Several important ham radio projects of the last decade successfully used interesting and easy solution of RF pre-amplifier, based on the original experiment implemented by Bill Carver, W7AAZ. Quad JFET gain block with paralleled J310 JFET´s in source gate transformer feedback topology achieves interresting key parameters as a low NF, high OIP3, good gain and reverse isolation. Martein, PA3AKE, used this pre-amplifier based on SMT in his high mode mixer frontend project.
Implementing this project I tried to measure some of key parameters (except of NF) of pre-amplifier used in the selectivity board. Home made N2PK VNA with MyVNA software gives results which are shown in the following pictures. As can be seen, the required parameters have been obtained and the project is in this respect very well repeatable.
Pre-Amplifier Gain 12,3dB
Input Impedance 48,5 / 0,7 Ohm
Input Return Loss -36,5 dB
Reverse Isolation -46 dB
I measured OIP3 as shown in the picture. As the RF signal sources two home made N2PK VNA analyzers in the generator mode were used. Using two PCs and MyVNA software from G8KBB allows to control both of them independently.
It was also interesting to verify the quality of the new spectrum analyzer USB-SA44B which I bought recently from Test Equipment Plus. Parameters as dynamic range, RBW and floor noise well meet the needs of ham radio enthusiasts.
As a prevention against generating own IM products, I used 20 dB attenuator at the input of Spectrum Analyzer. Means that the pre-amplifier main output signal level was really close to 0 dBm. In such a case measured level defference between main signal and IM product dividing by 2 give to us OIP3 value.
Next pictures show IM measuring results for different space between both 9 MHz signals. I started with space of 10 kHz, next picture shows space of 1 kHz, then 100 Hz and finaly 50 Hz space between both RF signals.
The results are very similar. The difference between main signal level and IM product level moves between 76.1 dBc and 80.8 dBc for different spaces. OIP3 of the Quad JFET pre-amplifier is +38 dBm in worse case. If the gain of pre-amplifier is 12,3dB, this very easy and cheap solution achieves a very good +25,7 dBm of IIP3.
To achieve high OIP3 drain current (Ids) must be set to 22-24 mA per device. Pre-amplifier in Martein´s project was designed and optimized for frequencies around 9 MHz, but it maintains very good parameters in wider frequency range.