LPF on TG

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Sam
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Joined: Sat Apr 28, 2018 6:49 pm

LPF on TG

Post by Sam » Wed Mar 02, 2011 5:44 pm

In the process of installing the new Mixer 3 amps, I have been playing with the TG output, which at the moment comes directly from mixer 3 with no buffer amp. I checked the output level at 100 MHz (running it to an HP analyzer), then inserted a 700 MHz low pass filter and checked again. The output level dropped 1.3 dB. So the filter must have 1.3 dB insertion loss, right?



Wrong. The actual insertion loss is 0.2 dB. The remaining loss comes from the filter reflecting LO2 and LO3 components back into the mixer. (Or, you could look at it as the filter shorting those components to ground). Due to phasing, the reflected components reduce the mixer output of 100 MHz by 1.1 dB; with a different reflection phase they might increase the output, and the effect might even be more than 1.1 dB.



Suppose you had two DUTs, one a 700 MHz low pass filter and one a 700 MHz high pass filter, both with insertion loss of 0.2 dB at 100 MHz. If you attach the low pass filter you get 0.2 dB of true insertion loss plus the bogus 1.1 dB decrease in output. If instead you attach the high pass filter, you get the correct 0.2 insertion loss, because the high frequency components are largely unaffected.



The point is that the measurement of a DUT at low frequency will be affected by its performance at high frequency, which is very undesirable. A side-benefit of the TG amp is that it isolates the output and prevents these reflections from having any significant effect. But an alternative is simply to leave a low-pass filter in place for all measurements, which is workable as long as you are not concerned with frequencies above the passband. The low pass filter has the advantage of also reducing any stray 1013.3 signal and also reducing 3G components that might be interpreted as 1G. Finally, the filter reduces the LO3 component, which causes a 10.7 MHz spur.



I use a MiniCircuits LFCN-575D low pass filter, which really kicks in at around 700 MHz. It is similar to a large SMT ceramic capacitor, but has its ground terminal on the bottom, extending up the sides. You build the board with connectors sticking out the ends like an attenuator. Multiple copies of the tiny board have been included in several of the recent PCB buys. (I think it is marked "LFCN".) I frequently use one on the TG and one on the MSA input. I suspect that some people who bought those boards have extras, so if anyone is interested in one you could post an inquiry here.



Sam W.

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