8. TEMPERATURE COMPENSATION
8.1
GENERAL GUIDANCE
The L-17C does require some temperature trimming for many applications. However, provided
that stable voltage regulators are used (
10 mV in dual-tracking over temperature), any
temperature- induced deviations are linear, and are therefore easy to correct.
Before we discuss temperature compensation of the L-17C, it is important to note the following
general points:
1. All setups must be mechanically sound and very clean. If the baseline output voltage on a
unit varies by more than the equivalent of 0.2 dB when the board is pressed with a pencil eraser,
the unit is very unlikely to pass temperature tests, due to the mechanical stresses caused by the
temperature changes. Similarly, any dirt, flux or other chemicals on the board can cause
electrochemical problems over temperature.
2. The unit must be checked in a compact box to ensure temperature uniformity. Not only are
the IC and the voltage regulators both sensitive to thermal gradients, but detector diodes are
among the best thermoelectric generators known. Also, because of small resistances and
potentials that can arise in box joints (especially when oxide films and/or dissimilar-metal fasteners
are involved), it is best if the four box walls are cut from a single metal block, or at least bent from
one piece and fastened at a single joint. Also, the box walls must be well-connected to the PC-
board ground plane.
3. All RF connections must be clean. In particular, if an anodized housing is used, make sure
that ground contacts on the RF are good. In order to achieve a proper ground when attaching
connectors to the box, "star" or cutting washers should be used to cut through oxide films.
4. The unit should not be temperature-tuned with pots in place; the pots used in selecting the
optimum room-temperature resistor values must be replaced by the appropriate fixed resistors.
This is because pots can change their values erratically when temperature- cycled, and often
exhibit temperature-induced hysteresis.
TEMPERATURE BEHAVIOR OF THE L-17C
If the amplifier were constructed with perfectly-matched components, the log transfer curves would
exhibit the behavior shown in Fig. 8.2 (over). Ideal units would show no temperature-induced shift
at zero input signal, and would also give a zero output level with zero input signal. The gains of
A1, A2, and A3 are not temperature-dependent, as they are fed back. However, the gains of the
log stages are inversely proportional to T, the absolute temperature. Consequently, at lower
temperatures they will enter the logging range at lower input voltages. For a temperature range of
-55
0
C to +85
0
C, this will give rise to a spread between the transfer curves of about 2 dB.
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