CMX994 Datasheet, PDF (37/61 Pages) CML Microcircuits – Local Oscillator

Direct Conversion Receivers
CMX994/CMX994A/CMX994E
8.1.2
DC Offsets
To provide an approximate correction of dc offsets use the following procedure: with the attenuation set at
minimum (i.e. max gain) measure the offset at the CMX994/CMX994A/CMX994E I or Q output, then the table
in section 7.4.1 can be used to give the required offset correction. The range double bit ($12, b2) can be used
to increase the correction range if required. Note that at high attenuation settings there may be an additional
error due to residual offsets from within the VGA as explained below.
The dc offsets in the CMX994 baseband path are shown in Figure 18. The voltages are only shown on the Q
path but the same considerations apply to the I path. Note that the dc offsets in I and Q paths will be
different because they relate to random offsets in a number of components; note also that the I and Q paths
are differential and that further details of the dc offset correction mechanism are given in section 6.2.1.
+
I Channel
+
-
V_mo
+
V_corr
+
-
V₀₁
Amplifier #1
Gain = +15
to -27dB
In 6dB steps
V_{offset Q Channel}
V₀₂
Amplifier #2
Gain =
+22dB
Where:
V_mo = dc offset at the output of the mixer
V_O1 , V_O2 = input offsets of amplifiers 1 and 2 respectively
V_corr = correction voltage (value set in Rx Offset register, $13)
V_offset = dc offset present at the output
Figure 18 I/Q Path dc Offsets
Referring to Figure 18, the approximate dc offset for a given gain setting can be calculated as:
V_offset = ( (V_mo + V_corr + V₀₁).G₁ .G₂ ) + ( V_{O2 .} G₂ )
The offset V_O2 can be estimated by setting the attenuation in Amplifier #1 to -42dB (G₁ = -27dB) and then
measuring V_offset , thus V_O2 = (V_offset / G₂ ). The sum of (V_mo + V₀₁) can then be estimated given that the gain
setting of G₁ and V_corr are known.
Assuming V_offset is minimised using V_corr at minimum attenuation then increasing the attenuation may result in
an increase in V_offset, however the error is typically ±15 mV (±55 mV absolute maximum). An example of the
variation in dc offsets with gain control is shown in Figure 19.
It should be noted that as the attenuation is increased the steps of the offset correction mechanism reduce
as the reciprocal of the attenuation. The result is that at maximum attenuation the offset correction steps
are
25mV x 10^(-42/20) = 0.2mV.
 2015 CML Microsystems Plc
Page 37 of 70
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