Low noise pre-amplifier
Latest results
10/12/2008
The following is an extract from my workshop notebook and shows the results from measurements on a randomly selected 23cm VLNA kit from the latest batch. This was built up to check consistency of results etc.
The indicated NF, using the 'Martlesham' HP8970A and HP346A noise head is 0.26dB with an insertion gain of 37.5dB at 1296MHz. As usual, the gain peak is below the optimum NF frequency as can be seen in the frequency response rows. Here the gain is a maximum at 1200MHz. It may have peaked higher slightly below 1200MHz, but the run was only started at 1200MHz.
The sensitivity of the nf and gain to input return loss was checked by inserting, in turn, a 1 inch and then a 5/8 inch long SMA adaptor between the noise head and the pre-amplifier input.
As can seen, the figures change only slightly, being mainly the additional loss of the adaptor added to the pre-amplier's noise figure, at the measurement reference plane of the input connector. This shows that the input return loss of the pre-amp is reasonable (to be measured for this unit) and this has ensured that the measurement uncertainty is acceptably low. Indeed, the small change seen is within the normal range of measurement change for the instrument. Some other pre-amplifier designs show enormous changes in nf and gain with this test!
Finally, the pre-amplifier was checked on a second set of HP8970A gear (system 2, still within its calibration period) and the results noted. These agree, within a small margin, with the results from system 1.
The photo below shows the 23cm preamp input (yet again) as this may be the source of one of the most common problems achieving lowest noise figure with least uncertainty in the measurement. It also shows clearly how the coils L1 and L2 MUST be wound and orientated. The PTH holes for the source lead of Tr1 MUST be tinned. The holes nearest Tr1 are the most important. There MUST be a short length of Tr1 source lead left as shown. That provides the required source inductance.
I plan to repeat the measurements for latest build 13cm and 9cm pre-amplifiers and post the results here.
