Zonge GDP-32 - Gains

RECEIVER SETUP
Section 6, Page 29 May 2002
6.5 GAINS
HOW GAINS ARE DEFINED
There are two ways of expressing gains in the GDP-32
II
: as a multiplier, and as a binary gain
number. Gains are expressed by the relation G = 2
g
(where “G” is actual multiplicative gain,
and “g” is the binary gain number.) For example, if the multiplicative gain “G” is 64, then the
binary gain “g” is 6, since 64=2
6
.
The receiver design employs three gain stages, G0, G1 and G2. An input attenuator is also used.
Thus, the Gains for each stage are:
G0 = 2
g0
G1 = 2
g1
G2 = 2
g2
A = 2
a
where gains can be:
G0 = {1, 4, 16}
g0 = {0, 2, 4}
G1,G2 = {1, 2, 4, 8, 16, 32, 64}
g1,g2 = {0, 1, 2, 3, 4, 5, 6}
and the attenuator can be:
A = {1 (“OUT”) 0.125 (“IN”)}
a = {0 (“OUT”) -3 (“IN”)}
The total receiver gain is G = G0 x G1 x G2 x A (or =g0+g1+g2+a.) Hence the gain can vary
from G=0.125 (g=-3) to G=16,384 (g=16.) The “Bits of Resolution” section (Section 6.6)
discusses the various definitions of “gain”.
GAIN-SETTING STRATEGY
Whether done automatically or manually, setting gains should follow a standard approach.
1. The GDP-32
II
senses the total signal level and automatically sets the Attenuator IN or
OUT. If the voltage exceeds ±3.5 V at unity gain, the Attenuator will be automatically
set to “IN”.
2. Any self potential (SP) present in the signal is offset.
3. The receiver bypasses the anti-alias filter and adjusts the first gain stage (G1). [Note: In
the manual mode, bypassing the antialias filter is not possible.] Since the G1 Gain stage
comes before the anti-aliasing filter, this process prevents front-end saturation.
G1 increases as required to adjust the signal amplitude to ±1.0V or less for NOISY Gain mode,
or ±2.25V or less for STANDARD gain mode. NOISY Gain mode provides sufficient headroom
so that noise spikes and SP drift should not saturate the system. G1 can be increased to a
maximum of 64. (G0 can be set to 1, 4 or 16)
4. The receiver engages the anti-aliasing filter and adjusts Gain stage G2 until the input to
the A/D is equal to or less than the maximum voltage permitted for the gain mode
selected.
NOTE: Regardless of the frequency being used, the automatic gain setting routine samples a
burst of the incoming signal at 32 kHz to check for low frequency radio noise. This prevents
input amplifier saturation when using low (IP) frequencies.
NOTE: The NOISY Gain mode is the default mode for all programs except TEM and MT/AMT,
and is used for most applications.