LTM9004
22
9004fa
For more information www.linear.com/LTM9004
applicaTions inForMaTion
With the RF AGC set for minimum gain, the receiver must
be able to demodulate the largest anticipated desired
signal from the handset. This requirement ultimately sets
the maximum signal the LTM9004 must accommodate at
or below –1dBFS. Assuming a handset average power of
+28dBm, the minimum path loss called out in the specifi-
cation is 53dB. The maximum signal level is then –25dBm
at the receiver input, or –30dBm at the LTM9004 input.
This is equivalent to –14.6dBFS peak.
There are several blocker signals detailed in the UMTS
system specification. The sensitivity may degrade to no
more than –105dBm in the presence of these signals. The
first of these is an adjacent channel 5MHz away, at a level
of –42dBm. This amounts to a peak digitized signal level of
–11.6dBFS. The resulting sensitivity is then –112.8dBm.
The receiver must also contend with a –35dBm interfer-
ing channel ≥10MHz away. The RF frontend will offer no
rejection of this channel, so it amounts to –6.6dBFS peak,
and the resulting sensitivity is –109.2dBm.
Out of band blockers must also be accommodated, but
these are at the same level as the inband blockers which
have already been addressed.
In all of these cases, the typical input level for –1dBFS
of the LTM9004 is well above the maximum anticipated
signal levels. Note that the crest factor for the modulated
channels will be on the order of 10dB to 12dB, so the
largest of these will reach a peak power of approximately
–6.5dBFS at the module output.
The largest blocking signal is the –15dBm CW tone ≥20MHz
beyond the receive band edges. The RF frontend will offer
37dB rejection of this tone, so it will appear at the input of
the LTM9004 at –32dBm. Here again, a signal at this level
must not desensitize the baseband module. The equivalent
digitized level is only –41.6dBFS peak, so there is no effect
upon sensitivity.
Another source of undesired signal power is leakage from
the transmitter. Since this is an FDD application, the re-
ceiver described herein will be coupled with a transmitter
operating simultaneously. The transmitter output level is
assumed to be ≤+38dBm, with a transmit to receive isola-
tion of 95dB. Leakage appearing at the LTM9004 input is
then –42dBm, offset from the receive signal by at least
130MHz. The equivalent digitized level is only –76.6dBFS
peak, so there is no desensitization.
One challenge of direct conversion architectures is 2nd
order linearity. Insufficient 2nd order linearity will allow
any signal, wanted or unwanted, to create DC offset or
pseudo-random noise at baseband. The blocking signals
detailed above will then degrade sensitivity if this pseudo-
random noise approaches the noise level of the receiver.
The system specification allows for sensitivity degrada-
tion in the presence of these blockers in each case. Per
the system specification, the –35dBm blocking channel
may degrade sensitivity to –105dBm. This is equivalent
to increasing the effective input noise of the receiver to
–148.2dBm/Hz. The 2nd order distortion produced by the
LTM9004 input is about 18dB below this level, and the
resulting predicted sensitivity is –116.6dBm.
The –15dBm CW blocker will also give rise to a 2nd order
product; in this case the product is a DC offset. DC offset
is undesirable, as it reduces the maximum signal the A/D
converter can process. The one sure way to alleviate the
effects of DC offset is to ensure the 2nd order linearity of
the baseband module is high enough. The predicted DC
offset due to this signal is <1mV at the ADC input.
Note that the transmitter leakage is not included in the
system specification, so the sensitivity degradation due
to this signal must be held to a minimum. The 2nd order
distortion generated in the LTM9004 is such that the loss
of sensitivity will be <0.1dB.
There is only one requirement for 3rd order linearity in
the specification. In the presence of two interferers, the
sensitivity must not degrade below –105dBm. The inter-
ferers are a CW tone and a WCDMA channel at –44dBm
each. These will appear at the LTM9004 input at –29dBm
each. Their frequencies are such that they are 10MHz and
20MHz away from the desired channel, so the 3rd order
intermodulation product falls at baseband. Here again, this
product appears as pseudo-random noise and thus will
reduce signal to noise ratio. For a sensitivity of –105dBm,
the allowable 3rd order distortion referred to the receiver
input is then –148.2dBm/Hz. The 3rd order distortion
produced in the LTM9004 is about 23dB below this level,
and the predicted sensitivity degradation is <0.1dB.