Useful Information - AOR SDU5600 Operating Manual

Section 7

7. Useful Information

The SDU5600 has been designed to operate with a wide-band companion receiver such as the
AR5000A. Therefore, there are some differences in operating characteristics with the unit when
compared with the spectrum analysers which are designed for more general testing / measuring
applications. It is advisable that you take into account the following information while making yourself
familiar with the SDU5600.
Signal strength
The signal strength which the SDU5600 indicates is measured at the RF input of the SDU (not the signal strength of the companion
receiver at the antenna terminal). If the radio provides a gain / loss from the aerial input to IF output, the calibration of displayed signal
strength will be affected. Most receivers provide a POSITIVE gain of +10dB or so. The receivers AGC and attenuator will also affect
results.
Frequency characteristics of the companion receiver
The SDU5600 operates with a wide span of frequency (up to 10MHz band span). It must be noted that the companion receiver's RF
circuit and / or IF filter configuration may affect the spectrum monitoring with some interference being observed. In particular, when the
wider band span has been selected such as a 10MHz, the far ends of the band edges are most affected depending on characteristics
of the receiver in use. The AR5000A is usable over 10MHz bandwidth, as is the AR3000A, however the output level of the AR8600/
MK2 drops beyond 4MHz or so.
Excessive input from the receiver
While receiving strong signals (e.g. broadcast station) with an external aerial connected to the companion receiver, the noise floor
may appear to be incorrectly lifted due to distortion from the receiver. Use of the attenuator of the receiver may help reduce these
phenomena... however, generally speaking the front end selectivity of the companion receiver is beneficial compared to a directly fed
spectrum analyser.
Selection of RBW
When monitoring the signals of broader bandwidth with the narrower RBW selected, the displayed signal strength may appear
reduced (less accurate) due to the fact that only part of the energy from the broader bandwidth can pass through the narrower
sampling filter, resulting in loss of signal strength to some extent. When monitoring wider signals (such as FM broadcast), use a wider
RBW such as 128kHz.
AGC action of the companion receiver
It must be noted that the AGC action of the companion receiver may affect the spectrum monitoring by the SDU5600. It may be
observed that the overall signal strength has dropped when abrupt changes in receiving conditions have occurred (for example, when
quickly tuning from a weak signal to a strong signal). This is because the input from the receiver's IF is significantly reduced by AGC
action. Switching the companion receivers AGC off may eliminate this effect but the reception will become unstable and distorted with
AGC off.
Image signal
Occasionally false signals (spurious image signal) can be spotted on the display, sometimes moving at random, sometimes to the
opposite direction while monitoring the spectrum. This is due to the image signals which are generated by characteristics of the
companion receivers superheterodyne circuit design or intermodulation products, ALL receivers will present this affect to some
degree.
With the SDU5600 operating at its widest sweep range (10MHz) when coupled to an AR5000A, an unwanted spurious signal can be
observed approximately 4.8MHz away from the centre frequency (moves slightly with AR5000A frequency). This is present at all
frequencies although it will jump to the other side of the screen when the AR5000 is tuned above 1.4GHz.
The spurii appears to be leakage of the 2nd local oscillator DDS signal at 5.8MHz to 6.7MHz (hence when viewed from the IF
frequency of 10.7MHz becomes 4.9 to 4.0MHz away). This is not a problem during normal receive with the AR5000A as the 10.7MHz
filter removes any signal but is present at the IF output taken before the filter. It is also not a problem if the sweep width on the
SDU5600 is reduced slightly.
To address this, select a bandwidth of about 9.8MHz rather than 10MHz, so the spurii drops off the edge of the display. The spurii is
usually in the area of -70dBm to -60dBm.
The AR8600/MK2 produces a 455kHz image, its a compromise of design and has to be worked around.
RF earth
Of course all electrical equipment produces some noise, especially where large displays are involved. Connecting a good RF earth to
the equipment can help with general low level spurii... the AC adapter is not earthed, avoid earth loops.
Minimum frequency span
The SDU5600 has a minimum frequency span of 160kHz (0.160MHz). It is not possible to set the frequency span narrower than
160kHz.
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