Variation in the sensitivity of the receiver with interfacing

Hi,

This is to bring to notice that we have observed a variation in the threshold of the receiver if the interfacing is not proper, my observation is as follows:

1. In our receiver, the RF is 14.5GHz, first down conversion is at 595 MHz so LO1 is 13.9GHz, the scheme of LO1 is a fractional PLL followed by a doubler and an amplifier, doubler and an amplifier is an separate entity and is connected to PLL through a semi rigid cable, in this mode of connection we observed a variation in the threshold then we put a 3dB pad at the output of the PLL, immediately we got an improvement in the threshold( Here in the doubler and amplifier entity, we have kept isolators at the input and output), what may be the reason of the improvement !, Plz guide,.

I would suspect either an oscillation (the PLL output stage, or the mixer LO amp stage if it is active) or some sort of ground loop making the PLL wonky. Try an inside/outside DC block at the LO port and see if that also solves the problem (a ground loop problem).

is the PLL signal going anywhere else? In rare cases inband spurs can be generated at the INPUT of an amplifier, and end up screwing up another part of a system

Sounds VCO Pulling Effect because of Doubler.
PLL may not pull-in the carrier in high pulling effects and therefore VCO signal can have some ringings about carrier.
I guess an isolation amplifier should be used before doubler.

It will be helpful if you can offer a block diagram with details of power levels. You did not define the "threshold". I would prefer to see how receiver noise figure changes.

Frequency multipliers used as local oscillators require a careful filtering of the output signal. I often witnessed that unwanted harmonics caused considerable changes in receiver NF and IF spectrum.

Microwave mixers are often sensitive to LO input power while their LO frequency bandwidth can handle a pulling of LO frequency source. PLO source should have a buffer to prevent the backward action of its load. And the frequency multiplier, doubler or tripler, is also sensitive to its input power.

Did you try to check the LO spectrum when you insert the 3-dB pad as you wrote? Did you see the IF spectrum before and after the modification?

I do guess this is a VCO pulling effect , by introducing a 3dB pad at the output of PLL, ISOLATION between PLO and doubler and driver amplifier entity is increased by another 6 dB , hence input power variation to doubler is reduced .

Yes, there is a filter immediate after doubler and after that a driver amplifier to drive the mixer, and the unwanted harmonics are 40dBc below the required the carrier.
Lo spectrum spurs are reduced below the noise floor after the insertion of 3dB pad, and the output power at IF increased after that.



How do the unwanted harmonics of the frequency multiplier is a cause of considerable change in receiver NF and IF spectrum?

These harmonics can reach to the VCO by ground paths and they impact VCO.
I mean, GND must be very clean and any fluctuation on GND will shift/impact the VCO frequency.They impact also LNA NF by the same method unless you have used differential topology.

The unwanted LO harmonics can convert other than desired frequency band into the IF as well as unwanted signals may overload the IF amplifier and following stages if unfiltered.

Mixers generally cannot operate without RF, LO and IF filters. LO frequency multipliers are used to save money but their specifics must be cared for.

Power amplifiers needed in LO multipliers also utilize commercial MMICs which are often designed as wideband devices. Their own noise entering a mixer can and does affect mixer noise figure.

If your receiver has a LNA before the mixer, take care that no stray signals from LO multiplier can enter its input. Overloading a LNA is often a problem, increasing receiver NF and generally affecting the whole receiver.