JRG-74

**Comparisons of simulated and measured emission spectra of**

**linear beam output devices**

**1 Introduction**

At the European Space Agency's Research and Technology Centre ESTEC,
radar unwanted emission tests were performed in August 2005. The
performed tests were focused on the measurement of unwanted emissions
from radar pulsed Travelling Wave Tube Amplifiers (TWTA). The objectives
for these tests were:

-   To analyse what discrepancies potentially arise between the theory
    and practical case;

-   To check if the design objective as described in Annex 8 to
    Recommendation ITU-R SM.1541 can be reached.

In order to fulfil these objectives, both unwanted emissions from
modulated and unmodulated pulsed were measured;

**2** **Measurement setup**

In Figure 1 a block diagram is shown for the microwave measurement setup
(non-radiated testing). The TWTA is a completely self-contained unit
(for mounting in a standard 19-inch rack) that provides all necessary
power supplies, forced-air cooling, RF components and controls. The
frequency span for this TWTA is from 2 -- 4 GHz.

With respect to measurements on existing radars, with this measurement
setup quite some freedom is available for selecting different waveforms,
i.e. non-chirp signals vs. chirped (linear FM). Results are to be
compared with analytical expressions from Fourier analysis adapted for
different radar pulse modulations.

### ![](media/image1.png){width="5.375in" height="4.0256944444444445in"}FIGURE 1

There may also be a need to include a spectrum analyser before the TWTA
(this would be interesting in order to investigate the added spectral
components by the tube). The chirp bandwidths to be investigated are up
to 10 MHz (this would be the maximum for the used waveform generator).
The attenuation numbers in the coupler-blocks (10 dB and 30 dB) are
indicative and depend on several factors (mainly choice of components).
The power meters before and after the tube are for control purpose
(input range and tube saturation point) and initial check of the tube
(gain).

**3 Performed measurements**

The measurement set-up was initially used to investigate unwanted
emissions for unmodulated pulses with different pulse length (e.g. 1, 2,
5, 10 μs**).** This way, experience was gained in the measurement of
unwanted emissions and easy comparison with Fourier analysis could
readily be made. The spectrum analyser was required to sweep up to 1 GHz
w.r.t. the carrier. The measurement will therefore be out-of-band and
spurious and will not include any higher order harmonics of the carrier
(although these have been checked during the testing).

The following table reflects the performed tests for linear FM modulated
pulses.

  ----------------------------- ------------ -------------- ---------------
                                **Chirp **   **(MHz)**      ** **

  **Pulse Length (μs)**         **1**        **5**          **10**

  **1**                         []{.mark}    []{.mark}      

  **5**                         []{.mark}    []{.mark}      

  **10**                                                    

  **50**                                                    

  **100**                                                   
  ----------------------------- ------------ -------------- ---------------

If the possibility would exist to work with different pulse rise- and
fall-times (mostly dependent on the tube characteristics), this would be
a bonus. In any case the pulse rise- and fall times would need to be
characterised before and after the tube (oscilloscope). Unfortunately,
due to the tube characteristics and the fact that the tube was driven
into saturation, this was not possible. The pulse rise- and fall-times
for all investigated pulses were \~10 ns.

**4 Fourier Analysis**

For the case of radar emissions, a method has been developed that makes
use of spectral representations based on Fourier analysis in order to
give an accurate estimation of the unwanted emission power levels for
different radar pulses. The output of this analysis tool has been
validated.

In order to arrive to accurate spectral representations of modulated and
unmodulated radar signals, first the shape of the pulse needs to be
taken into account. Here, trapezoidal pulses will be used where pulse
rise- and fall-times are similar. The radar spectra can be compared to
the OoB mask for primary radars as contained in Figure 25 of Annex 8 to
Recommendation ITU-R SM.1541 (page 48). Since the mask is expressed in a
frequency separation as a percentage of the --40 dB bandwidth (B~-40~),
B~-40~ is obviously dependent on the radar parameters and needs to be
calculated according to formulas in Annex 8 (Rec. ITU-R SM.1541).

The sample plot in Figure 2 makes a direct comparison of the simulated
spectral representation for System 2 from Rec ITU-R M.1463 and the
envelope mask from SM.1541

FIGURE 2

![](media/image2.jpeg){width="5.65in" height="4.2375in"}

**3 Conclusion**

The measurements were done using a spectrum analyzer in its 'maximum
hold' setting, which means that the measurements can basically be seen
as peak envelopes. It can be clearly seen from the figures in the two
annexes that the theoretical spectra are very close to the actual
measurements using the same parameters. Where this was not the case, it
was often due to insufficient measurement resolution. In the laboratory
setup a frequency sweeper has been used (waveform generator in the block
diagram) that is expected not to have sufficient spectral purity to
reach the required noise floor to conduct measurements around 100 MHz
away from the carrier.

Generally, it can be concluded that for the cases under study the
Fourier analysis method is suitable for the estimation of unwanted
emission power levels.

Regarding the design objective, it can be concluded that for the
investigated setup, different pulses and modulation, the design
objective could not be reached in most cases. This result is supported
by both the simulated as measured spectra. In order to reach the design
objective additional pulse shaping may be required -- which may not be
possible or desired for a TWTA solution when driven in saturation.

**ANNEX 1: Results for trapezoidal unmodulated pulses**

![](media/image3.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image4.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image5.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image6.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image7.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image8.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image9.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image10.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image11.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image12.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}

![](media/image13.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}

**ANNEX 2: Results for trapezoidal linear FM pulses**

![](media/image14.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image15.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image16.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image17.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image18.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image19.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image20.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image21.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image22.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image23.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image24.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image25.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image26.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}![](media/image27.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}

![](media/image28.jpeg){width="5.766666666666667in"
height="4.321527777777778in"}