is at S9, the IM3 measurements can be expected to deviate from the third order law dto the AGC when IM3 is about 50 dB below the test signals. This means that the IP3 measurements should be unaffected by thefront end AGC at levels where the third order law usually is accurate within 0.5 dB.

Figure 13 shows the measured result for the FT1000D at 20 kHz and at 100 kHz. The frequency stability is very good when anexternal fan is used to prevent the built-in fan from going on and off so it is possible to follow the IM3 product deep down into thnoise. The bandwidth was 0.5 Hz with an S-meter averaging time of 10 minutes for the lowest readings where the IM3 product wonly about 6dB above the noise in 0.5 Hz bandwidth.

The raw data ft1000_20.txt and ft1000_100.txt with the set-up of figure 11 and ft1000_20att.txt and ft1000_100att.txtwith a 20 attenuator added were processed with calim.fand the output from gencal.f(see above) to produce the data files ft1000a20.txt anft1000a20att.txt for 20 kHz separation and ft1000a100.txt and ft1000a100att.txt for 100 kHz separation.

There are several deviations from the third order law. First of all, adding a 20 dB attenuator forces the notch filter to work with 20dB higher signal levels which will make the intermodulation caused by the quartz crystals to be 60 dB higher than without the

attenuator. The attenuator will of course attenuate the intermodulation products by 20 dB but still the ratio signal to IM3 will bedegraded by 20 dB.

Fig.13. IM3 in a FT1000D. Red is at a frequency separation of 20 kHz while green is at 100 kHz. Circles are with the set-up of

figure 11 while '+' are with a 20 dB attenuator added.

It is quite clear from figure 13 that power levels of about -23 dBm with the attenuator included are perturbed by intermodulationexternal to the FT1000 when the frequency separationis 20 kHz. Near this point adding a 2dB attenuator to the 20 dB attenuatoractually increased the IM3 product. IM3 from the notch filter goes down by 2 dB while IM3 from the FT1000 goes down by 6 dB


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What happens to the summed IM3 depands on the exact amplitude and phase relations. It is clear from the red + marks on fig13 that the IM3 level out from the notch filter is at about -85 dBm for 20 kHz frequency separation when the signal pair is at -2dBm. The red circles show that the FT1000 intermodulation is at -50 dBm for -2 dBm input. With the IM3 produced in the notchfilter about 45 dB below the IM3 produced in the FT1000 when measuring without the 20 dB attenuator one might think themeasurement is accurate. The amplitude ratio corresponding to 45 dB is about 180. Since the phase relation between the twointermodulation sources is unknown, the error is +/- 179/180 in amplitude which converts to 0.05 dB. A small error, but having thIM3 from the signal source suppressed by 20 dB only as is often considered satisfactory means that the amplitude uncertainty is 1% which gives an uncertainty of 20 % or +/- 0.8 dB in the IM3 level. At a frequency separation of 100 kHz there is much lessintermodulation produced in the quartz crystals and the test signal is probably accurate over the entire range of figure 13 when th20 dB attenuator is not used.

At 20 kHz there is a significant increase in IM3 level at about -15 dBm. This is the level where the S-meter just reaches zero. It isan artifact or a measurement error, there is a phenomenon inside the FT1000 that raises the IM3 level by about 2 dB. At this signlevel the AGC can be switched off without any effect at all on signal levels except that one does not have to average out the smaamplitude variations that comes from the AGC system. At higher signal levels where AGC is active, the signal levels are perfectlstable with 3 decimals, the amplitudes are kept constant with a feed-back loop, the AGC. When the feed-back is broken because signals do not reach the AGC threshold, there is a noise voltage that modulates the receiver gain at random causing a leveluncertainty of about 0.05 dB. Switching the AGC off removes this uncertainty. As can be seen in the data files, the gain of theFT1000 increases by about 0.5 dB when the signal pair is added at 20 kHz frequency separation. This increased gain is causedexclusively by the nearest signal which is spaced 20 kHz away. The other test signal 40 kHz away does not affect the receiver gaThis phenomenon, that the gain increases when a strong signal is added at a frequency separation of 20 kHz is unaffected by the Rgain setting up to S-meter readings of about S7, close to the point where one can see input impedance changes due to the front enAGC.

It is clear that the FT1000 deviates slightly from the third order law at a frequency separation of 20 kHz. The discrepancy betweeIP3 values extrapolated from S5 and from the noise floor in 500 Hz bandwidth (-126 dBm) is about 1 dB so for all practical purpthe FT1000 IM3 is well characterized by an IP3 value of +22 dBm. The noise figure is 19.3 dB

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Date post:	07-Apr-2018
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SM 5 BSZ - Inter Modulation in Receivers

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