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K-Band Focal Plane Array:
Integrated Downconverter Modules
1/30/2009
Matt Morgan
National Radio Astronomy Observatory
The current GBT IF Transmission System has only 8 channels, requiring outputs
to be multiplexed in order to service 7 dual-pol pixels simultaneously.
Two downconverters are needed to implement the IF multiplexing scheme.
The second downconverter simply omits the latter mixing stage.
Dimensions of IDM1 are 5.0 x 3.4 x 1.0 inches
Dimesnions of IDM2 are 3.0 x 3.4 x 1.0 inches
Problem #1: Low Gain and RF Bandwidth (IDM1)
Rev: January 2007
Rev: May 2007
"Specifications subject to change without notice." – No kidding.
Low LO Amplifier Gain Impacts Mixer Bandwidth
IDM2 Does Not Have Low Gain/Bandwidth Problem
Problem #1: Low Gain and RF Bandwidth (IDM1)
- Caused by improper biasing of ALH444 amplifier
- gate voltage was not servo'd due to unusual bipolar gate supply
- Fixed in IDM2, which has identical RF and LO1 components, by
adjusting the bias resistors.
- Long-term solution: servo the negative gate voltage, as has
already been done for other amplifiers in this module.
Problem #2: IF Channel Crosstalk (IDM1)
- excessive IF ripple
- independent of RF/LO frequency
- NOT present in IDM2, in which IF
circuit paths are channelized
IDM2 Does Not have Ripple or Isolation Problem
IF Channels located in PCB cavity on IDM1
Solution: Isolate IF channels as they are in IDM2
Problem #3: Spurious Mixing Products (IDM1)
Known Spurs
(LO1 is generated by a doubler in the LO Distribution System)
Tricky, because I need:
- sharp cutoff between 7.7 GHz and 10.1 GHz
- VERY deep rejection over 25-35 GHz (LO1 range)
- moderate rejection all the way up to 100 GHz (LO1 harmonics)
NOT easy to do in one filter!
Deep Stop-Band Low-Pass Filter for Spur Elimination
Alternative Low-Pass Filter for Spur Elimination
Cascade combination of three filters – to be used only if the preferred
solution is not manufacturable.