The time delay tempco of an RF cable and depends on the cable dielectric and construction.
It can be as high as 0.5ps/C/m for some cables. Thus the lengths of RF cables used should be kept short and wherever possible cables connecting the sources to the mixer inputs should have the same length, and have the same temperature. Since a well designed DMTD system has a resolution of 0.01ps or less at the mixer RF input frequency, ensuring that cable delay tempcos match is critical to achieving such performance.
A 10MHz isolation amplifier having a reverse isolation of 120dB has a worst case phase shift variation due to load VSWR variations of 100fs. To ensure that phase shift due to cross talk and load VSWR is 10fs or less the reverse isolation and crosstalk need to be greater than 140dB.
A significant reduction in phase shift due to load VSWR is achieved if the the length of the cable connected the isolation amplifier to the load is close to an odd multiple of one-half the signal wavelength (λ/2) in the cable.
The phase shift tempco of a state of the art 10MHz isolation amplifier can be as low as 1ps/C.
Using a bandpass filter with a Q of 10 will degrade this to 30ps/C or worse depending on the filter component tempcos. Achieving a system stability of 10fs or better requires stabilising the isolation amplifier temperature to better than 10mK when no bandpass filter is used. When a bandpass filter is used the temperature of the bandpass filter would need to be stabilised to better than 300μK to maintain the same performance. Whilst stabilising the isolation amplifier temperature to 10mK or so is challenging it can be done. Stabilising a bandpass filter temperature to better than 300μK is very expensive and very difficult to achieve.
To achieve the best long term performance it is essential to control the temperature of critical components by placing them in a temperature controlled enclosure. Achieving the required temperature stability (10mK or better for some components) requires a well designed oven with low internal temperature gradients and high internal temperature stability.
A method of anticipating the effect of ambient temperature changes and adjusting the oven power to compensate: 1) Analysis of High Performance Compensated Thermal Enclosures
A modern implementation of the passive thermally boostrapped oven technique used decades ago in portable standard cell air bath ovens: