Updated June 28,
here to go to our main page on receivers
Page is under construction!
New for July 2008! Often
in microwave systems there is a need for two or more receive channels.
This could be because dual polarization is required, or in a monopulse
radar, when sum and azimuth or elevation are required on at the
same time (often the AZ/EL channels can be switched into a single
The image below of a four-channel
receiver was used by permission from Liam Devlin, who also contributed
some of the discussion on this page. Thanks!
for high isolation
One requirement for dual-channel
receivers is channel-to-channel isolation. This isolation is limited
by the antenna isolation for a dual-pol system; typically the "system
guy" will try to specify the receiver isolation to be 10 dB
better than the antenna, for example, 35 dB id the antenna provides
25 dB. If the system guy want 40 dB isolation between receiver channels,
tell him (her) to go take a hike!
One "sneak path" associated
with dual receiver channels is the LO signal path because a mixer
can leak its RF signal out the LO port. The two LOs must come from
the same source, by way of a splitter. If a Wilkinson splitter is
used, the isolation of the two branches is determined by your ability
to control the mismatch on the common port. The way to fix this
is to add LO amplifiers on each LO path, or isolators, either way
the leakage out of either mixer will be blocked! (We'll soon add
a picture to help you understand this...)
In order to save space, dual-path
MMICs are often employed (limiters,
LNAs, phase shifters, etc..), but these are the culprits in designing
for high isolation. Here's a Microwaves101
rule of thumb:
Generally, channel isolation is limited to around 30 dB for dual
channel MMICs. For a full receive chain, 25 dB isolation is a good
The general procedure is to terminate
input of B and output of B and measure gain through channel A as
a reference. The input to B while terminating input A, take the
"gain" (the cross-channel transmission) data and SUBTRACT
the second measurement from the first measurement. (so the "isolation"
is positive in dB).
To help visualize how this works, imagine each channel had a gain
of 50dB. Assume the channel to channel isolation is 30 dB. We then
measure from input B to output A (input A and output B terminated
in 50 ohms).
If we consider the case where all of the coupling (which limits
the isolation) takes place at the input. The measured transmission
(from input B to output A) is thus made up of -30 dB isolation plus
50 dB gain resulting in 20 dB measured transmission. To get your
30 dB isolation you take the 50dB gain and subtract the 20 dB cross
channel transmission. (If you do it the other way around you get