Signal Processing In The If Stage; If Agc Processing - Kenwood TS-990S In-Depth Manual

06 DSP

Signal Processing in the IF Stage

The IF AGC processing for signal reception is the core of the TS‑990S.
In the TS‑870, TS‑2000S, and TS‑590S, the IF AGC processing is the most actively innovated algorithm in the DSP
signal processing and the tuning in the analog stages. We have found new tasks each time we create an innovation and
solve them through hardships.
The IF AGC processing we have been innovating through inheritance of our technological tradition represents our
KENWOOD tones.

IF AGC Processing

As described above, the TS‑990S can simultaneously receive two bands by using the main band and the sub band.
The basic scheme (block) of the IF AGC processing is the same for both the main band DSP and the sub band DSP.
However, concerning the analog‑step frequency configuration, down‑conversion is applied for the main band and
conditional down‑conversion/up‑conversion switching system is applied for the sub band, and the applicable roofing
filter types are different.
For both the main band and the sub band, the passband width of the preceding stage (analog stage) may be wider than
the final passband width of the DSP. The IF AGC processing was designed carefully so as to influence interfering signals
to the desired signal. The analog circuit characteristics of the main band are different from those of the sub band, thus
their response characteristics are respectively tuned.
AGC loops are placed before and after the interference rejection, such as an IF filter, manual notch filter, etc. The AGC
loop of the preceding stage functions to prevent signals whose level is higher than the reference from being entered into
the A/D converter for the IF input and is called the out‑band AGC loop. The AGC loop of the succeeding stage has the
same AGC performance as a traditional one and is called the in‑band AGC. With the in‑band AGC operated after the IF
filter and interference rejection, the desired signal can be highlighted.
The basic approach to AGC response characteristics is to control the AGC amplifier gain with an ultra‑high‑speed
attack, as well as conventional transceivers do, to control the gain without causing unnecessary amplitude fluctuation,
and to reduce factors causing fatigue felt by the long‑term listening.
One of the factors causing fatigue felt from listening are a momentary signal level overshoot by high‑speed attacks. This
phenomenon is inevitable in highlighting weak signals and minimizing distortion on the receiver. However, demodulation
with the phenomenon causes the sound quality to be hard, and the high‑processing‑speed release setting is of no use.
The sound quality at attacking is based on the comprehensive characteristics including the characteristics of the AGC
amplifier in the analog stage as well as the characteristics of the AGC loops in the preceding and succeeding stages and
the IF filter. In the TS‑990S, to drastically change the response characteristics for the out‑band AGC following the status
of the out‑band and in‑band AGC gain control and to realize ideal attack characteristics by two types of AGC loops having
different characteristics for the in‑band dual loop AGC (the single loop AGC for AM mode) are solutions for designing tasks.
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Fig. 57 IF AGC Processing Block Diagram