2. GENERAL ARCHITECTURE
2.1
OVERVIEW
In this sub-section, we discuss general features of the L-17C's architecture, which is shown in Fig.
1(a). The standard pin-out configuration for the 36-pin version is shown in Fig. 1(b). Detailed
discussions of the individual sections of the L-17C are given in the following sub-sections.
The IC consists of 3 cascaded linear amplifiers, A1, A2 and A3, seven log stages, and an output
amplifier. There is an extra log stage for linear extension. The transfer curve enters the
logarithmic regime when the input to the first log stage reaches 18 mV. Each log stage is active
for 6 dB (12 dBV) for a linear input, giving an overall range of approximately 84 dBV for a linear
input. With a tunnel- diode detector, it is possible to achieve a dynamic range of up to 50 dBm, if
full advantage is taken of the linear extension capability.
The gain of A1 is adjustable; the gains of A2 and A3 are normally x4 and x16 respectively, and will
not need adjustment if a normal log transfer function is desired. A satisfactory transfer curve will
be obtained provided that the quiescent balanced outputs of the linear amplifiers remain within 3 to
4 mV of ground. To set the start of logging to correspond to the desired RF power, the gain of A1
should be adjusted appropriately, as described below.
It is easy to estimate the input voltage at the start of logging, as is illustrated in the following
example. If the input amplifier gain is set to 10, the onset of logging occurs at an input voltage of
V
in
, where V
in
is given by:
V
in
x 10 x 4 x 16 = 18 mV
Therefore, V
in
= 28
V.
For an application such as a detector log video amplifier (DLVA) using a tunnel diode detector with
K = 700, this corresponds to an input power of -44 dBm.
(1)
Note that with this setting, the overall
gain (assuming an output slope of 50 mV/dB) is:
10 x 4 x 16 x 6.5 = 4160. The factor 6.5 represents the voltage gain between the input of
the log stages and the overall output, for a transfer slope of 50 mV/dB.
In order to shift the onset of logging to higher input powers, the gain of A1 can be reduced.
However, this amplifier is very quiet, and it should not be run at a gain less than 4 if TSS
(tangential signal sensitivity) is an important concern. It is better to switch off the current in the
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(1)
For applications other than DLVAs, please see section 3 below. For conversion purposes, non-
DLVA users should note that a detector with K = 700 gives an output voltage of 700
V at an input
power of -30 dBm, and since a detector is a square-law device at low powers, V
in
is proportional to
input power.
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